ABSORBER is a photovoltaic device where the material readily absorbs photons to generate charge carriers (free electrons or holes).

ABSORPTION occurs when incident light is absorbed by matter and in the process produces thermal, electrical or chemical effects.

ABSORPTION COEFFICIENT "ALPHA" is the depth at which short waves absorbed, i.e. at which the photons in solar radiation penetrate matter at the various wavelengths of the spectrum. The "alpha" of semiconductors is relative to the wavelength.

ABSORPTIVITY of a roof specifies the portion of the solar radiation absorbed by the roof and is converted into heat.

ACCEPTOR is a dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, thus providing a hole which can accept a free electron.

ACTIVATED SHELF LIFE is the period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

ACTIVATION VOLTAGE/S is the voltage/s at which a charge controller will take action to protect the batteries.

ADJUSTABLE SET POINT is a feature that allows the user to adjust the voltage levels when a charge controller becomes active.

AGEING EFFECT is also called degradation. The efficiency of amorphous silicon solar cells drops when exposed to light until it has reached the stable value (at around 10 to 30 perce of its initial value) over the course of its service life. The efficiency of "cascading" or "tandem" cells helps here considerably.

AGM (Absorbent Glass Mat) separator battery is a battery where the electrolyte (a mixture of water and sulfuric acid) is bound to a fiberglass mat. This is a type of so-called closed lead acid battery. A gas mixture (hydrogen and oxygen) is always generated when lead acid batteries are charged, and in normal operation this is internally recombined to form water. This removes the need for regularly refilling the battery cells with water, which is why these batteries are often described as "low maintenance" or even "maintenance free". AGM batteries are available from many different manufacturers for a wide range of applications. They usually have very good high current properties but are not very charge-cycle resistant.

AIR MASS (sometimes called Air Mass Ratio) is equal to the cosine of the Zenith Angle - that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation that travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness). "Equivalent air mass" is a measurement of the distance that light passes through the atmosphere relative to the shortest distance air mass 1.0 when sunlight is directly incident on the equator. In other space, the air mass is zero; in Germany at 48 degrees latitude, the average density of the atmosphere is air mass 1.5. The air mass value represents a specific spectrum of sunlight.

ALTERNATING CURRENT (AC) is a type of electrical current where the direction of which is reversed at regular intervals or cycles i.e. its polarity constantly changes. In the power grid of most countries (including Germany but excluding USA and Japan), the alternating current as a frequency of 50 Hz (Hertz) i.e. it assumes the positive or negative value of an ideally sinusoidal half-wave 50 times in one second. Alternating voltage or current is generated by rotating diesel generator or photovoltaic inverters. Electricity transmission networks use AC because voltage can be controlled with relative ease.

AMBIENT TEMPERATURE is the temperature of the surrounding area.

AMORPHOUS is a state of an element where the atoms do not have a defined crystal structure, but are merely in a disordered array.

AMORPHOUS SEMICONDUCTOR is a non-crystalline semiconductor material that has no long-range order.

AMORPHOUS SILICON is a thin-film, silicon material having no crystalline structure. ASI (a-Si) is the abbreviation for amorphous silicon. The atoms in amorphous material form an irregular structure (amorph: Greek for formless) i.e. randomly arranged rather than arranged in a crystal. Due to the great ability of a-Si to absorb light, a solar cell made of this material requires layer thickness of only a few µm (microns or micrometers). These cells are thus also called "thin cells" using thin-film technology. It is manufactured in a process where layers of doped silicon are deposited on a substrate.

AMORPHOUS SILICON SOLAR CELLS consist of very thin layers of silicon. Rather than using very expensive crystalline silicon, several thin layers of silicon (with a maximum thickness of 5 micron) are applied to a carrier medium in a "vapor-depositing" or sputtering process. As these processes occur at relatively low temperatures (starting at 200 degrees C), less energy is required. When small amounts of hydrogen are added, the unsaturated bonds that occur during vapor-deposition are partially neutralized. Amorphous solar cells are thus called a-Si cells. In addition to the material and energy savings, amorphous silicon has other advantages: it absorbs sunlight better (direct semiconductor, lower energy input-required absorption); the manufacturing process is relatively simple; and the ration of Si and Ge atoms can be varied to adjust the size of the band-gap. Another advantage of amorphous cells is that the cells can be easily arranged in series to provide higher voltages. Two disadvantages that amorphous solar cells have compared to crystalline solar cells are the lower efficiency and degradation under exposure to solar radiation. However, high ambient temperatures in the summer partially compensate for this ageing effect. The greater band-gap that amorphous solar cells have in comparison to crystalline silicon also makes them more sensitive to changes in the high-energy range of the solar spectrum. Amorphous solar cells are thus currently used for the smallest applications (pocket calculators, clocks, etc.).

AMORTIZATION TIME - See energy payback

AMPERAGE INTERRUPT CAPABILITY (AIC)is the rating of direct current fuses capable of interrupting the highest possible current.

AMPERE (amp) is a unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

AMPERE HOUR METER is an instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

ANGLE of the surface of the solar generator to the horizontal plane is called "beta". The angle at which the installed module's surface deviates from the horizontal plane results from the stands used or the angle of the support surface (roof).

ANGLE OF INCIDENCE is the angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but of the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90 degrees.

ANNUAL SOLAR SAVINGS (of a solar building) is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

ANODE is the positive electrode in an electrochemical cell (battery). It is also the earth or ground in a cathodic protection system and is also the positive terminal of a diode.

ANTI-REFLECTION COATING (or Layer) is a thin coating of a transparent material applied to a solar cell surface that reduces the light reflection and increases the light transmission (absorption). Unlike coloured crystalline cells, "normal" crystalline cells have an anti-reflection layer that gives the cells their typical blue colour. This layer directs sunlight to the light-sensitive layer, thus increasing the absorption rate and giving the cells their dark blue appearance. To reduce optical losses, a thin anti-reflection coating is added to crystalline silicon solar cells. It ensures that the cell absorbs more light, otherwise the surface of the silicon would otherwise reflect 30 % of the incident sunlight. The thickness of the anti-reflection layer gives the cell its colour. Standard solar cells are blue because of the absorption rate is the highest at this thickness. The thickness of this layer can, however be varied to make the cells appear gold, red, or various hues of green and blue, though the efficiency of the cells is reduced in the process.

APPERTURE AREA is the glass surface of a collector through which the solar radiation can enter. The aperture area is the reference unit for the collector efficiency according to DIN 4757 or in EN 12975.

ARRAY CURRENT is the electrical current produced by a photovoltaic array when it is exposed to sunlight.

ARRAY OPERATING VOLTAGE is the voltage produced by a photovoltaic array when it is exposed to sunlight and is connected to a load.

ARRAY RATIO is a measure of the quality of the PV generator under operation that takes into account losses of the direct current side due to mismatches, cabling, and sub-optimal ambient conditions. It describes the ratio between the measured direct current yield and the theoretical direct current yield under standard test conditions.

AUTONOMOUS PV SYTEMS are off-grid systems not connected to the public grid.

AVAILABILITY is the quality or condition of a photovoltaic system being available to provide power to a load. It is usually measured in hours per year. One minus availability equals downtime.

AZIMUTH ANGLE is the angle between true south and the point on the horizon directly below the sun. It specifies how many degrees the surface of the thermal collector or photovoltaic module deviates from the exact south-facing direction.

BALANCE OF SYSTEM represents all components and costs other than the photovoltaic modules or array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

BAND GAP / BAND DISTANCE (in a semi-conductor) is the energy difference between the highest valence band and the lowest conduction band. The electrons in an atom can only have certain energy states; they have a defined energy band. Between the two most extreme bands - the valence band, which is the highest range of electron energy, and the conduction band, which contains free electrons - there is a gap where no electrons can dwell. Non-conductors have an especially large gap, while metals (conductors) have overlapping bands. The band gap of the semiconductor determines the maximum efficiency of the solar cell.

BAND GAP ENERGY is the amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to free state, and thus promote it from the valence to the conduction level.

BARRIER ENERGY is the energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier.

BATTERY (in a solar system) is an electrochemical storage system comprising of two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series / parallel arrangement to provide the required operating voltage and current levels.

BATTERY AVAILABLE CAPACITY is the total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage.

BATTERY CAPACITY is the maximum charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

BATTERY CELL is the simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all of the above.

BATTERY CYCLE LIFE is the number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

BATTERY ENERGY CAPACITY is the total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers that it is to the battery industry where capacity usually refers to ampere-hours.

BATTERY ENERGY STORAGE is the storage of energy using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

BATTERY LIFE is the period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and /or years, depending on the type of service for which the cell or battery is intended.

BIFACIAL CELL is a solar cell that can absorb light on both sides, i.e. using rear illumination from light reflected of the while wall of a building.

BIPV (Building-Integrated Photovoltaics) is a term used for the design and integration of photovoltaics (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical façades, replacing view glass, spandrel glass, or other façade material; into semi-transparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

BLOCKING DIODE is a semi-conductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but no backwards.

BORON (B)is a chemical element commonly used as the dopant in photovoltaic device or cell material.

BOULE is a sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.

BTU (British Thermal Unit) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit; and one BTU is equal to 252 calories.

BYPASS DIODE is the diode connected parallel to the feed direction across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) becomes reverse biased. It protects these solar cells from thermal destruction or "hot-spot effect" in case of total or partial shading of individual solar cells while other cells are exposed to full light. Ideally, each solar cell should have a bypass diode. However, in practice, bypass diodes are used in every 15 to 20 solar cells, except in special modules for shaded environments.

CADMIUM TELLURIDE (CdTe) is a polycrystalline thin-film photovoltaic material.

CADMIUM TELLURIUM (CdTe) is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production.

CAPACITY FACTOR is the ratio of the average load on (or power output of) an electricity generating unit or system to the capacity rating of the unit or system over a specified period of time.

CAPTIVE ELECTROLYTE BATTERY is a battery having an immobilized electrolyte (gelled or absorbed in a material).

CATHODE is the negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

CATHODIC PROTECTION is a method of preventing oxidation of the exposed metal in structures by imposing a small electrical voltage between the structure and the ground.

