Technical terms from the field of solar energy explained in detail. Please select a letter and find the explanation you are looking for.
Absorption — If the intensity of light is reduced when it passes through matter, we speak of absorption. Matter absorbs, i.e. takes up, the energy. Since blue cells absorb more light than red, yellow or green cells, almost all solar cells are blue.
AC — Short for ‘alternating current’, electrical current whose polarity constantly alternates. Our power network contains AC with a frequency of 50 Hz (Hertz), it assumes a negative and a positive value 50 times each per second. In the solar plant AC is generated through an inverter .see alternating current.
Acidic texturing — Method that is part of the production of photovoltaic cells which leads to efficiency increases through chemical surface treatment.
Activated Shelf Life — The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.
AIC — See amperage interrupt capability.
Air mass (sometimes called air mass ratio) — 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 travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).
Alkaline—The term ‘alkaline’ refers to the ratio of negatively charged hydroxide ions (OH–) and positively charged protons (H+). Alkaline materials contain more hydroxide ions.
Alternating Current (AC) — A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.
Ambient Temperature — The temperature of the surrounding area.
Amorphous Semiconductor — A non-crystalline semiconductor material that has no long-range order.
Amorphous Silicon — A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.
Ampere, or amp, is the unit of measurement of the electrical current, abbreviated ‘A’. The force of an electric current is the number of charges flowing per time unit. Electrical current is measured as the force with which two parallel electrical lines attract each other. It is indicated as current with the symbol ‘I’ . See also Ohm’s Law .
Ampere Hour Meter — An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).
Angle of Incidence — 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 if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
Annual Solar Savings — The 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.
The positive pole of an electrical circuit. The electrons travel from the negative pole (cathode) to the anode (positive pole). The definition as positive and negative poles was established when the actual direction in which the electrons travel was not yet known.
Antireflection Coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.
Anti-reflective layer—The anti-reflective layer is a layer of only a thickness of only a few millionths of a millimetre, consisting of silicon nitride (SiN) and minimising reflection losses. Light that is reflected from the surface of a solar cell cannot be absorbed , and hence does not contribute to the generation of electricity. In solar cells, the anti-reflective layer therefore increases the light yield and the efficiency .
Array — see photovoltaic (PV) array.
a-Si—a-Si stands for ‘amorphous silicon’. Atoms in amorphous matter, such as amorphous silicon , are irregular and seem to be randomly arranged; Greek amorphos means ‘shapeless’. By contrast, the microscopic shape of crystalline matter is regular.
Autonomous System — See stand-alone system.
Azimuth Angle — The angle between true south and the point on the horizon directly below the sun.
Back contact cell — In this photovoltaic cell, a new method is used to attach the contact (accumulator) only to the back of the photovoltaic cell , which reduces the shading by the small conductor lines at the front.
Balance of Applications — Represents all components and costs other than the photovoltaic modules/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 Energy (Eg) — The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.
Base Load — The average amount of electric power that a utility must supply in any period.
Battery — 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. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.
Battery Available Capacity — 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 Cell — 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 the above.
Battery Energy Capacity — 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 than it is to the battery industry where capacity usually refers to ampere-hours.
Battery Energy Storage — Energy storage 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 — 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.
BIPV (Building-Integrated Photovoltaics) — A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.
Blocking Diode — A semiconductor 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 not backwards.
Boule — 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.
Breakage rate — The percentage of wafers or cells that break during processing and production. Reducing the breakage rate is therefore part of the optimisation of a production plant.
BSW – Bundesverband Solarwirtschaft e.V. (Federal Association of Solar Business) — The BSW represents the interests of German solar companies in Berlin vis-à-vis politicians and the public. In 2006 the BSW has over 600 members and represents more than 80% of the domestic production of solar plants. The BSW was created on 28 March 2006 through a merger of the Bundesverband Solarindustrie (BSi) e.V. (Federal Association of Solar Industry) and the Unternehmensvereinigung Solarwirtschaft (UVS) e.V. (Association of Solar Industry Companies). See also www.solarwirtschaft.de .
Btu (British Thermal Unit) — The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.
Bypass Diode — A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.
A diode that is switched parallel to a solar cell , conducting the electrical current of all remaining solar cells past a single shadowed solar cell (when they are connected in series). The diode thus prevents a hot spot .
