Glossary - Solar Energy & Photovoltaics

This is the most comprehensive glossary of terms related to solar photovoltaic energy online. If you see any errors or would like to see anything added to this list, please let us know.

Absorption factor n. See absorption coefficient.

Absorption coefficient n. The fraction of incident solar irradiance that is absorbed by a photovoltaic cell. This in turn contributes significantly to the cell temperature under operational conditions. Light with a shorter wavelength (and more energy) has a large absorption coefficient and light with a longer wavelength (and less energy) has a low absorption coefficient.

Absorption depth n. The distance into a material at which light drops to ~36.79% (or more precisely, 1/e) of its original intensity. It is the inverse of the absorption coefficient. Light with a shorter wavelength (and more energy) has a shorter absorption depth and light with a longer wavelength (and less energy) has a longer absorption coefficient.

Absorption factor n. See absorption coefficient.

Actual battery capacity n. Depending on how a battery’s control electronics are programmed, the actual battery capacity varies from nominal capacity.

Air mass (AM) n. The length which light travels through the atmosphere to reach a solar cell divided by the shortest possible length for light to travel (i.e. when the sun is directly overhead). It can be expressed as 1 divided by the cosine of the zenith angle.

Alternating current n. An electric current that reverses its direction periodically, typically at 50 or 60 hertz. In electric applications it usually has a sinusoidal waveform.

Amorphous silicon (a-Si) n. The non-crystalline allotrope of silicon. It is deposited as a thin film to produce a-Si solar cells.

Amortization period n. The period of time it takes for a solar installation to pay back its initial cost. Subsequent to this period, the installation generates profit.

Ampere (A) n. The SI (International System of Units) base unit of electric current. A current of one ampere is equivalent to one coulomb of electric charge going past a given point per second.

Ampere hour (Ah) n. A unit of electric charge equal to 3,600 coulombs.

Angle of incidence n. The angle between the sun’s rays and a line perpendicular to a given solar module’s surface. Thus, a module that faces directly toward the sun has an angle of incidence of 0° and is capturing the most possible energy.

Anode n. An electrode in a photovoltaic cell through which electric current flows into the cell, meaning it is the negative terminal. Thus, the electrons flow out of the anode because electric current is, by convention, opposite to the direction of electron flow.

Anti-reflective coating (ARC) n. A thin layer of dielectric material applied to the surface of a photovoltaic cell to minimize reflection of electromagnetic energy.

Array n. A group of multiple photovoltaic modules, usually connected to the same inverter.

Atomic layer deposition (ALD) n. A thin film deposition technique used in the production of thin film photovoltaic cells. Most ALD reactions use two chemicals, called precursors, which react with a surface to deposit a highly conformal, self-limiting, nanometer-thick thin film.

Azimuth n. The angle between the point on the horizon directly beneath the sun and true south.

Back contact solar cell n. A cell in which all or part of the front contact grids have been moved to the rear of the device to achieve higher efficiency.

Back surface field (BSF) n. A highly doped region at the rear surface of a solar cell that passivates the rear surface, thus minimizing the impact of rear surface recombination.

Balance of System (BOS) n. All the components of a photovoltaic installation including modules, wiring, switches, mounting racks, inverters, and batteries.

Band gap n. The minimum change in energy required to excite an electron that is stuck in its bound state in the valence band of a semiconductor into a free state where it can jump to the conduction band.

Base load n. The electricity generation resources on a given grid that operate continuously without downtime. Base load power plants only stop for maintenance or unexpected outages. Also see peak load.

Battery capacity n. The amount of electric charge a battery is able to deliver at its rated voltage.

Battery management system (BMS) n. Manages and optimizes the way a battery is used in a PV system. A BMS may also attempt to extend the life of the battery by preventing unfavorable operating conditions such as deep discharge and overcharge.

Bifacial solar cell n. A double-sided solar cell.

Bi-directional inverter n. An inverter that functions in both directions. Thus, it can draw AC power from the grid and feed it as DC power into the battery, on top of its regular function of converting DC power from the PV modules or battery into AC power.

Blocking diode n. A diode wired in series with PV modules and a battery. It allows energy to pass from the modules into the battery, but prevents energy from the battery flowing back out into the modules.

Building-integrated photovoltaics (BIPV) n. Photovoltaic panels mounted on a building in place of conventional building materials, such as the facade, windows or roof of the building. BIPV replaces the building materials and is not merely mounted on top of them, like it is in rooftop applications.