CELL (Battery) is a single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. The word 'Cell' can also be used as in the case of photovoltaic (PV) cell.

CELL BARRIER is a very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. It is also called Depletion Zone or Space Charge.

CELL JUNCTION is the area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the centre of cell barrier or depletion zone.

CHARGE is the process of adding electrical energy to a battery.

CHARGE CARRIER is a free and mobile conduction electron or hole in a semi-conductor.

CHARGE CONTROLLER is a component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status. It is used as a connecting element in stand-alone systems and is connected between photovoltaic generators and accumulators (batteries).

CHARGE FACTOR is a number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. It is usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. It is related to Charge Rate.

CHARGE RATE is the current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

CHEMICAL VAPOR DEPOSITION (CVD) is a method of depositing thin semi-conductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more or which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

CIGS is the abbreviation for Copper Indium Gallium diSelenide, a semiconductor material for thin solar cells.

CIS is the abbreviation for general chalcopyrite films of copper indium selenide (CuInSe₂).

CLEFT (Cleavage of Lateral Epitaxial Films for Transfer) is a process for making inexpensive Gallium Arsenide (GaAs) photovoltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

CLOUD ENHANCEMENT is the increase in solar intensity caused by reflected irradiance from nearby clouds.

COLLECTOR is an elementary component of a collector system that converts the solar radiation into heat as efficiently as possible. The heat is collected and transported by a heat transfer medium and transferred to a storage tank by a heat exchanger. A flat collector consists of a well-insulated housing that contains an absorber. The housing is covered with a sheet of solar glass.

COLLECTOR CIRCUIT (or Solar Circuit) is the circuit between the collector and a collector system storage tank that consists of collector, piping, heat exchanger, pump and safety equipment.

COLLECTOR CIRCUIT YIELD is the thermal energy that is transferred from the collector circuit (solar circuit) to the solar storage tank. In most cases, a specific collector circuit yield is specified. An annual collector circuit yield of more than 400 kWh/m2 is a good result for a flat collector system; an amount greater than 500 kWh/m2 is excellent result for a large-scale system.

COLOURED SOLAR CELLS - In addition to conventional solar cells which are generally dark blue (crystalline cells) or grayish brown (amorphous cells), coloured cells are also sold. The colours range from gold to magenta (red), violet, green, light blue, black and gray. Unlike coloured crystalline cells, "normal" crystalline cells have an anti-reflection layer that gives the cells their typical blue colour. This layer directs sunlight to the light-sensitive layer, thus increasing the absorption rate and giving the cells their dark appearance. Depending on the thickness of the anti-reflection layer, the light incident on the light-sensitive layer is reduced to a certain wavelength. This not only gives the cells their colour, but also reduces efficiency by a few percentage points. The efficiencies of coloured mono-crystalline cells - 12 to 15 percent - are nonetheless still comparable to those of conventional polycrystalline solar cells. It is however, difficult to keep the hues constant across the entire anti-reflection surface to ensure colour consistency.

COMBINED COLLECTOR is a photovoltaic device or module that provides useful heat energy in addition to electricity.

CONCENTRATOR is a photovoltaic module, which includes optical compounds such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

CONDUCTION BAND (or Conduction Level) is an energy band in a semi-conductor in which electrons can move freely in a solid, producing a net transport of charge.

CONDUCTOR is the material through which electricity is transmitted, such as electrical wires, or transmission or distribution line.

CONNECTION - The beginning and end of the connection cables between interconnected solar cells are fed through the back of the module and inserted in the module's socket. The cables for each module string are connected to the strong diodes in the generator junction box. The main direct current cable goes from this "J box" to the switch box, which houses all of the required fuses and safety devices.

CONSUMER APPLICATIONS of solar cells are in mobile solar products such as small systems for pocket calculators, watches, cell phones, etc. and in special products such as car and boat roofs, etc.

CONTACT STRIPS are roughly 5mm wide metallic conductors on crystalline solar cells (generally two per cell) through which free electrons travel. They thus provide the electrical connection and connect multiple cells to each other.

CONTACT RESISTANCE is the resistance between metallic contacts and the semi-conductor.

CONVERTER is a unit that converts direct current (dc) voltage to another dc voltage.

COPPER INDIUM DISELENIDE (CuInSe2 or CIS) is a poly-crystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and /or sulfur) for thin cells. Thin cells consist of several layers of variously doped copper indium diselenide. This technology has proven to be especially hard to manage, but in the meantime industrial production has begun. The desire to have greater efficiency, especially in aerospace applications has led to the use of other semiconductor materials. The use of Gallium Arsenide (GaAs) or Cadmium Telluride (CdTe) as the basic material provided this increase in efficiency. Although serially produced GaAs cells have reached efficiencies of 20%, no broad use in civilian applications is expected due to the high production costs.

CRYSTALLINE SILICON (c-Si) is a type of photovoltaic cell made from a slice of single-crystal silicon or poly-crystalline silicon. The atoms in crystalline silicon are arranged in crystals. If a single crystal is drawn from molten silicon, this crystal can be sliced into mono-crystalline wafers and used in mono-crystalline solar cells. If the molten silicon is allowed to solidify into blocks, crystals arranged in many directions are formed, displaying the poly-crystalline (multi-crystalline) "snowflake" structure typical of such solar cells.

CURRENT AT MAXIMUM POWER (IMP) is the current at which maximum power is available from a module.

CHEMICAL VAPOR DEPOSITION (CVD) is a term used in thin-cell photovoltaics. Here, the sour material (called the pre-cursor) is first chemically restructured under high steam pressure. For instance, silicon is a solid, but the compound of silicon with four hydrogen atoms (SiH4 or Silane) is gaseous at room temperature. Often, the pre-cursor is bonded with organic chains consisting only carbon and hydrogen in one molecule. This procedure is called metal organic chemical vapor deposition or MOCVD. The now gaseous material then enters a chamber containing the substrate to be coated. When the substrate is heated, the molecule decomposes, the original pre-cursor is deposited, and the rest remains gaseous and is removed. The most important CVD application in thin-cell photovoltaics is the deposition of amorphous silicon. A plasma supports the decomposition of silane at low substrate temperatures (generally around 200oC), which is why this process is called plasma enhanced chemical vapor deposition or PECVD. Unlike cathode atomization, the conditions here are designed to prevent the electrode from atomizing. If the pre-cursor is a fluid and can be chemically made into a fluid, it can basically be sprayed onto the heated substrate like a lacquer.

CUTOFF VOLTAGE is the voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.

CYCLE is the discharge and subsequent charge of a battery.

CZOCHRALSKI PROCESS is a method of growing large size, high quality semi-conductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

DANGLING BONDS is a chemical bond associated with an atom on the surface layer of a crystal. This bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface.

DAYS OF STORAGE is the number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.

DC-to-DC CONVERTER is an electronic circuit that converts direct current voltages (e.g. photovoltaic module voltage) into other levels (e.g. load voltage). It can be part of a maximum power point tracker.

DEEP-CYCLE BATTERY is a battery with large plates that can withstand many discharges to a low state-of-charge.

DEEP DISCHARGE is the discharging of a battery to 20% or less of its full charge capacity.

DEPTH OF DISCHARGE (DOD) is the ampere-hours removed from a fully-charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

DENDRITE is a slender thread-like spike of pure crystalline material, such as silicon.

DENDRITE WEB TECHNIQUE is a method for making sheets of poly-crystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.

DEPLETION ZONE (or Cell Barrier) is the term derived from the fact that this microscopically thin region is depleted of charge carriers (free electrons and hole).

DESIGN MONTH is the month having the combination of insolation and load that requires the maximum energy from the photovoltaic array.

DIFFUSE INSOLATION is the sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. It is the opposite of direct insolation.

DIFFUSE RADIATION is the radiation received from the sun after reflection and scattering by clouds, atmospheric molecules, particles and ground. The share of solar radiation is basically equally reflected in all directions by clouds, fog, etc. in the atmosphere and is thus incident on a surface without casting a shadow.

DIFFUSION FURNACE is the furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.

DIFFUSION LENGTH is the mean distance a free electron or hole moves before recombining with another electron or hole.

DIODE is an electronic device that allows current to flow in one direction. See Blocking Diode and Bypass Diode.

DIRECT BEAM RADIATION is the radiation that reaches the solar generator / absorber directly from the direction of the sun. The share of the radiation that falls directly incident on the earth's surface casts shadows without any reflections and refractions. It is measured by a pyrheliometer with a solar aperture of 5.7 degrees to transcribe the solar disc.

DIRECT CURRENT (DC) is a type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relative low voltage and high current. DC can be converted to alternating current for use in typical 120 volt or 220 volt household appliances. Solar cells generate direct current. An inverter can be used to convert direct current into alternating current, which can then be fed to the public grid, for instance at 230V and 50 Hz in Germany.

DIRECT RADIATION is solar radiation that reaches the Earth's surface on a direct path from the sun. Total radiation is diffuse radiation added to direct solar radiation.

DIRECT INSOLATION is the sunlight falling directly upon a collector. It is the opposite to Diffuse Insolation.

DISCHARGE is the withdrawal of electrical energy from a battery.

DISCHARGE FACTOR is the number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity. i.e. C/5 indicates a discharge factor of 5 hours.

DISCHARGE RATE is the rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

DISCONNECT is the switch gear used to connect or disconnect components in a photovoltaic system.

DISTRIBUTED ENERGY RESOURCES (DER) is a variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.

DISTRIBUTED GENERATION is a popular term for localized or on-site power generation.

DISTRIBUTED POWER is the generic term for any power supply located near the point where the power is used. It is the opposite of central power. See Stand-Alone Systems.

DISTRIBUTED SYSTEMS are systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

DONOR (in a photovoltaic device) is an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electricity conductivity over that of an undoped semiconductor.

DONOR LEVEL is the level that donates conduction electrons to the system.

DOPANT is a chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).

DOPING is the addition of dopants to a semiconductor. Impurities with other electrical properties are introduced to a semiconductor material - such as silicon. It is dependent on the impurities applied either an abundance of electrons (negatively charged, n-type doping) or holes (positively charge, p-type doping). In both cases, the conductivity is considerably increased.