Cadmium Telluride (CdTe) — A polycrystalline thin-film photovoltaic material.
Capacity Factor — 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 — A battery having an immobilized electrolyte (gelled or absorbed in a material).
Cathodic Protection — A method of preventing oxidation of the exposed metal in structures by imposing a small electrical voltage between the structure and the ground.
Cd — see cadmium.
CdTe — see cadmium telluride.
Cell (battery) — 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. (Sometimes the terms cell and battery are used interchangeably). Also see photovoltaic (PV) cell.
Cell Barrier — 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. Also called depletion zone or space charge.
Charge — The process of adding electrical energy to a battery.
Charge Controllers — 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.
Charge Factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.
Charge Rate — 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) — A method of depositing thin semiconductor 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 of 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.
Cleavage of Lateral Epitaxial Films for Transfer (CLEFT) — 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.
Clouding — Obstruction to the incidence of light and to light absorption through uneven surface structures or contacts on the inside surface of a photovoltaic cell . See also shading
Cloud Enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.
Concentrator — A photovoltaic module, which includes optical components 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.
Conductor — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.
Contact Resistance — The resistance between metallic contacts and the semiconductor.
Conversion Efficiency — See photovoltaic (conversion) efficiency.
Converter — A unit that converts a direct current (dc) voltage to another dc voltage.
Current — See electric current.
Cycle — The discharge and subsequent charge of a battery.
Czochralski Process — A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.
Dangling Bonds — A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface.
Deep-Cycle Battery — A battery with large plates that can withstand many discharges to a low state-of-charge.
Deep Discharge — Discharging a battery to 20% or less of its full charge capacity.
Degradation — ‘Degradation’ or ‘ageing’ refers to the decline of a solar cell’s efficiency over time. Mainly solar cells made of amorphous silicon are affected by the ageing process. Laboratory experiments have shown that after about 1,000 sun hours the efficiency has achieved a stable degree of saturation
DENA — Deutsche Energie-Agentur (German Energy Agency). This government agency promotes saving energy and regenerative energies. It implements national and international campaigns, informs end uses and develops strategies for future energy supply. It was founded by the federal government in autumn of 2000 and is headquartered in Berlin.
Depth of Discharge (DOD) — 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%.
Dendritic Web Technique — A method for making sheets of polycrystalline 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.
DGS — The Deutsche Gesellschaft für Sonnenenergie e.V. (German Association of Solar Energy) was founded in1975 in Munich. Its main objective is the transformation of the energy industry through the broad-based introduction of renewable energies. More at http://www.dgs.de/
Diffuse Radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground.
Diffusion Furnace — Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.
Electronic semiconductor component which lets the electrical current pass in only one direction. If a power line is compared to a water pipe, then the diode would be a (one-way) valve in the water pipe. Diodes are made of semiconductor crystals and have a typical resistance. A solar cell therefore is basically a large, illuminated diode.
Direct Beam Radiation — Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.
Direct Current (DC) — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.
Discharge — The withdrawal of electrical energy from a battery.
Discharge Factor — A 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. Related to discharge rate.
Disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.
Distributed Energy Resources (DER) — 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 — A popular term for localized or on-site power generation.
Distributed Power — Generic term for any power supply located near the point where the power is used. Opposite of central power. See stand-alone systems.
Distributed Systems — 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, 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 electrical conductivity over than of an undoped semiconductor.
Dopant — 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 — DopingBy the precise addition of small amounts of so-called doping atoms, the electrical conducting ability of a semiconductor can be significantly enhanced. Doped semiconductors are used for the production of pn junctions in solar cells and electronic components.The addition of dopants to a semiconductor.
Downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
Duty Cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.
Efficiency — The efficiency η (pronounced ‘eta’) of a solar cell or a module is defined as the ratio between the power output and the power input (the absorbed light). In other words, it indicates how well the solar cell converts light into electricity. In addition, the efficiency of the module indicates how much energy is lost by connecting the individual cells in the module.
Electric Circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.
Electrical field — An electrical field is generated when current flows through a conductor. The field exerts a force on charge carriers; due to their different charges, electrons and electron holes in semiconductor material move in opposite directions in an electrical field, therefore they can be separated by a field. In most solar cells the pn junction generates an electrical field.
Electrical grid — An integrated system of electricity distribution, usually covering a large area.