Busbars n. Strips of conductive metal that run along PV cells to carry electric charge.

Bypass diode n. A diode that allows current to pass around one or more shaded cells if they are receiving less sunlight than other cells, thus preventing hot spots from occurring.

C-rate n. A measure of how quickly a battery, with regards to its capacity, is discharged. A C-rate of 1C means the battery discharges completely within one hour. The energy available to be drawn is also dependent on the C-rate.

Cadmium telluride (CdTe) n. A crystalline compound formed from cadmium and tellurium used to make thin film photovoltaic cells.

Calendar life n. The time an inactive battery can be stored before it reaches a critical balance capacity.

Cathode n. An electrode in a photovoltaic cell through which electric current flows out of the cell, meaning it is the positive terminal. Thus, the electrons flow into the cathode because electric current is, by convention, opposite to the direction of electron flow.

Cell n. A photoelectric cell which generates an electric current when exposed to light, usually from the sun. Cells are connected and packaged into a solar photovoltaic module.

Cell efficiency n. The percentage of incident energy in the form of sunlight reaching a solar cell that is converted into electrical output.

Charge controller n. Limits the rate at which electric current is fed to or drawn from a battery or storage system, in order to prevent overcharging and overvoltage, which may damage a battery or reduce its lifespan.

Concentrated photovoltaics (CPV) n. The use of lenses or curved mirrors to concentrate a large amount of sunlight onto a smaller area of photovoltaic cells.

Concentrated solar power (CSP) n. The use of lenses or curved mirrors to concentrate a large amount of sunlight onto a smaller area, where it is converted to heat and used to drive a steam turbine to generate electricity.

Copper indium (di)selenide (CIS) n. See Copper indium gallium (di)selenide (CIGS).

Copper indium gallium (di)selenide (CIGS) n. A semiconductor material comprising copper, indium, gallium and selenium that is used to make thin film solar photovoltaic cells.

Crystalline silicon (c-Si) n. Silicon in crystalline form. This includes monocrystalline silicon and polycrystalline silicon.

Cross-linking n. The use of cross-links (bonds that link one polymer chain to another) in encapsulants in order to alter the properties of the material.

Crucible n. A container made of ceramic or metal in which metals or other substances are melted.

Curie point n. Also known as the Curie temperature. It is the temperature where a material’s permanent magnetism changes to induced magnetism.

Czochralski process n. A method of growing single crystals used to produce monocrystalline silicon.

Copper zinc tin sulfide (CZTS) n. A semiconducting compound which can be used as the absorber layer in a thin film solar cell. Unlike CIGS and CdTe solar cells, which contain cadmium, CZTS is made up of non-toxic elements.

Damp-heat test n. A test developed by the International Electrotechnical Commission (IEC) that determines how much a solar module’s efficiency degrades subsequent to extended periods of heat and humidity. The most recent version of the test (EN 60068-2-78:2013) specifies that a module should be subjected to 1,000 hours of 85⁰C and 85% relative humidity (RH). Some testing bodies, such as TÜV Rheinland, conduct more rigorous “IEC-like” tests, which last for 2,000 hours instead of 1,000 hours, to test for greater durability.

Degradation n. The process of a solar module losing efficiency as it is exposed to the elements over time. Common contributors to degradation include reduced adherence of contacts or corrosion due to water vapor; metal migration through the p-n junction; and deterioration of the anti-reflection coating.

Depth of discharge (DOD) n. The battery capacity that has been discharged, expressed as a percentage of maximum capacity. A discharge to at least 80% is termed as a deep discharge.

Diamond wire n. Wire impregnated with diamond dust used to cut wafers. Diamond wire saws can significantly lower wafer thickness and minimize wasted silicon dust.

Diffuse radiation n. Solar radiation (i.e. sunlight) that has been scattered by particles in the Earth’s atmosphere. On overcast days with significant cloud cover, direct sunlight is reduced meaning that most visible light is diffuse radiation. Some solar modules perform better than others in diffuse light conditions. Also see direct radiation and reflected radiation.

Diffusion n. The scattering of sunlight by particles in the Earth’s atmosphere.

Diode n. A semiconductor device with two terminals, which allows current to flow in one direction only. The anode is the electrode through which current flows into the diode, and the cathode is the electrode through which current flows out of the diode. Note that electron flow is, by convention, opposite to the electric current, so electrons flow from the anode toward the cathode.