DOWNTIME is the time when the photovoltaic system cannot provide power for the load. It is usually expressed in hours per year or in percentage.

DRY CELL is a cell (battery) with a captive electrolyte. It is a primary battery that cannot be recharged.

DUTY CYCLE is the ratio of active time to total time. It is used to describe the operating regime of appliances or loads in photovoltaic systems.

DUTY RATING is the amount of time an inverter (power conditioning unit) can produce at full rated power.

EDGE-DEFINED FILM-FED GROWTH (EFG) is a method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upwards by capillary action through a mold.

EFFICIENCY is generally the percentage at which a technical system converts one type of energy into another (desired) type. It thus indicates the ratio of useful output to power consumed in the conversion process. The efficiency of a solar cell indicates how much incident solar energy is converted into electricity. The efficiency a given cell can attain depends on the material used and the manufacturing process. In photovoltaics, a distinction is made between the efficiency of cells, the somewhat lower efficiency of modules, and the slightly lower efficiency of entire systems due to losses in other system components. In a heat collector, the efficiency is defined as the ratio of the heat output of the collector (output) compared to the irradiation level in the collector plane (input).

ELECTRIC CIRCUIT is the path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.

ELECTRIC CURRENT is the flow of electrical energy (electricity) in a conductor, measured in amperes.

ELECTRICAL GRID is an integrated system of electricity distribution, usually covering a large area.

ELECTRICITY is the energy resulting from the flow of charge particles, such as electrons or ions. The strength of the charge quantity flowing through a conductor (e.g. in the form of electrons flowing through a copper cable) is referred to as electrical current. The unit of measure for current is ampere or "A".

ELECTROCHEMICAL CELL is a device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.

ELECTRODE is a conductor that is brought in conducting contact with a ground.

ELECTRODEPOSITION is an electrolytic process in which a metal is deposited at the cathode from a solution of its ions.

ELECTROYLYTE is a nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.

ELECTRON is an elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.

ELECTRON VOLT (eV) is the amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 volt; equivalent to 1.603 x 10 ^ -19; a unit of energy or work.

ENERGY is the capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

ENERGY AUDIT is a survey that shows how much energy used in a home, which helps find ways to use less energy.

ENERGY CONTRIBUTION POTENTIAL is the recombination occurring in the emitter region of a photovoltaic cell.

ENERGY DENSITY is the energy represented by an electron in the band model of a substance.

ENERGY PAYBACK is also called energy amortization period. It indicates how long a system needs to generate the energy needed to equal that consumed for its manufacture and installation. After this period, the system has a positive energy balance.

ENERGY YIELD is the amount of energy that a system can produce.

EPITAXIAL GROWTH is the growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.

EQUALIZATION is the process of restoring all cells in battery to an equal state-of-charge. Some battery types may require a complete discharge as a part of the equalization process.

EQUALIZING CHARGE is a continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.

EQUINOX is the times of the year when the sun crosses the equator and night and day of equal length; usually occurs on March 21st (spring equinox) and September 23rd (fall equinox).

EVA (Ethylene Vinyl Acetate) is a transparent polymer plastic that is resistant to light and high-temperatures. It is used as a casting compound for the lamination of solar cells. Only composite glass with an immediate layer of PVB is considered shatterproof. Solar modules with an intermediate layer of EVA are thus not standard products.

EXTRINSIC SEMICONDUCTOR is the product of doping a pure semiconductor.

FAÇADE INSTALLATION is a PV system that is mounted on a building façade or permanently integrated into the façade.

FEED-IN TARIFF is the tariff for feeding electricity produced from renewable energy sources into the general power supply grid.

FERMI LEVEL is the energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a semiconductor, the Fermi level is in the band gap.

FILL FACTOR is the ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. It is a key characteristic in evaluating cell performance.

FINAL ENERGY CARRIERS are the physical materials that contain the energy consumers purchase, such as heating oil, electricity, wood chips, etc. They are derived from secondary and primary energy sources minus conversion and distribution losses, internal consumption, and non-energetic consumption. Final energy carriers are converted into collective energy.

FIXED TILT ARRAY is photovoltaic array set in at a fixed angle with respect to the horizontal.

FLAT-PLATE ARRAY is a photovoltaic (PV) array that consists of non-concentration PV modules.

FLAT-PLATE PHOTOVOLTAICS (PV) is a PV array or module that consists of non-concentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.

FLAT ROOF INSTALLATION - In order to achieve an optimum energy yield, the PV modules on a flat roof in Germany must be mounted with a tilt of approximately 30 degrees using a supporting structure. For countries near the equator, a 5 degree tilt is only necessary to allow draining of rain water and self-flushing of dirt on the module surface.

FLOAT CHARGE is the voltage required to counteract the self-discharge of the battery at a certain temperature.

FLOAT LIFE is the number of years that a battery can keep its stated capacity when it is kept at float charge.

FLOAT SERVICE is a battery operation in which the battery is normally connected to an external current source; for instance, a battery charger which supplies the battery load < under normal conditions, while also providing enough energy input to the battery to make up for its internal quiescent losses, thus keeping the battery always up to full power and ready for service.

FLOAT-ZONE PROCESS is a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised, the molten interface beneath the coils becomes single crystal.

FOSSIL ENERGY SOURCES / FUELS are raw energy sources - oil, coal and gas - that were created over millions of years. They are not, however, available in unlimited amounts, and their combustion causes environmental impacts. A distinction is made between primary fossils energy sources (such as brown coal) and secondary fossil energy sources (such as gasoline and diesel). Unlike these energy sources, renewable energy is available in infinite amounts in human terms.

FREQUENCY is the number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

FREQUENCY REGULATION indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed 1%.

FRESNEL LENS is an optical device that focuses light like a magnifying glass: concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

FULL SUN is the amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 watts per sq meter).

GALLIUM (Ga) is a chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.

GALLIUM ARSENIDE (GaAs) is a crystalline, high-efficiency compound used to make certain types of solar cells and semiconductor devices.

GASSING is the evolution of gas from one or more of the electrodes in the cells of a battery. Gassing is commonly due to local action self-discharge or from the electrolysis of water in the electrolyte during charging.

GASSING CURRENT is the portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid. This current increases with increasing voltage and temperature.

GEL battery is a type of battery in which the electrolyte (a mixture of water and sulfuric acid) is bound into a gel. This is a type of so-called closed lead acid battery. A gas mixture (hydrogen and oxygen) is always generated when lead acid batteries are charged, and in normal operation this is internally recombined to form water. This removes the need for regularly refilling the battery cells with water, which is why these batteries are often described as "low maintenance" or even "maintenance free". Gel batteries are available from many different manufacturers for a wide range of applications. There are gel batteries for high-current applications but also for cycle operation with very high cycle resistance.

GENERATOR in Photovoltaics (or Solar Generator) refers to all of the interconnected PV Modules as a whole.

GIGAWATT (GW) is a unit of power equal to 1 billion watts; 1 million kilowatts, or 1,000 megawatts.

GLASS / GLASS LAMINATE is a laminate made in a process whereby solar cells are embedded between front and rear panes of glass either by means of a layer of EVA (Ethylene Vinyl Acetate) or a cast resin. If EVA is used, the manufacturing process is basically the same as with glass /plastic laminates. If cast resin is used, the cells are aligned on the conventional rear pane using spacers to ensure that they are perfectly centered on the glass once the module has been filled with cast resin. This approach offers one main benefit over plastic laminates: the solar cells are protected from mechanical loads on the glass from wind, snow, etc. as they are within the module's "neutral zone". In particular, the electric connections between the cells are well protected. This technology allows for solar panels of up to 2 m x 3 m to be manufactured. The cast resin used has been modified to make it more transparent and give it more adhesive strength and long-term stability. Architects like to use glass /glass laminates as an aesthetic design element as the two main components - the glass and the solar cell - both offer a number of possible combinations in terms of shape, colour, size, transparency, and design.

GLASS / PLASTIC LAMINATE is a laminate made between a front pane of glass and a rear layer of tedlar. A layer of EVA (Ethylene Vinyl Acetate) is used to attach these laminates to the actual solar cells. This assembly is then baked into compound in a laminator under pressure and at high temperatures. Most of the solar panels sold today are glass /plastic laminates with 36, 72 or 144 solar cells. For this reason, glass /plastic laminates are often called standard panels or modules. Depending on the solar cells used, the units thus produced have outputs of 50 to 300 watts. Glass /plastic laminates are manufactured under great competition; to keep prices low, they are almost exclusively made in large-scale production with a surrounding aluminium or stainless-steel frame.

GLOBAL RADIATION (or Radiant Energy or Total Solar Radiation) is the sum of direct, diffuse, and reflected solar radiation incident on the Earth's surface. A horizontal plane is usually the reference. The Earth's atmosphere reduces solar radiation by absorbing, reflecting, and dispersing it, thus reducing the intensity at the Earth's surface to 1000 W /m2 at noon on a summer day in Germany under clear skies. This corresponds to the energy content of 100 litres of heating oil or 100 m3 of gas.

GREEN POWER is power generated from renewable energy sources.

GRID is the fine "finger" structure made of metal on the front side of the solar cell that serves as an n-contact.

GRID-CONNECTED SYSTEM(or Grid-Interactive System) is a solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid. In industrialized countries, consumers get their power almost exclusively from the grid; hence, any considerable amount of power from photovoltaics will have to be fed to the grid. Grid-connected PV Systems are connected to the grid via inverters to feed either all of the power or just the excess power to the grid. Grid-connected applications are found on the roofs of homes and businesses with up to 1 MW power; above 1 MW power, one speaks of photovoltaic power plants.

GRID LINES are metallic contacts fused to the surface of the solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.

GROUND MOUNTING refers to a PV system that is installed on an open surface, for example a field.

HARMONIC CONTENTis the number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.

HEAT TRANSFER MEDIUM - Fluid in a collector circuit of a solar energy system that transports the heat from the collector to the storage tank. The heat transfer medium is usually a mixture of water and glycol to ensure frost protection.

HETEROJUNCTION is a region of electrical contact between two different materials.