Electrochemical Cell — 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 — A conductor that is brought in conducting contact with a ground.
Electron — 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.
Electric particles with a negative charge, whose movement in a conductor, such as a wire, is called ‘current flow’. Therefore electric particles that are at rest do not constitute electrical current./p>
Electron Volt (eV) — 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 — 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 — A survey that shows how much energy used in a home, which helps find ways to use less energy.
Energy Density — The ratio of available energy per pound; usually used to compare storage batteries.
Energy Levels — The energy represented by an electron in the band model of a substance.
Epitaxial Growth — 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 Charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.
Equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).
Fermi Level — 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.
Fixed Tilt Array — A photovoltaic array set in at a fixed angle with respect to horizontal.
Flat-Plate Photovoltaics (PV) — A PV array or module that consists of nonconcentrating 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.
Float Life — The number of years that a battery can keep its stated capacity when it is kept at float charge.
Float Service — 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 — 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.
Float-Zone Process — In reference to solar photovoltaic cell manufacture, 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 a single crystal.
Fossil ResourcesThe fossil resources oil, gas and coal were formed millions of years ago, when the remnants of plants and animals were deposited on the ocean floor and over the years were covered by sediments such as clay and sand. Underneath the sediments, pressure and temperature increased, converting the organic material into hydrogen and carbon compounds. The carbon was withdrawn from the atmosphere. When these resources are burned today, it is released as CO2 , causing the greenhouse effect. There is only a limited supply of fossil resources: according to the IEA, fossil oil, for example, will last approximately another 40 years, fossil gas about 50 years, uranium about 60 years and coal about 100 to 120 years.
Frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).
Fresnel Lens — 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 — The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).
Ga — See gallium.
GaAs — See gallium arsenide.
Gassing — The evolution of gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from local action self-discharge or from the electrolysis of water in the electrolyte during charging.
Gassing Current — 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.
Gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
Grain boundary — When molten metal solidifies, fields are formed that can be recognised under a microscope and look like grains. The collision of two grains is called ‘grain boundary’. The size of the grains can be controlled through the speed of the cooling process. In multicrystalline silicon as the source material of solar cells, the boundaries between the individual grains represent obstacles in the transport of the charge. Therefore the efficiencies of monocrystalline solar cells are typically higher than those of multicrystalline cells.
Grid — See electrical grid.
Grid-Interactive System — Same as grid-connected system.
Harmonic Content — 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.
Heterojunction — A region of electrical contact between two different materials.
Hole — The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.
A term from the physics of semiconductors. When an electron is released through the impact of light rays and moves towards an electrical pole, a spot without an electron is left in its former place in the crystal lattice – a hole. Since these spots without an electron are immediately taken by another electron, it looks as if this hole were moving. Therefore the hole can be regarded as a moving, positive-charged particle.
Homojunction — The region between an n-layer and a p-layer in a single material, photovoltaic cell.
Hot spot —Hot spot of a solar module, which is generated on a module when a single solar cell in a series connection is shadowed (see shading ). Such a cell behaves like an (Ohm) resistance and can heat up to the point of destruction when the current of the module’s other cells flows through it. In that case the solar cell burns out like an overcharged resistance.
Hydrogenated Amorphous Silicon — 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.
Indium Oxide — A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction solar cell.
Infrared Radiation — Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.
Ingots — Blocks made from raw silicon , from which silicon wafers are cut.
Input charges — Amount which according to the REA is paid for the input of regeneratively produced electricity into the public electricity network. The rate is different for all types of renewable energies, such as wind, solar energy, waterpower and geothermal energy, and is reduced by 5% every year for solar energy. In 2006 it is between 40.6 cents (large outdoor plants) and 51.8 cents (roof systems) per kWh for solar plants
Input Voltage — This 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 — The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. See diffuse insolation and direct insolation.
Inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid. Photovoltaic modules generate DC . An inverter transforms DC into AC . This makes it possible for the electrical energy produced with solar energy to be used by end consumers with 230 volts of alternating voltage or to feed it into the public power network. Central inverters are used in large photovoltaic systems, and string inverters in small photovoltaic systems.
ISPRA Guidelines — Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.