Direct current (DC) n. An electric current flowing in one direction only.

Direct radiation n. Solar radiation (i.e. sunlight) that hits the earth without being scattered by clouds or particles in the Earth’s atmosphere. Also see diffuse radiation.

Dopant n. A substance, such as boron or phosphorus, added to a semiconductor material so as to alter its conductive properties.

Dual-axis tracking n. Tracking that tilts on both the up-down axis and left-right axis. Also see tracking.

Dye-sensitized solar cell (DSSC) n. A thin film solar cell with a photo-sensitized anode, usually titanium oxide (TiO2) or zinc oxide (ZnO) covered with an organic sensitizing dye.

Economic payback n. The time taken for a solar photovoltaic system to have paid off all its system costs with the income it has generated. Subsequent to the economic payback period, the system is then generating profits for its owner.

Efficiency of a battery n. The product of the coulombic efficiency and the voltage efficiency of a battery. The coulombic efficiency is the amount of charge that enters the battery during the charging cycle divided by the amount of charge that exits the battery during the discharge cycle. The voltage efficiency is the voltage during charging divided by the voltage during discharging.

Electrolyte n. A chemical compound that ionizes when dissolved or molten to produce an electrically conductive liquid or gel.

Emitter wrap through (EWT) n. A high-efficiency back-contact solar cell technology, in which the metallization for the bus bars and fingers is moved to the back of the solar cell. Also see metallization wrap through.

Encapsulant n. A material used for encapsulating (i.e. enclosing) a solar cell. Often different encapsulants are used for the front and back side of a solar cell.

Energy density (Wh/kg or Wh/l) n. The amount of energy stored in a system or region of space per unit of mass or volume.

Energy payback n. The time it will take for a solar photovoltaic system to generate an equal amount of energy to that which was used in its construction and mounting. Subsequent to the energy payback period, the system is generating excess electricity for its owner.

Equalization n. An overcharge performed on a flooded lead acid battery that is fully charged, in order to remove sulfate crystal build-up on the plates and reverse various other negative chemical effects. Various manufacturers recommend equalization anywhere from monthly to yearly.

Ethylene vinyl acetate (EVA) n. The copolymer of ethylene and vinyl acetate, often used as an encapsulant due to its strong adhesive properties.

Export tariff n. An added payment for electricity that is exported to the grid, rather than consumed on-site.

Feed-in tariff (FIT) n. An economic policy designed to promote active investment in renewable energy technologies. Typically a FIT comprises long-term contracts that guarantee a certain price for energy from renewable sources. Often they are different depending on the type of renewable energy technology utilized.

Float service n. Charging energy into a battery at the same rate as its self-discharge rate, hence keeping it in a fully charged state. Also called float charge or trickle charge.

Flywheel n. A form of energy storage that utilizes a spinning wheel. Advanced flywheel systems have high strength rotors suspended by magnetic bearings in a vacuum enclosure.

Gallium arsenide (GaAs) n. A compound of gallium and arsenic used to make single crystal thin film solar cells.

Generation capacity n. The maximum rated output of all the inverters in a power plant. Also see nameplate capacity.

German renewable energy act (EEG) n. A German law designed to stimulate renewable energy development that came into effect in the year 2000. Prior to its implementation, there was a total of 30 MWp of solar photovoltaics capacity on the German grid, which has since climbed to 32,400 MWp by the end of 2012. In just seven years, small-scale system prices in Germany fell from €5,100/kWp (Q2 2006) to €1,698/kWp (Q2 2013), shaving off a full two thirds of their cost.

Gigawatt (GW) n. A unit that is equal to one billion watts. See watt.

Gigawatt hour (GWh) n. A unit that is equal to one billion watt hours. See watt hour.

Gigawatt peak (GWp) n. A unit that is equal to one billion watts peak. See watt peak.

Gravimetric energy density (Wh/kg) n. The ratio of the storage capacity of a battery or cell to its weight.

Greenhouse effect n. The trapping of solar infrared radiation in the lower portions of the Earth’s atmosphere.

Grid n. An interconnected electricity network for delivering electricity to consumers.

Grid-connected adj. Used to describe a photovoltaic system that is connected to the grid, as opposed to an off-grid system.

Grid-tied adj. See grid-connected.

Hermetic seal n. An enclosure that is airtight.

High concentration photovoltaics (HCPV) n. Concentrated photovoltaics that concentrate sunlight to intensities of 1,000 or more suns. See concentrated photovoltaics (CPV).