HIGH VOLTAGE DISCONNECT HYSTERESIS is the voltage difference between the high voltage disconnect set point and the voltage at which the full photovoltaic array current will be reapplied.

HOLE is the vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

HOT SPOT EFFECT - If several cells in a solar module are shaded, the ones not shaded still produce electricity, which flows through the series connections to the shaded cells that now act as blocking diodes. The great resistance causes the cells to overheat; they may even be destroyed in the process. To prevent such "hot spots" from forming, bypass diodes are used to pass the current through.

HOMOJUNCTION is the region between an n-layer and a p-layer in a single material, photovoltaic cell.

HYBRID SYSTEM is a solar PV system that combines with other forms of generation, usually a diesel generator.

HYDROGENATED AMORPHOUS SILICON is amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.

INCIDENT LIGHT is the light that shines onto the face of a solar cell or module.

INDIUM OXIDE is a wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. It is often used as a front contact or one component of a heterojunction solar cell.

INFRARED RADIATION is an electromagnetic radiation whose wavelengths lie in the same range of 0.75 to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

INGOTS are mono-crystalline ingots made from pure poly-crystalline silicon (known as poly-silicon) in a separate pulling area using the Czochralski process. The silicon is melted down in a crucible and held at a temperature marginally above its melting point. A mono-crystalline silicon rod, known as the seed crystal, is then dipped into the molten silicon. As the rod is pulled upwards and rotated, it grows into a mono-crystalline ingot with the same crystal orientation as the seed crystal. The diameter of the ingot is directly related to the speed with which the seed crystal is withdrawn from the "melt".

INPUT VOLTAGE is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

INSOLATION (or irradiation) is the amount of solar energy received on a given area measured in kilowatt-hour per sq m (kWh / sq m) - this value is equivalent to sun peak hours.

INTERCONNECTis a conductor within a module or other means of connection that provides an electrical interconnection between the solar cells.

INTRINSIC LAYER is a layer of semiconductor material, used in a photovoltaic device, whose properties are essentially those of the pure, undoped material.

INTRINSIC SEMICONDUCTOR is an undoped semiconductor.

INVERTER is a device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid. An Inverter is also called frequency converter.

ION is an electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged and a gain makes the particle negatively charged.

IRRADIANCE is the amount of solar power striking on a given area. It is a measure of the intensity of sunshine and is given in units of watts (or kilowatts) per square metre (W / sq m).

IRRADIATION (or insolation) is the amount of solar energy received on a given area measured in kilowatt-hour per sq m (kWh / sq m) - this value is equivalent to sun peak hours.

ISLANDING refers to a phenomenon where a sufficient number of inverters are connected to the grid and when the grid supply to that area fails, then the inverters would interact with each other such that the voltage and frequency will become a reference to each other so that the passive protection would not operate and continue to maintain power onto the grid that should otherwise be dead. Active protection is required for such a situation in order to shut down the inverters to prevent islanding.

ISPRA GUIDELINES are guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra in Italy.

I-TYPE SEMICONDUCTOR is a semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V CURVE is a graphical presentation of the current versus the voltage from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.

JOULE is a metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 Btu equals 1,055 joules.

JUNCTION is a region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).

JUNCTION BOX [of a photovoltaic (PV) generator] is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

JUNCTION DIODE is a semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.

KILOWATT (kW) is a standard unit of electrical power equal to 1000 units, or to the energy consumption at a rate of 1000 joules per second.

KILOWATT-PEAK (kWp) is the system's peak electrical output, under standard test conditions of 1000W/m2 irradiance, air mass of 1.5 spectrum, 25 degree C cell temperature.

KILOWATT-HOUR (kWh) is one thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh = 3,600 kJ.

LANGLEY (L) is the unit of solar irradiance measured as one gram calorie per square centimeter. 1 L = 85.93 kWh /m2.

LAMINATE is a frameless solar module. For the sensitive, fragile solar cells to withstand loads and weathering over 20 years or longer, they are embedded in EVA and laminated between two glass panes (glass /glass laminate) or between a front glass pane and a rear layer of plastic (glass /plastic laminate).

LASER (Light Amplification by Stimulated Emission of Radiation) describes the amplification of light through the stimulated emission of radiation. A typical laser emits light in a narrow and well-fefined beam and with a well-defined wavelength (or colour). This is in contrast to a light source such as the incandescent light bulb, which emits in almost all directions and over a wide spectrum of wavelength. These properties can be summarized in the term of coherence. The most widespread use of lasers is in optical storage devices such as compact disc and dvd players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers and laser pointers. In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are also used for military and medical applications.

LATTICE is the regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

LEAD-ACID BATTERY is a common battery with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.

LIFE is the period during which a system is capable of operating above a specified performance level.

LIFE-CYCLE COST is the estimated cost of owning and operating a photovoltaic system for the period of its useful life.

LIGHT-INDUCED DEFECTS are defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

LIGHT TRAPPING is the trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.

LIGHTNING PROTECTION - A PV system generally does not increase the risk of a lightning strike. For safety reasons and to prevent damages, PV systems are, however, constructed according to lightning protection standards.

LINE-COMMUTATED INVERTER is an inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a power failure in the power grid, the photovoltaic system cannot feed power into the line.

LIQUID ELECTROLYTE BATTERY is a battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. It is also called a flooded battery because the plates are covered with the electrolyte.

LOAD is the demand on energy producing system; the energy consumption or requirement of a piece or group of equipment. It is usually expressed in terms of amperes or watts in reference to electricity.

LOAD CIRCUIT is the circuit that comprises of the wire, switches, fuses, etc. that connect the load to the power source.

LOAD CURRENT (A) is the current required by the electrical device.

LOAD RESISTANCE is the resistance presented by the load. See 'resistance'.

LOSSES reduce the theoretically possible efficiencies of solar cells for several reasons:

1. The contact strips on the top of the solar cell reflect part of the incident light. These strips are kept as slender as possible to minimize losses. However, the contact resistance between the semiconductor layer and the contact impedance requires a large contact surface. In addition, the distance between the contact strips should not be too great, lest the losses due to resistance be too great on the charge carriers << path through the semiconductor layer.
2. Furthermore, there are reflection losses for the transition of solar radiation from the air to the semiconductor material due to the different refraction coefficients. These losses are clearly reduced when the surface is coated with an anti-reflection layer. The cell surface can also be structured (roughened).
3. Short-wave light generally does not penetrate the semiconductor material as deeply as long-wave light. Hence, the exploitation of the share of short-wave light is decisive for the design of the top semiconductor layer. The more this layer is doped, the thinner it should be, as the charge carriers recombine very quickly in such layers. The light absorbed here produces little current in the solar cell.
4. Great short-circuit currents, open circuit voltage, and fill factors require a great diffusion length. However, charge carriers like to recombine at imperfections. i.e. at defects in the crystals or impurities. Hence, the quality of the crystals in the starting material has to be great and fulfill the highest purity requirements.
5. The surface of the semiconductor also represents a large area of imperfections in the crystal structure. Various techniques are used to passivate such imperfections on the surface and in the volume, thus reducing the drop in efficiency.
6. Additional losses occur when energy travels from the solar cell. Resistance losses occur when the charge carriers travel to the contacts and then through the connecting lines. Defects in solar cells may also lead to local short circuits between the front and the back of the cell.
7. These losses currently amount to some 10% in highly efficient silicon solar cells from industrial production.

LOW VOLTAGE CUTOFF (LVC) is the voltage level at which a charge controller will disconnect the load from the battery.

LOW VOLTAGE DISCONNECT is the voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.

LOW VOLTAGE DISCONNECT HYSTERESIS is the voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.

LOW VOLTAGE WARNING is a warning buzzer or light that indicates the low battery voltage set point has been reached.

MAINTENANCE-FREE BATTERY is a sealed battery to which water cannot be added to maintain electrolyte level.

MAJORITY CARRIER is the current carrier, either a free electron or hole that is in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

MAXIMUM POWER POINT (MPP) is the point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is about 0.45 volts.

MAXIMUM POWER POINT TRACKER (MPPT) is the means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

MAXIMUM POWER TRACKING is operating a photovoltaic array at the peak power point of the array's I-V curve where maximum power is obtained. To ensure maximum output, a transformer integrated in the inverter sets the MPP voltage for the current irradiance so that the solar generator automatically runs at the MPP. This technique is also called peak power tracking.

MWP - The suffix 'p' denotes that a solar cell, a solar module, or a solar generator is at its nominal or maximum or 'peak' output under standard test conditions.

MICROGROOVE is a small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

MINORITY CARRIER is a current carrier, either an electron or a hole that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device. If the semiconductor is n-conductive, the electrons are majority carriers and the holes are minority carriers.

MINORITY CARRIER LIFETIME is the average time a minority carrier exists before recombination.

MIS means Metal Insulator Semiconductor.

MODIFIED SINE WAVEis a waveform that has at least three states (i.e. positive, off, and negative). It has less harmonic content than a square wave.

MODULARITY is the use of multiple inverters connected in parallel to service different loads.

MODULE is an array of solar cells that increase output. In order to provide current and voltage at useful levels, multiple solar cells are connected and embedded between panes of glass or plastic to protect them from mechanical stress and weathering. The modules are either embedded in a metal frame or sold without frames as laminates. Any number of solar modules can be combined to create a solar generator of any size. The output of a single solar cell is low. However, technical systems require high output. Solar cells are thus connected to create solar modules. The well known principles of series and parallel connections are used in this process. In series connections, the positive electrodes are connected to the negative poles of the other cell. If multiple cells are connected in series, the total voltage is equal to the sum of the individual voltages. In contrast, the current from the parallel voltage depends on the weakest cell, which is why cells connected in parallel always have to have the same size and current-voltage characteristic. The parallel connection basically increases current. Negative pole is connected to negative pole and positive pole to positive pole. The sum of the individual currents produces the total current, with voltage remaining constant.

MODULE DERATE FACTOR is a factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

MODULE ORIENTATION- In Central Europe, a module facing the south at a 35 degree angle to the ground generates the most power over the course of the year. Deviations from southwest to southeast only produce relatively minor losses. Ideally, the modules should be at a 45 degree angle.