ISE — ISE, the Fraunhofer Institute of Solar Energy Systems in Freiburg, is one of the leading institutes of solar technology in Germany. In addition to photovoltaics, ISE deals with solar construction, chemical energy conversion, energy storage and rational energy use. The ISE researcher and institute director, Prof. Joachim Luther, received the German Environment Award in 2005 for his outstanding merits in the field of solar energy. See also www.ise.fhg.de .
I-Type Semiconductor — 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 — 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.
Junction — 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 — A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.
The energy produced by a solar plant is indicated by the amount of electricity in watts produced per hour. The kilowatt hour is the product of constant power generation times the duration of power generation
Kilowatt-Hour (kWh) — 1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.
Kilowatt peak (kWp) — Unit of measurement for the nominal capacity of a photovoltaic cell or a photovoltaic module . This value is measured under standardised test conditions. While these conditions do not reflect the true conditions in an installed system, they make a direct comparison of the output of different modules possible. Measurements are made at a module temperature of 25 degrees and a radiation of 1,000 W/m².
Langley (L) — Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.
Lattice — The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.
Lead-Acid Battery — A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.
Life — The period during which a system is capable of operating above a specified performance level.
Life-Cycle Cost — The estimated cost of owning and operating a photovoltaic system for the period of its useful life.
Light Trapping — 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.
Line-Commutated Inverter — 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 failure in the power grid, the photovoltaic system cannot feed power into the line.
Liquid Electrolyte Battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.
Load Circuit — The wire, switches, fuses, etc. that connect the load to the power source.
Load Resistance — The resistance presented by the load. See resistance.
Low light behaviour — Performance of the photovoltaic cell during low radiation, for example when the sky is cloudy. Q-Cells solar cells possess particularly good low light properties, which increases their total efficiency .
Low Voltage Warning — A warning buzzer or light that indicates the low battery voltage set point has been reached.
Maximum Power Point (MPP) — 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 at about 0.45 volts.
Microgroove — A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.
Minority Carrier — 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.
Modularity — The use of multiple inverters connected in parallel to service different loads.
Module — See photovoltaic (PV) module.
Monolithic — Fabricated as a single structure.
Movistor — Metal Oxide Varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.
MPP tracking — MPP is the abbreviation for ‘maximum power point’ . ‘Tracking’ is the term for the adjustment of the energy take-off so that a solar plant is always operated at the maximum power point. This avoids loss of electrical power and is part of the basic equipment of a charge controller and an inverter
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) — This organization sets standards for some non-electronic products like junction boxes.
NEC — See National Electrical Code.
N doping — By specifically adding, small amounts of so-called doping atoms to a silicon lattice, its electric conductivity can be significantly increased. Doped semiconductors – doped silicon – are used to produce pn junctions in solar cells and electronic components.
Network input — The REA obliges power network operators to pass the electricity from renewable energy sources from their networks on to the consumers and to pay the charges established by the REA (see input charges ) for the electricity. This charge is then allocated to all electricity customers nationwide. The input charge varies for the different renewable energy sources, such as sun, wind and water, and decreases each year by fixed percentage points.
Network-connected plants — Energy production plants are usually connected to the general power network. The opposite of this are insular plants, which are only used for supplying buildings, for example, that are not connected to the network. These solar plants then store the energy in batteries. The advantage of network operation is that the energy will be consumed even if the producer does not currently need the energy.
Network connection — Connection of a photovoltaic plant to the power network using an inverter ; the goal is to feed the electricity produced in the PV plant completely or partially
NEMA — See National Electrical Manufacturers Association.
Nickel Cadmium Battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.
Normal Operating Cell Temperature (NOCT) — The estimated temperature of a photovoltaic module when operating under 800 w/m2 irradiance, 20 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.
Ohm — 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.
One-Axis Tracking — A system capable of rotating about one axis.
Open-Circuit Voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.
Operating Point — The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.
Orientation — Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.
Outgas — See gassing.
Panel — See photovoltaic (PV) panel.
Parallel Connection — 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.
Passivation — A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.
Peak Demand/Load — The maximum energy demand or load in a specified time period.
Peak Power Tracking — see maximum power tracking.
Peak Sun Hours — 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 Watt — 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.
Photocurrent — An electric current induced by radiant energy.
Photoelectric Cell — 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 — 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 — A particle of light that acts as an individual unit of energy.