Hot spot n. A region in a photovoltaic module with a higher temperature than the surrounding temperature. More specifically, it occurs when one or more cells generates less current than the other cells in a series of cells, due to partial shading, cell damage or interconnection failure. Consequently, this portion of the series becomes reverse biased and dissipates power as heat. This phenomenon is mitigated with the use of a bypass diode.

Hybrid system n. A system that combines more than one type of energy generation (e.g. solar and wind).

Hydrogen fuel cell n. An electrochemical cell in which liquid hydrogen reacts with oxygen to generate electricity.

Indium tin oxide (ITO) n. A solid solution of indium(III) oxide (In2O3) and tin(IV) oxide (SnO2). It is often used as a transparent conducting oxide (TCO) in thin film solar applications.

Ingot n. A metal cast into a given shape that is suited to further processing.

Inverter n. An electrical power converter that takes direct current (DC) as an input and outputs alternating current (AC).

In-line diffusion n. A diffusion process that is carried out in an in-line continuous furnace on the production line, rather than in a batch tube furnace.

Insolation n. The amount of solar radiation falling upon a given area, usually measured in watt hours per square meter (Wh/m²).

Ion n. An atom or molecule that has gained or lost one or more valence electrons, giving it a net positive or negative electrical charge.

Irradiance n. The power of electromagnetic radiation per unit area falling on a given surface, usually measure in watts per square meter (W/m²).

Irradiation n. See insolation.

Jawaharlal Nehru National Solar Mission (JNNSM) n. An Indian scheme enacted on January 11, 2010, with the target of deploying 20 gigawatts of grid-connected solar power by 2022 in India.

Kilowatt (kW) n. A unit that is equal to one thousand watts. See watt.

Kilowatt hour (kWh) n. A unit that is equal to one billion watt hours. See watt hour.

Kilowatt peak (kWp) n. A unit that is equal to one billion watts peak. See watt peak.

Laminate n. Flexible solar panels that can be bonded directly to a surface such as a roof.

Large-scale adj. Used to describe a solar photovoltaic installation above a specified capacity, which varies depending on the market. For example, the Australian Capital Territory designates systems above 200 kW as large-scale, whereas large-scale systems are classified as greater than 5 MW in Victoria, Australia.

Levelized cost of energy (LCOE) n. The price at which electricity generated from a given source must be sold in order for the project to break even over its lifetime. This factors in all the costs over a project’s lifetime and makes it possible to compare the cost of different energy generation technologies.

Light trapping adj. Used to describe a solar cell in which light is “trapped” and bounces back and forth many times. This allows thinner cells to still derive energy from longer wavelength light.

Lithium-ion battery n. A rechargeable battery in which lithium ions move from the anode to the cathode during discharge and back during charge.

Load simulation n. A process carried out by a computer program that models a PV system and a building’s energy requirements, so as to optimize the size of the system.

Low-light conditions n. Conditions in which there is little solar radiation. These conditions are also simulated in photovoltaic module testing procedures, such as PV+Test.

Maximum continuous discharge current n. The maximum current at which a battery can be discharged continuously. This is usually defined by the manufacturer to prevent excessive discharge rates that can damage the battery or reduce its capacity.

Maximum power point tracking (MPPT) n. The process of optimizing the power output of a solar photovoltaic system to work most effectively with a storage system or grid, via the use of a DC to DC converter.

Maximum power point tracking charge controller n. A high frequency DC to DC converter that optimizes the current and voltage output of a photovoltaic system to maximize the current, and thus power, fed into the battery. Also see maximum power point tracking.

Megawatt n. A unit that is equal to one million watts. See watt.

Megawatt hour (MWh) n. A unit that is equal to one million watt hours. See watt hour.

Megawatt peak (MWp) n. A unit that is equal to one billion watts peak. See watt peak.

Metallization n. The process of coating a layer of metal onto a non-metallic surface.

Metallization wrap through (MWT) n. A high-efficiency back-contact solar cell technology, in which the metallization for the bus bars is moved to the back of the solar cell. Also see emitter wrap through.

Module n. A packaged, interconnected assembly of solar cells.

Module efficiency n. The efficiency at which a module converts incident sunlight into electric energy. For example, if 100 watts of sunlight hits a module’s surface and it outputs 15.6 watts of electric energy, then it has a module efficiency of 15.6%. Note that a module’s efficiency is always slightly lower than the efficiency of each of the cells that make it up, mainly due to the unused spaces between cells.