MONO-CRYSTALLINE means made of one crystal. The basic material for mono-crystalline solar cells is cut from the single crystal or "ingot", which is drawn from the molten silicon.

MONO-CRYSTALLINE SOLAR CELLS depends on crystalline silicon as the most important material. Crystalline silicon is made in several steps. First, quartz (SiO2) is molten into metallurgic silicon of 98% purity. This silicon must be further purified for use as a semiconductor element. "Electronic-grade" silicon is created when distilled with trichlorsilane (SiHCl3) in several stages, with the silicon allowed to crystallize afterwards. Then, p-conductive mono-crystalline or poly-crystalline material is made. The compact single crystals (ingots) are either made in the Czochralski method (CZ) or the Float Zone method (FZ). Poly-crystalline material is made by pouring the molten silicon into blocks or by drawing sheets out of the molten silicon. Then, silicon wafers some 250 micron thick are cut from the compact material. These wafers are doped with phosphor atoms to increase conductivity. Contacts are applied to the front and back, and an anti-reflection coating is added to produce solar cells.

MONOLITHIC means fabricated as a single structure.

MOVISTOR is Metal Oxide Varistor. It is used to protect electronic circuits from surge currents such as those produced by lightning.

MULTI-CRYSTALLINE is the description of photovoltaic semiconductor material composed of variously oriented, small, individual crystals. It is sometimes referred to as polycrystalline or semi-crystalline.

MULTI-JUNCTION DEVICE is a high efficiency photovoltaic device containing tow or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

MULTI-STAGE CONTROLLER is charging controller unit that allows different charging currents as the battery nears full stage-of-charge.

NATIONAL ELECTRICAL CODE (NEC) contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV System.

NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION (NEMA) is the organization that sets standards for some non-electronic products like junction boxes.

NET ENERGY BALANCE is the difference derived when the sum of all the energy needed for the construction, operation, and dismantling of the energy system is subtracted from the energy that the system generates over its service life. An energy converter with a negative energy balance consumes more energy than it generates.

NICKEL CADMIUM BATTERY is a battery containing nickel and cadmium plate and an alkaline electrolyte.

NOCT - Normal Operating Cell Temperature.

NOMINAL VOLTAGE is a reference voltage used to describe batteries, modules or systems i.e. a 12-volt or 24-volt battery, module or system.

NOMINAL OPERATING CELL TEMPERATURE (NOCT) is the estimated temperature of a photovoltaic module when operating under 800 w/m2 irradiance, 20 deg C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

N-CONDUCTIVE LAYER is the semiconductor material that is negatively charged. The silicon is doped with impurities to produce an abundance of electrons.

N-TYPE is the type of negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons.

N-TYPE SEMICONDUCTOR is a semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity e.g. phosphorus in silicon.

N-TYPE SILICON is a silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.

OFF-GRID SOLUTION is a PV system that is not connected to the public grid.

OFF-GRID SYSTEMS are closed power supply systems in which all of the consumers receive power from the power generators connected; in particular, there is no connection to the public grid.

OHMis a measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

ON-ROOF INSTALLATION. For on-roof installations, the solar modules are installed parallel to the roof covering using special mounting system.

ONE-AXIS TRACKING is a system capable of rotating about an axis.

OPEN-CIRCUIT VOLTAGE (VOC) is the maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing. The open circuit voltage of a single crystalline silicon solar cell is around 0.5 volts under full sunlight.

OPERATING POINT is the current and voltage point produced when a photovoltaic module or array is connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

ORIENTATION is the placement with respect to the cardinal directions, N-S-E-W; azimuth angle is a measure of orientation from north.

OUTPUT is a physical parameter labeled Positive or Negative; it is defined as the quotient of work and time. The SI Unit for output is Nm/s = 1 watt.

OVERCHARGE is the forcing of current into a fully charged battery. The battery will be damaged if overcharged for a long period.

PACKING FACTOR is the ratio of array area to actual land area or building envelope area for a system; and for a module, it is the ratio of total cell area to the total module area.

PARALLEL CONNECTION is a way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.

PARALLEL OPERATION - When a PV unit is connected to the grid, which acts as a sort of battery: when there is too little sunlight, power is taken from the grid; where the PV unit produces more than is locally consumed, the extra power is fed to the grid. The parallel operation of PV units means that the grid can replace batteries as no storage is needed.

PASSIVATION is a chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.

PEAK DEMAND / LOAD is the maximum energy demand or load in a specified time period.

PEAK POWER POINT is the operating point of the I-V (current-voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.

PEAK SUN HOURS is the equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

PEAK WATTS is a unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (WP) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.

PERFORMANCE RATIO (PR) - A system's PR is relative to the specifications of the solar module, the inverter, and the cabling. The PR reflects the ratio of the system's actual and nominal output; it is generally between 0.7 and 0.8 for modern systems. An important factor in the determination of PR is the distribution of annual irradiation between the relevant value range of 100 to 1000 W/m2. PR is a measurement of the quality of PV systems in operation regardless of the location and orientation. PR is defined as the generator's actual (measured) yield of alternating current in relation to its nominal (= theoretically possible under standard test conditions) direct current yield.

PHOSPHORUS (P) is a chemical element used as a dopant in making n-type semiconductor layers.

PHOTOCURRENT is an electric current induced by radiant energy.

PHOTOELECTRIC CELL is a device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.

PHOTOELECTROCHEMICAL CELL is a type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

PHOTON is a particle of light that acts as an individual unit of energy; a quantum of electromagnetic radiation; and an elementary particle of light that transports solar energy as a bundle of light energy at the speed of light. In a solar cell's PN junction, electrons absorb photons. The energy of the light quanta releases the electrons from their original position.

PHOTOVOLTAICS (PV) is the technology that converts sunlight into electricity directly. The term "photovoltaics" was coined from a combination of the Greek word for "Light" and the name of Italian researcher "Volta". Beckquerel discovered this photovoltaic effect in 1839 and Bell Laboratories developed the first solar cell in 1954.

PHOTOVOLTAIC APPLICATIONS - Examples of PV output applications are:

1. Earth satellites (1 to 20 kW): TV satellites, weather satellites, space probes, manned space stations.
2. Small applications < 1 kW: clocks, radios, measuring devices (temperature, radiation), pocket calculators.
3. Solar Home Systems (SHS) < 100 W: basic power supply for homes (lighting & telecommunications).
4. Commercial Mini-grids (50 W to 10 kW): lighthouses, buoys, TV transmitters and relay stations, off-grid consumers (restaurants, farms, hikers' huts), parking meters, motorway telephones, and drinking water pumps in developing countries.
5. Distributed, Grid-connected PV Systems (1 kW to 30 kW): apartments, demonstration units on schools and other buildings.
6. PV Power Plants > 30 kW: demonstration and test systems.

PHOTOVOLTAIC ARRAY is an interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

PHOTOVOLTAIC CELL is the smallest semiconductor element within a PV module which performs the immediate conversion of light into electrical energy (dc voltage and current). It is also called a solar cell.

PHOTOVOLTAIC CONVERSION EFFICIENCY is the ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

PHOTOVOLTAIC DEVICE is solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in the device and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multi-crystalline, or amorphous forms.

PHOTOVOLTAIC EFFECT is the phenomenon that occurs when photons, the 'particles' in a beam of light, knock electrons loose from the atoms they strike. In this process, the photon energy is converted into potential and kinetic energy of the electrons. The electrons absorb the photons' entire energy quantum, which is defined as the product of Planck's quantum of action and the frequency of the photons. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

PHOTOVOLTAIC ELECTRICITY GENERATION is the generation of electricity directly using solar radiant energy. Photovoltaic electricity generation has decisive benefits:

1. No emission (no noise or waste gases);
2. Long service life (no moving parts - for solar modules, warranties of 20 years and more are granted);
3. Little environmental impact ( operation and disposal of silicon solar cells are completely harmless);
4. Raw materials (silicon is the second most common element on the Earth's crust);
5. Application Range (photovoltaics can be used with almost any output, from small applications such as watches to large facilities of several MW power).

PHOTOVOLTAIC GENERATOR is the total of all PV strings of a PV power supply system, which are electrically interconnected. The individual modules are first connected in a series (in a row) to form strings and these strings are then connected parallel to the PV generator to achieve sufficiently high voltages and currents which can be fed into the public grid using inverters.

PHOTOVOLTAIC MODULE is the smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, and protective devices such as diodes, intended to generate direct current power under un-concentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

PHOTOVOLTAIC PANEL is often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules i.e. laminate string of modules used to achieve a required voltage and current.

PHOTOVOLTAIC PRINCIPLE describes the formation of an electrical current in a semiconductor when charge carriers are excited (internal photon effect) during the irradiance of light (photons). If the charge carriers are extracted, electrical energy is gained in the form of electrical current.

PHOTOVOLTAIC PUMP SYSTEMS - In many arid regions of developing countries, a stable supply of drinking water is a life-threatening problem. The same holds true for some humid areas where the available surface water is often dirty and not suitable for human consumption. The actual pump generally consists of an asynchronous motor and a centrifugal pump inserted into the well as one unit (submerged pump). In some cases, no battery is used; the PV generator directly powers the inverter. In principle, electricity does not have to be stored in a pump system as it is easier to store the pumped water for times when there is little sunlight.

PHOTOVOLTAIC SYSTEM is a complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components. Various components are required for the photovoltaic use of solar energy by means of solar modules, including terminal boxes, inverters, charge regulators, and batteries. Multiple solar cells connected and packaged in a ready-to-use system are called a solar module. A typical model has 36 cells connected in series with a total voltage of 18 volts. The module's efficiency is lower than that of the individual cells as no power is generated between the cells and on the margins of the module. Inverters convert the direct current that the unit produces into grid-compatible alternating current. Charge regulators and batteries store the electricity from the units.

PHOTOVOLTAIC-THERMAL SYSTEM is a photovoltaic system that, in addition to converting sunlight into electricity, collect the residual head energy and delivers both heat and electricity in usable form. It is also known as total energy system.