The photon is a light quantum, the smallest energy unit of light. The term was coined by Albert Einstein. The elementary particle photon moves at the speed of light, and when it encounters a solar cell, its energy is directly converted to electricity. Hence the term ‘photovoltaics’
Photovoltaic(s) (PV) — Pertaining to the direct conversion of light into electricity.
Photovoltaics deal with the conversion of radiation energy, in particular, solar energy, into electrical energy, and has been used for energy supply (initially via satellites) since 1958. The name consists of the components photo – Greek for ‘light’ – and volta – after Alessandro Volta, the pioneer in electricity
Photovoltaic (PV) Array — 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 (PV) Cell — The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.
Photovoltaic (PV) Conversion Efficiency — The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.
Photovoltaic (PV) Device — A 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 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, multicrystalline, or amorphous forms.
Photovoltaic (PV) Effect — The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. 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 (PV) Generator — The total of all PV strings of a PV power supply system, which are electrically interconnected.
Photovoltaic (PV) Module
— 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
unconcentrated sunlight. The structural (load
carrying) member of a module can either be the top layer (superstrate)
or the back layer (substrate). Photovoltaic
A photovoltaic or solar module consists of several solar cells that are connected with one another. They are placed between two glass or plastic discs to protect them from the weather conditions. To achieve more usable voltages, several photovoltaic cells are connected with each other in a solar module . Typically photovoltaic modules are attached inside a frame on the roof or to a supporting frame.
Photovoltaic (PV) Panel — 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., a laminate string of modules used to achieve a required voltage and current).
Photovoltaic-Thermal (PV/T) System — A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system.
Physical Vapor Deposition — 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 on a substrate.
P-I-N — A semiconductor photovoltaic (PV) 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.
Pocket Plate — A plate for a battery in which active materials are held in a perforated metal pocket.
Polycrystalline — See Multicrystalline.
Power Conditioning Equipment — Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.
Power Conversion Efficiency — The ratio of output power to input power of the inverter.
Power Density — The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).
Power Factor (PF) — 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 energy consumption — Primary energy consumption is the entire energy consumed by a national economy. Especially in electrical power generation, regenerative energies such as solar energy, water and wind power increasingly replace the traditional energies of brown coal, coal and nuclear power. Today their share is already at over 10% in Germany and is to increase to one quarter by 2020
Projected Area — The net south-facing glazing area projected on a vertical plane.
Pulse-Width-Modulated (PWM) Wave Inverter — A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.
PV — See photovoltaic(s).
Pyranometer — An instrument used for measuring global solar irradiance.
Quad — One quadrillion Btu (1,000,000,000,000,000 Btu).
Qualification Test — 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.
Rated Battery Capacity — 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.
Recombination — 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 nonradiative, where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting phonons. Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.
Controller — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.
Remote Systems — See stand-alone systems.
Reserve Capacity — The amount of generating capacity a central power system must maintain to meet peak loads.
Resistance (R) — The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat 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.
Reflection losses — Light that is reflected from the surface of a solar cell and therefore can no longer contribute to the generation of electrical current. Hence anti-reflection layers are applied onto solar cells.
Re-doping Switch of the wafer surface from a positive to a negative type of conductor, which is achieved by diffusing phosphorus particles into the wafer surface. As a result, the silicon wafer has two separate layers, one positive-conductive and one negative-conductive layer.
Renewable energy — ‘Renewable energy’, also called ‘regenerative energy’, is a term for energy from resources that are replaced and, from what we know, inexhaustible. Regenerative energy is essentially used as solar energy, biomass, the earth’s heat, waterpower and wind energy. An important criterion is that renewable energy is provided without generating CO2 .
Renewable Energy Act (REA) — Since April 2000, the REA has determined the charges for electricity from solar, wind, water, geothermal and biomass energy. The parliamentary majority, made up of Social Democrats, the Green Party and the Christian Democrats, tried to promote the regenerative energies with this act, triggering a boom that has surpassed many expectations. In early 2006, 130,000 persons were employed in the renewable energy sector, and the Federal Ministry of the Environment, Environmental Protection and Reactor Safety expects that about 260,000 people will be employed in the sector by 2020, and that regenerative produced electricity will make up 25% of total consumption in Germany by then.
Ribbon (Photovoltaic) Cells — 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.
RMS — See root mean square.