Molten salt battery n. A electric battery, which uses high temperature molten salt as an electrolyte.

Molybdenum n. A chemical element with atomic number 42 and chemical symbol Mo, which is used in CIGS solar cell fabrication.

Mono-like adj. Describes a type of crystalline silicon ingot used to manufacture solar cells, which is largely monocrystalline, although with some polycrystalline regions. The technology was first developed by BP in 2006 and produces cells with higher efficiencies than polycrystalline silicon at lower costs than monocrystalline silicon.

Monocrystalline silicon (mono-Si) n. Silicon in a crystalline lattice that is completely unbroken and continuous to its edges.

Monosilicon n. See monocrystalline silicon.

Mounting system n. The components that fasten solar modules in place, whether it be on a roof, carport, facade or the ground. This includes mounting racks, cross beams, fasteners, clamps and any other components which function solely to mount solar photovoltaic modules.

Multicrystalline silicon n. See polycrystalline silicon.

n-type cell n. A solar cell made from the rarer n-type silicon instead of the more dominant p-type silicon. Both types of cell are derived from a silicon feedstock, although during crystallization phosphorus is used as a dopant to produce n-type silicon crystals, whereas boron is usually used to produce p-type silicon crystals. The advantages of n-type cells are that they do not suffer from light-induced degradation (LID) and they are less sensitive to impurities present in the silicon feedstock.

Nameplate capacity n. The total rated power output of all photovoltaic panels in a power plant. This is the theoretical maximum output of the panels in a power plant, but in practice this is rarely, if ever, reached. Also see generation capacity.

Net metering n. A government policy in various electricity markets that allows generators of renewable energy to feed excess generated electricity into the grid for a price. Most net metering policies reimburse the generator with either the retail rate, a fraction of the retail rate, or compensation at avoided cost.

Nominal battery capacity n. Amount of electrical charge that can be stored without taking into account that it should not be discharged to 100%.

Nominal voltage n. The reference voltage of a battery.

Off-grid adj. Used to describe a renewable electricity generating system which is not connected to the electricity grid.

On-grid adj. Used to describe a renewable electricity generating system which is connected to the electricity grid and can feed electricity into the grid.

One-axis tracking n. See single-axis tracking.

Open circuit voltage (VOC) n. The difference in electrical potential between the two terminals of a battery or solar cell when it is not connected to an external load. In the case of a solar cell, it is the maximum voltage available, which occurs at zero current.

Organic solar cell n. Any solar cell that is made from organic (i.e. carbon-based) polymers. Also see dye-sensitized solar cell (DSSC).

p-n junction n. The boundary between a p-type semiconductor material and an n-type semiconductor material. Electrons flow from the n-type side to the p-type side and holes flow from the p-type side to the n-type side, because the n-type material has a high electron concentration and the p-type material has a high hole concentration.

p-type cell n. A solar cell made from the more dominant p-type silicon instead of the rarer n-type silicon. Both types of cell are derived from a silicon feedstock, although during crystallization boron is usually used to produce p-type silicon crystals, whereas phosphorus is used as a dopant to produce n-type silicon crystals. The advantages of p-type cells are that they are less sensitive to cosmic rays, which meant they were much better suited toward space applications such as satellites. Thus, p-type solar cells have become the dominant technology in the market. Also see n-type cell.

Panel n. See module.

Panel efficiency n. See module efficiency.

Passivation n. The process of adding a passivating surface layer to a solar cell, such as silicon nitride, silicon dioxide, aluminum oxide, or titanium dioxide. This reduces surface recombination and enhances the cell’s efficiency by prolonging the effective lifetime of the charge-carrier. Also see surface recombination.

Payback time n. This term is used in the context of solar photovoltaics to refer to either the time taken for the energy payback or the time taken for the economic payback of a given solar system. See energy payback and economic payback.

Peak load n. The electricity generation resources on a given grid that operate intermittently. These include resources such as wind and solar energy that are unpredictable, and peaking power plants that are only run at times of high demand, such as during summer afternoons when the use of air conditioning increases.

Peak shaving n. The process of shifting electricity consumption from periods of peak demand, when the electricity prices are also at their peak, to off-peak periods in order to consume the same amount of electricity for a lower cost. This can be achieved by scheduling certain energy-intensive devices, such as dishwashers, washing machines or dryers, to run in off-peak periods.