PHYSICAL VAPOR DEPOSITION (PVD) is a method of depositing thin semiconductor photovoltaic films. With this method, physical processes such as thermal evaporation or bombardment of ions are used to deposit elemental semiconductor material (e.g. semiconductor wafers) on a substrate as compared to chemical vapor deposition. PVD is used in the manufacture of items including semiconductor devices, aluminized PET film for balloons and snack bags, and coated cutting tools for metalworking. The methods are characterized by the following points:

1. Gas (vapor) generation of the layer-forming particles.
2. Transport of the vapor to the substrate (also called the target).
3. Condensation of the vapor on the substrate and layer formation.

There are two groups of PVD methods: vaporization and sputtering. Both methods are based on a purely physical process. During vaporization, atoms and ions are transferred into the gas phase through different vaporization procedures, e.g. via thermal, laser impulse or electron beam. Sputtering means that the initial material is atomized by ion bombardment and converted into a gas.

P-I-N is a semiconductor photovoltaic device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.

PLASMA is a gas containing a significant portion of ionized molecules and free electrons. It is sometimes described as the fourth aggregate state of matter. The ratio of ionized to neutral molecules is typically in a range from a ten-millionth to a ten-thousandth. The plasma glows, and its colour is characteristic of its composition and its state. Plasmas are suitable both for material deposition at comparatively low temperatures and also for material ablation via ion bombardment.

PLATES are metal plates, usually lead or lead compound, immersed in the electrolyte in a battery.

P/N is a semiconductor photovoltaic (PV) device structure in which the junction is formed between a p-type semiconductor and an n-type layer.

POCKET PLATEis a plate of battery in which active materials are held in a perforated metal pocket.

POINT-CONTACT CELL is a high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection.

POLYCRYSTALLINE is a term that describes photovoltaic cells consisting of many crystals of different sizes and with different orientations.

POLYCRYSTALLINE SOLAR CELL is a photovoltaic cell made from crystalline silicon. First, molten silicon is poured into a mold. The mold cools to form a block, creating crystals of different orientation. These various orientations give the surface of these cells their lively appearance, somewhat like marble. These blocks are then sliced into wafers. Polycrystalline solar cells are less expensive - thanks to their less complicated production processes, but they also have a lower efficiency of up to 16 %. The purer the silicon in the cell, the greater will be the cell efficiency.

POLYCRYSTALLINE SILICON (or Poly-silicon) is a material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon. Poly-silicon occurs when molten silicon is poured into molds and left to solidify. The resulting blocks are then cut into wafers some 300 to 500 microns thick for further processing.

POWER is the energy provided or consumed per unit of time. The measurement unit of power is watt (W) or kilowatt (kW). Thus 1 kW = 1,000 W = 1,000 J/s.

POWER CONDITIONING is the process of modifying the characteristics of electrical power e.g. for inverting direct current to alternating current.

POWER CONDITIONING EQUIPMENT is an electrical equipment or power electronics equipment, used to convert power from a photovoltaic array into a form suitable for subsequent use. It is a collective term for inverter, converter, battery charge regulator, and blocking diodes.

POWER CONVERSION EFFICIENCY is the ratio of output power to input power of the inverter.

POWER DENSITY is the ratio of the power available from a battery to its mass (watt /kg) or volume (watt /litre).

POWER FACTOR (PF) is the ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

PRIMARY BATTERY is a battery whose initial capacity cannot be restored by charging.

PRIMARY CIRCUIT (or Collector Circuit or Solar Circuit) is the first closed loop of the heat source (collector). It is the circuit in which energy is generated.

PRIMARY ENERGY is "raw" energy contained in energy sources that has not undergone any conversion, such as coal, crude oil, crude gas, nuclear fuels, hydropower, solar radiation, etc. Primary energy sources can be to some extent be used directly by the end user. In most cases, primary energy is, however converted into secondary energy sources in one or more conversion steps.

PROJECTED AREA is the net south-facing glazing area projected on a vertical plane.

PSEUDOSQUARE (EQUROUND) is the shape of wafers or solar cells when round wafers are cut from a silicon block and then cut into a square with rounded edges.

P-CONDUCTIVE LAYER is positively charged semiconductor material. The silicon is doped with impurities to produce 'holes' or a lack of electrons.

P-TYPE SEMICONDUCTOR is a semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity e.g. boron in silicon.

PULSE-WIDTH-MODULATED (PWM) WAVE INVERTER is a type of power inverter that produces a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

PVB is the abbreviation for Polyvinyl butyral. It is the interlayer used for laminating 2 pieces of glass together.

PYRANOMETER (or Solarimeter) is an instrument used for measuring global solar irradiance.

PYRHELIOMETER is an instrument used for measuring direct beam solar irradiance. It uses an aperture of 5.7 degrees to transcribe the solar disc.

QUAD is one quadrillion Btu (1,000,000,000,000,000 Btu).

QUALIFICATION TEST is a procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.

QUARTZ SAND is the raw material used to make metallurgic silicon (99 % pure) through a heat reduction process.

RATED BATTERY CAPACITY is the term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See battery capacity.

RATED MODULE CURRENT (A) is the current output of a photovoltaic module measured at standard test conditions of 1,000 W/m2, Air Mass 1.5 and 25 deg C.

RATED OUTPUT is the maximum possible power output of a solar cell or module. The rated output is defined as the peak output at the maximum power point under standard test conditions (STC) and is indicated in watt peak (WP).

RATED POWER is rated power of the inverter. However, some units cannot produce rated power continuously. See duty rating.

REACTIVE POWER is the sine of the phase angle between the current and voltage waveforms in an alternating current system. See power factor.

RECOMBINATION is the action of a free electron falling back into a hole. Recombination processes are either radiative (where the energy of recombination results in the emission of a photon) or non-radiative (where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting photons). Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.

RECTIFIER is a device that converts alternating current to direct current. See inverter.

REFLECTION occurs when radiation bounces off the border between two materials - such as on a surface. Radiation that is not reflected either penetrates the material (transmission) or is absorbed by it.

REFLECTION LOSSES - Radiation that is reflected by the surface or a collector or module or solar cell can no longer contribute to the generation of heat or electricity.

REGENERATIVE ENERGY SOURCES are sources of renewable primary energy (such as hard coal, crude oil, biomass, and solar radiation) that naturally occurs, not the secondary and final energy that is converted from it.

REGULATOR prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.

REMOTE SUPPLY SYSTEMS are stand-alone solutions with applications similar to those of micro-grid and mini-grids. Unlike the micro-grids in developing countries, these systems are used in industrialized countries to power remote telecommunications stations, pumps, or parking meters. Such systems are usually installed where connections to the grid are prohibitively expensive. The constant supply of solar power, often in combination with batteries or diesel generators, is often the least expensive alternative for operators.

RESERVE CAPACITY is the amount of generating capacity a central power system must maintain to meet peak loads.

RESISTANCE (R) is the property of a conductor, which opposes the flow of an electric current resulting in the generation of head in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.

RESISTIVE VOLTAGE DROP is the voltage developed across a cell by the current flow through the resistance of the cell.

REVERSE CURRENT PROTECTION is any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See blocking diode.

RIBBON (PHOTOVOLTAIC) CELLS is a type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

ROOF-INTEGRATED INSTALLATION is an installation where the modules are integrated into the roof covering. To do so, the existing roof covering is removed or in case of new buildings, the planned module surface is cut out.

ROOF SLOPE is the angle of a roof with the horizontal surface. Roof slopes between 20 and 50 degrees are suitable to ensure the harmonic integration of a PV System. Corresponding mounting systems can bring the modules into a suitable position, even with an unfavorable roof slope.

ROLL-TO-ROLL is a manufacturing process for thin cells in which the carrier material rolls out and passes through the various processing steps on a belt before being rolled up again at the end of the process.

ROOT MEAN SQUARE (RMS) is the square root of the average square of the instantaneous values of an ac output. For a sine wave, the RMS value is 0.707 times the peak value. The equivalent value of alternating current I, that will produce the same heating in a conductor with resistance R, as a dc current of value I.

SACRIFICIAL ANODE is a piece of metal buried near a structure that is to be protected the structure from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.

SATELLITE POWER SYTEM (SPS) is the concept of providing large amounts of electricity for use on the Earth from one or more satellites in geo-synchronous earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. Therefore, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

SCHOTTKY BARRIER is a cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

SCRIBING is the cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.

SEALED BATTERY is a battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.

SEASONAL DEPTH OF DISCHARGE is an adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.

SECONDARY BATTERY is a battery that can be recharged.

SECONDARY CIRCUIT is the second closed loop of the heat source (collector). It is the circuit that transports the energy to the consumer device.

SECONDARY ENERGY is the energy converted from primary energy that is not yet useful energy. Secondary energy carriers such as electricity or hydrogen are used because they can be stored or converted better than primary energy carriers.

SELECTIVE COATING is a special coating on the absorber of a collector that absorbs nearly all of the (short-wave) solar radiation and simultaneously reduces the radiation losses that increase with the rising temperature by radiating (emission) the (long-wave) thermal emission.

SELF-DISCHARGE is the rate at which a battery, without a load, will lose its charge.

SEMICONDUCTOR is any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process. Solar-grade silicon is often a waste product from the semiconductor industry. This material is thus available in large quantities, environmentally friendly as a waste product, and easy for further processing. A semiconductor is thus a solid whose band gaps - and hence electrical conductivity - are greater than those in a conductor and an insulator. At very low temperatures, semiconductors do not conduct electricity; conductivity increases as temperature rise.

SEMI-TRANSPARENT SOLAR MODULES are partially transparent. They provide us with skin and visual protection from the intense heat of the sun even as they allow enough light to enter for lighting purposes.

SERIES CONNECTION is a way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

SERIES CONTROLLER is a charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

SERIES REGULATOR is the type of battery charge regular where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

SERIES RESISTANCE is the parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

SHADING - The most common problem for photovoltaics is cell shading. If a module is partially shaded, it does not produce electricity. However, the cells not shaded continue to produce voltage, creating a voltage with reverse polarity in neighboring shaded cells. When the cell's throughput voltage of 15 volts has been attained, it becomes conductive and acts as resistance. Electricity then flows, heating the cell, possibly to the point of destruction.