Roll-to-roll process — Low-cost industrial production process for thin-film solar cells on metal or plastic film. The flexible substrate is unwound from a roll, coated in the process chambers and rolled up again at the other end of such a production line.
Root Mean Square (RMS) — 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 — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.
Satellite Power System (SPS) — Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous 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. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.
Scribing — The cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.
Sealed Battery — 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 — 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 — A battery that can be recharged.
Semiconductor — 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.
Semiconductors acquire electrical conductivity under the influence of light rays. The possibility of influencing the conductivity through doping makes semiconductors interesting for the production of solar cells . A total of 90% of all solar cells produced worldwide are manufactured from the semiconductor material silicon , the second most frequent element of the earth’s crust.
Semicrystalline — See Multicrystalline.
Shallow-Cycle Battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.
Shelf Life of Batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.
Shading When clouds, trees or buildings cast shadows on a solar plant, this leads to reduced yields and, in rare cases, can cause damage through hot spots . This shadowing is not deliberate. Transparent solar cells can also be installed for shading in conservatories in addition to energy production. In this case the shading is intentional.
Shunt Controller — A 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 Controller — Type of a 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 — A commercial method of making purified silicon.
Silicon (Si) — 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's commonly found in sand and quartz (as the oxide).
Chemical element capable to bond with four neighbouring atoms. Silicon is the semiconductor which has been the most important for the semiconductor industry and photovoltaics so far. The raw material silicon oxide (sand) can be processed into monocrystalline, multicrystalline or amorphous silicon. See also monocrystalline silicon , multicrystalline silicon .
Silicon wafer — Silicon disc for the production of computer chips and photovoltaic cells .
Sine Wave — 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 — An inverter that produces utility-quality, sine wave power forms.
Single-Crystal Material — A material that is composed of a single crystal or a few large crystals.
Single-Crystal Silicon — Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.
Solar Cell — see photovoltaic (PV) cell.
Solar Constant — 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 — 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 — Electromagnetic energy transmitted from the sun (solar radiation). 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 Insolation — See insolation.
Solar Irradiance — See irradiance.
Solar Noon — The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky; equal to true or due, geographic south.
Solar Panel — See photovoltaic (PV) panel.
Solar Spectrum — 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 percent 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 — 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.
Space Charge — See cell barrier.
Specific Gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.
Spinning Reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.
Split-Spectrum Cell — 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 mulitjunction device.
Sputtering — A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.
Square Wave Inverter — 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.
Stand-Alone Applications — An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.
Standard Reporting Conditions (SRC) — 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.
Storage Battery — 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 — 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.
Sulfation — 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) — SMES technology 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 — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.
Superstrate — 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 Capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.
System Storage — See battery capacity.
Tare Loss — Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.
Temperature coefficient — Indicates how much the efficiency of a solar cell is reduced if the temperature is raised by 1°C. Since crystalline solar cells have a relatively high temperature coefficient, modules consisting of crystalline solar cells should always be sufficiently back-ventilated.
Temperature Compensation — 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°C from ambient temperature.
Temperature Factors — It 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.
Thermophotovoltaic Cell (TPV) — 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 — Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.
Tilt Angle — The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.
Total Harmonic Distortion — The measure of closeness in shape between a waveform and it's fundamental component.
Total Internal Reflection — 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 Array — 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) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.
Tray Cable (TC) - may be used for interconnecting balance-of-systems.
Two-Axis Tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).
Tunneling — Quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.
Ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.
Underground 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.
Uninterruptible Power Supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.
Utility-Interactive Inverter — 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.
Valence Level Energy/Valence State — Energy content of an electron in orbit about an atomic nucleus. Also called bound state.
Varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.
Vented Cell — A battery designed with a vent mechanism to expel gases generated during charging.
Vertical Multijunction (VMJ) Cell — 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. Also called a multiple junction cell. See multijunction device and split-spectrum cell.
Wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.
Watt — The rate of energy transfer equivalent to one ampere under an 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) — Measuring unit for the standard performance (power rating, or wattage rating) of a photovoltaic cell or a photovoltaic module under standard test conditions (STC ). Module prices are typically indicated in €/Wp. 1,000 watts peak = 1 kilowatt peak
Waveform — The shape of the phase power at a certain frequency and amplitude.
Window — 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.
Zenith Angle — the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).