SHALLOW-CYCLE BATTERY is a battery with small plates that cannot withstand many discharges to low state-of-charge.

SHELF LIFE OF BATTERIES is the length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

SHORT-CIRCUIT CURRENT (ISC) is the current flowing freely through an external circuit that has no load or resistance: the maximum current possible.

SHUNT CONTROLLER is the charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.

SHUNT REGULATOR is a type of battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.

SIEMENS PROCESS is a commercial method of making purified silicon.

SILICON (Si) is a semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It is commonly found in sand and quartz (as the oxide). Silicon is the most important raw material for solar cells, both in thin cell technology to produce amorphous cells and in wafers for crystalline cells.

SILICON SOLAR CELL uses highly pure semiconductor silicon as the classic raw material. Solar cell chips are thus made of mono-crystalline or polycrystalline silicon. Polycrystalline material consists of numerous small crystals connected at the edges. These edges are themselves impurities and thus also reduce the output of the cell. It thus comes as no surprise that mono-crystalline solar cells have the highest efficiency. The resulting ingots and blocks are cut into wafers some 300 to 500 microns thick for further processing. Solar cells are made using the same technology common in the semiconductor industry. An N zone is produced in the P-conductive basic substance by means of diffusion under high temperatures. Most of the 10cm x 10cm square silicon wafers then have a PN junction near the front side of the cell. A highly conductive metal layer is then added to the bottom of the cell and another layer to the top side as strips that conduct electricity without hampering energy yield considerably. The metal surface structures are applied in a screen printing method using a metal sheet. To reduce the reflection of light of the surfaces (top), a special anti-reflection coating is added. In combination with the surface of the silicon, this coating gives the cell its steel blue colour. The metal contacts on the top of the cell affect the cell's output. Applied in a screen printing procedure, these contact strips are some 200 microns wide. If the contact strips are grafted with the laser, they may be as thin as 20 microns. The area covered by these strips can thus be reduced.

SINE WAVE is a waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.

SINE WAVE INVERTER is an inverter that produces utility-quality, sine wave power forms.

SINGLE-CRYSTAL MATERIAL is a material that is composed of a single crystal or a few large crystals.

SINGLE-CRYSTAL SILICON is a material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.

SINGLE-STAGE CONTROLLER is a charge controller that redirects all charging current as the battery nears full state-of-charge.

SOLAR CELL is the smallest building block in photovoltaics and is the one that generates electricity from light. Basically, a solar cell consists of a P-conductive basic material and an N-conductive layer on top. A metal contact covers the rear of the cell, while metal strips are applied to the front to minimize losses from shading. Transparent conductive layers that cover the whole surface are also used. To reduce reflection losses, an anti-reflection layer is added to the cell's surface. This gives silicon solar cells their characteristic blue colour. The first silicon solar cell was made in 1954. Impurities are introduced to a thin wafer of highly pure silicon in a process called doping; boron and phosphorus are commonly used here to create two layers with different electric properties. When exposed to light, an electric field occurs in the junction region. Light thus creates free charge carriers in the silicon solar cell that are separated by the electric field. The result is electric voltage of capacity 0.5 volts at the contacts; if a consumer is connected, electricity flows. While the voltage between the solar cell terminals remains constant, the intensity of the current that the cell produces depends directly on the size of the cell and in particular, the incident luminance. Multiple solar cells are connected in series to provide technically interesting voltage. These cells are embedded in a pane of glass and a sheet of plastic. The arrangement is called a module.

SOLAR CELL OUTPUT depends on the following parameters: the size of the cell, the intensity of solar radiation and temperature. As the cells heat up under exposure to sunlight, their efficiency drops by around 0.5% per 1oC.

SOLAR CELL RATING - Standard Test Conditions (i.e. air mass = 1.5, irradiance = 1 kW/m2 and temperature = 25oC) are assumed for the rating of solar cells and modules to allow for comparison. The manufacturing process and basic material used determine the electric rating and the production cost of the solar module.

SOLAR CELL STACK - Thin film technology allows multiple, extremely thin partial cells to be stacked rather than just having one photovoltaically active layer. This increases the efficiency of the thinner individual cells; the ageing effect of a-Si stacks is also reduced. Furthermore, partial solar cells can be spectrally adapted so that each cell can effectively convert a different range of the solar spectrum.

SOLAR CELL TYPES - The material most commonly used for solar cells is currently mono-crystalline or polycrystalline silicon. The cells produced are thus called non-crystalline or polycrystalline cells. In the mid 1970's, amorphous silicon was also developed as a material for photovoltaics. In addition to one-layer solar cells, tandem solar cells are also being developed, especially with configurations based on amorphous silicon and amorphous silicon alloys with carbon and germanium. Here, multiple cells with different energy gaps and transparent contacts on both sides are stacked in a sequence. The MIS (Metal Insulator Semiconductor) inversion layer solar cell is another type of cell that is based on the effect of a layer of stationary positive charges on the surface of a P-doped layer. One speaks of an inversion layer - i.e. it is basically inverted - due to electric field from the stationary charges on the surface. The advantage of these cells is that they can be manufactured in only six steps at relatively low temperatures, while conventional mono-crystalline and polycrystalline cells require up to 17 individual steps. Concentrator cells are another possibility. These cells work with greater light intensity. Mirrors and line systems are used to concentrate the radiation. To attain high energy yields, such concentrator systems have to track the sun. The greater series resistance under concentrated radiation poses a problem. Hence, concentrator cells have to be highly doped and equipped with especially low-loss contacts.

SOLAR CONSTANT is the average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays: equal to 1353 watts per square meter or 492 Btu per square foot.

SOLAR COOLING is the use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners.

SOLAR ENERGY is the electromagnetic energy transmitted from the sun (solar radiation) and it makes up some 99.9% of all of the energy used on the Earth. The solar energy incident on the surface is weakened in the atmosphere and partially converted into other types of energy such as wind and hydropower. The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

SOLAR GENERATOR is the totality of all PV modules in a system. Large systems may be subdivided into several partial generators. To allow for greater output, solar modules are connected in series or in parallel. A row of modules connected in series is called a string.

SOLAR GRADE SILICON is the intermediate-grade silicon used in the manufacture of solar cells. It is less expensive than electronic-grade silicon.

SOLAR HOME SYSTEM (SHS) - Some 2 to 3 billion people worldwide have no access to electricity today. They live in developing countries in Africa, Asia and Latin America. Some 90% of their relatively low-energy demand is heat for cooking, with the other 10% for lighting and communication (radio). The use of SHS's provides power for this 10% through photovoltaics. The non-energy benefits of SHS's are much greater than their energy benefits as access to modern communication devices provides both better educational effects for individuals and generally accelerates regional development. The electrical design of SHS's is very simple. The system consists of a module, a battery, and a charge regulator with excess-discharge protection. For years, SHS projects have been conducted in countries like Mexico and India.

SOLAR INSOLATION refers to the amount of electromagnetic energy (solar radiation) incident on the surface of the earth. Basically that means how much sunlight is shining down on us. The values are generally expressed in kWh/m2/day. This is the amount of solar energy that strikes a square metre of the earth's surface in a single day.

SOLAR NOON is the time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky: equals to true or due, geographical south.

SOLAR RADIATION - The atmosphere is largely impenetrable for solar radiation; only wavelengths between 0.3 and 5.0 µm (called the optical window) and 10-2 to 102 m (called the radio window) can reach the Earth's surface. Solar Radiation is energy from the optical window comprising the range of visible light from 0.38 to 0.78 µm.

SOLAR RESOURCE is the amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

SOLAR SPECTRUM is the total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 % of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total solar radiation.

SOLAR THERMAL ELECTRIC SYSTEMS are solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.

SOLAR THERMAL ENERGY is the solar energy used for the direct generation of heat. The corresponding energy converter is know as a collector.

SOLAR YIELD specifies the utilizable solar heat. In other words, it is the remaining heat from the storage tank that can be used after deducting all thermal losses of the collector system.

SPECIFIC GRAVITY is the ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. It is used as an indicator of battery state-of-charge.

SPINNING RESERVE is the electric power plant or utility capacity on-line and running at low power in excess of actual load.

SPLIT-SPECTRUM CELL is a compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See multi-junction device.

SPUTTERING is a thin-cell production method used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate. In this process, the source material is a wafer several millimeters thick. The "sputter cathode" can be hot pressed. The cathode lies several centimeters over the substrate to be coated in a vacuum. The processing unit contains a rarefied gas, usually an inert gas such as argon. This gas prevents chemical reactions. The unit may also contain other gases that become part of the thin film (reactive sputtering). An electrical discharge creates a plasma from the gas, i.e. the atoms become electrically charged ions. In an electric field, these ions are accelerated to the cathode and atomize the source material upon impact with the cathode. This impact is similar to that of a pool ball on an array of resting balls, which are sent into different directions upon impact. The sputtered particles move to the substrate, where they are deposited as a thin film. While this may sound complex, the process is technically simple and the best, most inexpensive one for many applications. One convincing example is the glass industry, where panes of glass for insulating windows of 3 x 6 m2 are coated in a through-feed process. As the sputtering method has proven its worth in industrial use, it is also preferred in thin-film photovoltaics.

SQUARE WAVE is a waveform that has only two states i.e. positive or negative. A square wave contains a large number of harmonics.

SQUARE WAVE INVERTER is a type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency.

STAEBLER-WRONSKI EFFECT is the tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.

STANDS (for Mounting PV Modules) - The energy yield of photovoltaic modules is proportional to the incident solar energy. Hence, the surface of the modules ideally faces the sun. There are two basic types of installations: rigid stands and trackers that follow the sun in one of tow axes. PV modules that concentrate sunlight require tracking, but tracking also increases the energy yield of non-concentrating systems.

STAND-ALONE SYSTEM is an autonomous or hybrid photovoltaic system not connected to a grid. It may or may not have storage, but most stand-alone systems require batteries or some other form of storage.

STAND-OFF MOUNTING is the technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

STANDARD REPORTING CONDITIONS (SRC) is a fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.

STANDARD TEST CONDITIONS or STC are defined worldwide to allow for comparisons between various photovoltaic modules. The current, voltage, and output of a module typically tested in a laboratory are measure under an irradiance level of 1000 W/m2, a module temperature of 25oC, and an Air Mass 1.5 Solar Spectrum.

STANDBY CURRENT is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.

STARVED ELECTROLYTE CELL is a battery containing little or no free fluid electrolyte.

STATE-OF-CHARGE (SOC) is the available capacity remaining in the battery, expressed as a percentage of the rated capacity.

STORAGE BATTERY is a device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.

STRATIFICATION is a condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See equalization.

STRING is a number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

STRUCTURAL GLAZING is a glazing system that uses frameless panes of glass or PV laminates. The elements are glued to the metal profiles of the support structure so that no attachment system interrupts the glass surface (solely glass façade) or casts shadows outside.

SUBSTRATE is the physical material upon which a photovoltaic cell is applied. The substrate determines the mechanical properties of the module, such as flexibility.

SUBSYSTEM is any one of the several components in a photovoltaic system i.e. array, controller, batteries, inverter and load.

SULFATION is a condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

SUPERCONDUCTING MAGNETIC ENERGY STORAGE (SMES) is the technology that uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. It has been proposed as a storage option to support large-scale use of photovoltaics as a means to smooth out fluctuations in power generation.

SUPERCONDUCTIVITY is the abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

SUPERSTRATE is the covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

SURGE CAPACITYis the maximum power, usually 3-5 times the rated power, that can be provided over a short time.

SUNLIGHT is light from the sun. Solar radiation reaches the Earth's atmosphere with an intensity of 1367 W/m2. After being refracted and absorbed, only a maximum of 1060 W/m2 can be used at the surface. A reference value for sunny days is 1000 W/m2. The amount of solar energy varies by location.

SYSTEM AVAILABILITY is the percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

SYSTEM OPERATING VOLTAGE is the photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

TANDEM SOLAR CELL is a combination of three or more cells integrated optimally and electrically, with each cell sensitive to a different spectral range, in one component. Tandem solar cells have greater efficiencies than each of their individual components. Efficiencies of around 30% are possible with these cells, but they are difficult to produce. They consist of several layers of different semiconductor material stacked up as thin-film. The different spectral sensitivities of the various semiconductors are utilized. A material that is sensitive to the visible part of the solar spectrum is combined with one sensitive to the infrared range. For instance, a structure made of Gallium Arsenide (GaAs) and Gallium Antimonide (GaSb) has an efficiency of 30% under concentrated light from a lens. Tandem solar cells are stacked solar cells with partial cells covering only two different ranges of the solar spectrum.

TARE LOSS is the loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

TEMPERATURE COEFFICIENT specifies how much the open circuit voltage or power output and thus the efficiency of a solar cell or module decreases for each degree Celsius of temperature increase. Since the above sizes of crystalline solar cells have comparatively high negative temperature coefficients, modules, particularly those made of crystalline silicon cells, should be well-ventilated.

TEMPERATURE COMPENSATION is a circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than + 5 deg C from ambient temperature.

TEMPERATURE DIFFERENCE CONTROLLER controls the heat transport from the collector to the storage tank by switching the collector pump on an off. The control signal is the temperature difference at the collector outlet and in the lower part of the storage tank.

TEMPERATURE FACTORS is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.

TEXTURED SOLAR CELL is a PV cell that has undergone surface structuring i.e. roughening of the surface of the solar cell by mechanical or chemical means. This allows the incident sunlight to be exploited better. In crystalline solar cells, this is for instance done by created reverse pyramid structures.

THERMOPHOTOVOLTAIC CELL (TPV) is a device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.

THICK-CRYSTALLINE MATERIALS is a semiconductor material, typically measuring from 200-400 microns thick cut from ingots or ribbons.

THIN FILM is a layer of semiconductor material, such as amorphous silicon, copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells.

THIN FILM PHOTOVOLTAIC MODULE is a photovoltaic module constructed with sequential layers of thin film semiconductor materials. See amorphous silicon.

THIN-LAYER TECHNOLOGY is the name for the manufacturing technology of thin-film solar cells that are deposited directly onto a low-cost substrate (glass, metal foil, plastic foil). The advantages of thin-film technology lie within the material and energy savings during the industrial manufacturing process, the simple doping ability and the ability to produce large-scale solar cells with integrated series interconnection.

TILT ANGLE is the angle at which a photovoltaic array is set to face the sun relative to a horizontal position. Depending on the latitude of the installation location, the tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

TIN OXIDE is a wide band-gap semiconductor similar to indium oxide; used in hetero-junction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass.

TOTAL AC LOAD DEMAND is the sum of the alternating current loads. This value is important when selecting an inverter.

TOTAL HARMONIC DISTORTION is the measure of closeness in shape between a waveform and its fundamental component.

TOTAL INTERNAL REFLECTION is the trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.

TRACKING is the constant orientation of the solar module to face the sun whenever possible, like a sunflower. Tracking the sun increases the energy yield by approximately 30%. Here, a distinction can be made between horizontal tracking, vertical tracking (for modules installed at an angle), and a combination of the two.

TRACKING ARRAY is a photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) axis where the array tracks the sun east to west and (2) axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and indirect sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

TRANSFORMER is an electromagnetic device that changes the voltage of alternating current electricity.

TRANSPARENT SOLAR CELLS are made by modifying the production process for silicon solar cells. Tiny holes some 0.15 mm wide are punched into the base material (mainly standard polycrystalline silicon wafers) in an additional mechanical processing step, making the cells partially transparent. A 100 mm x 100 mm solar cell thus has some 40,000 such holes.

TRAY CABLE (TC) may be used for interconnecting balance-of-systems.

TRICKLE CHARGE is the charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

TRIPLE SOLAR CELL is a stacked solar cell covering three different ranges of the solar spectrum.

TWO-AXIS TRACKING is a photovoltaic array tracking system capable of rotating independently about two axes.

TUNNELING is a quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.

TYPICAL METEOROLOGICAL YEAR (TMY) is a collection of weather information that includes data about insolation for every hour of a typical year computed using historical weather data.

ULTRAVIOLET is the electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

UNDERGROUD FEEDER (UF) may be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

UNDERGROUND SERVICE ENTRANCE (USE) may be used within battery enclosures and for interconnecting balance-of-systems.

UNINTERRUPTED POWER SUPPLY (UPS) is the designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

USEFUL ENERGY is the energy available to fulfill a need after the final conversion in devices, such as for air-conditioning, cooking, information, transport, etc. It is the product of final energy minus the losses from the final conversion step, such as the light available after waste heat losses are deducted for lamps.

UTILITY-INTERACTIVE INVERTER is an inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.

VACUUM EVAPORATION is the deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

VACUUM ZERO is the energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

VALENCE BAND is the highest energy band in a semiconductor that can be filled with electrons.

VALENCE LEVEL ENERGY / VALENCE STATE is the energy content o an electron in orbit about an atomic nucleus. It is also called bound state.

VALUE CHAIN is the full process of the PV system - starting from purification of the PV material to complete installation. In the initial stage of the value chain, the ultra-pure silicon (poly-silicon) used for solar cell production is produced by passing quartz through several stages of purification. It is then either cast in polycrystalline silicon blocks or drawn with mono-crystalline silicon rods to form mono-crystalline ingots. The silicon blocks or ingots are sawn into wafers whose thickness is typically between 200 and 300 µm. To produce a functional solar cell, the manufacturer coats the wafers and applies metal contacts via which the electric current is made available. The module manufacturer produces PV modules by interconnecting a number of single solar cells to form a larger unit. The interconnected solar cells are fitted in a frame and covered with glass to provide weather resistance and long-term stability. In the final stage of the value chain, the system supplier adds the necessary electronics, storage and installation solutions, wires them up and connects them to the electrical circuits.

VARISTOR is a voltage-dependent variable resistor. It is normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

VENTED CELL is a battery designed with a vent mechanism to expel gases generated during charging.

VERTICAL MULTI-JUNCTION (VMJ) CELL is a compound cell made of different semiconductor materials in layers one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. It is also called a multiple junction cell. See multi-junction device and split-spectrum cell.

VOLT (V) is a unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

VOLTAGE is the amount of electromotive force measured in volts which exists between two points. It is the "push" behind electrical current conduction. While the nominal voltage is the voltage under nominal static conditions (STC in photovoltaics), the operating voltage reflects the current value under real operation.

VOLTAGE AT MAXIMUM POWER (VMP) is the voltage at which maximum power is available from a photovoltaic module.

VOLTAGE PROTECTION - Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

VOLTAGE REGULATION indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

VRLA stands for valve regulated lead-acid and is the designation for low maintenance lead-acid batteries also called recombinant batteries.VRLA batteries are commonly further classified as:

• Absorbent glass mat battery
• Gel battery

These batteries are often colloquially called sealed lead-acid batteries, but this term is misleading: a sealed battery would be a safety hazard due to overpressure risks when overcharging, and there is always a safety valve present, hence the name valve-regulated. Sealed is opposed to vented (also called flooded). Because VRLA batteries use much less electrolyte (battery acid) than traditional lead-acid batteries, they are also occasionally referred to as an "acid-starved" design.

WAFER is a thin slice of semiconductor (photovoltaic) material made by cutting it from a single crystal or ingot. Wafers suitable for use in solar cell production are thin slices of silicon, typically 200 to 300 µm thick. They are either drawn or cast (tape or foil silicon) directly from a silicon melt or cut from mono-crystalline or multi-crystalline ingots using tools such as wire saws.

WATT is the rate of energy transfer equivalent to one ampere under one electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

WATT PEAK (WP) is the peak (maximum) output of a cell or module or PV generator under Standard Test Conditions.

WAVEFORM is the shape of phase power at a certain frequency and amplitude.

WET SHELF LIFE is the period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

WINDOW is a wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

WIRE TYPES - See Article 300 of National Electric Code for more information.

WORK FUNCTION is the energy difference between the Fermi level and vacuum zero. This is the minimum amount of energy it takes to remove an electron from a substance into the vacuum.

ZENITH ANGLE is the angle between the direction of interest (of the sun) and the zenith (direction directly overhead).