Phase shifting transformer n. A device that controls the power flow through various lines in an electrical grid in order to prevent overload. It does this by inserting a voltage with a selected phase angle into the power system, which can redirect power to more desirable pathways that are less at risk of overload.

Photovoltaics (PV) n. The process of converting solar radiation into electricity by utilizing semiconductors that exhibit the photovoltaic effect.

Polycrystalline silicon (poly-Si) n. A material made up of small silicon crystals. Unlike monocrystalline silicon, it is not continuous and unbroken.

Polymer solar cell n. A flexible thin film solar cell made from polymers that exhibit the photovoltaic effect. Organic solar cells are examples of polymer solar cells, although inorganic polymers are also used.

Polysilicon n. See polycrystalline silicon (poly-Si).

Power purchase agreement (PPA) n. A contract between an electricity generator and an electricity purchaser that lays out the terms of what is expected of each party, including payment terms and how, when and how much electricity will be delivered to the purchaser. Like any contract, a power purchase agreement can be structured in many ways depending on the needs of the generator and purchaser of the electricity and the structure of the market within which they operate.

Power conversion system (PCS) n. A system used to convert electric energy from one form to another, either by converting from direct current to alternating current, altering the voltage or frequency, or incorporating a number of these processes. Examples of power conversion systems include DC to DC converters, inverters and voltage stabilizers.

Power management system (PMS) n. A management platform that governs energy distribution in a photovoltaic system, which may also include energy storage and can be on or off-grid. A power management system controls when energy is sent to or drawn from the battery, how energy is harvested from the photovoltaic system, and when energy is sent to or drawn from the grid.

Power-to-weight ratio n. See specific power.

Pump storage n. A form of hydroelectric energy storage also known as pumped storage, which is used for load balancing. It uses electricity in low-cost off-peak periods to pump water up to a higher elevation and then during peak periods of consumption, this water is used to generate hydroelectricity.

Pyranometer n. An instrument used to measure hemispherical solar radiation on a given plane.

Pyrheliometer n. An instrument used to measure solar direct normal irradiance (DNI) on a given plane.

Quasi-mono adj. See mono-like.

Rear contact solar cell n. See back contact solar cell.

Reactive power n. The power in an alternating current electric network that is temporarily stored in inductors and capacitors and returned back to the network, also known as volt-ampere reactive (VAR) power. Reactive power strongly influences voltage levels across the entire electric network.

Recombination n. The process whereby an excited electron recombines with a hole and the energy is lost to another form of energy than electric energy. There are three types of recombination: radiative recombination, Auger recombination and Shockley-Read-Hall recombination.

Resistance (Ω) n. The opposition of a material, usually a conductor, to the flow of electric current. Resistance is measure in ohms (Ω).

Reflected radiation n. Solar radiation that has reflected off non-atmospheric things, such as trees, terrain, buildings or the ground. Also see direct radiation and diffuse radiation.

Renewable energy credit (REC) n. A tax credit that incentivizes renewable energy uptake, also known as a renewable energy certificate (REC) in numerous U.S. states. In most markets, one renewable energy credit certifies that the holder produced one megawatt hour (MWh) of renewable electricity.

Residential system n. A solar photovoltaic system installed on a private residence. This term is also often used interchangeably with “small-scale system.”

Roll-to-roll process n. The process of printing thin film solar photovoltaic cells on a flexible roll of plastic or metal foil.

Screen-printing n. A process involved in manufacturing wafer-based solar photovoltaic cells. Screen printing is currently the dominant method of solar cell fabrication, due to its relative simplicity compared with other processes.

Self-discharge (%/month) n. The amount at which internal chemical reactions reduce the stored charge of a battery without any connection between the electrodes.

Selective emitter n. A thin layer on the front of a solar cell that only allows light of certain wavelengths through.

Semiconductor n. A material that has an electrical conductivity between that of an insulator and that of a metal. Solar photovoltaic cells utilize semiconductors that exhibit the photovoltaic effect, meaning they generate electric current when exposed to light.

Shading n. The shadows that fall upon a given solar photovoltaic system from surrounding objects such as trees or buildings.

Shadowing n. See shading.

Single-axis tracking n. Tracking on one axis to tilt a solar array toward the sun, so as to maximize solar irradiation and hence energy output. Single-axis trackers can be oriented on a horizontal, vertical or tilted axis, depending on the application.

Silicon carbide (SiC) n. A hard crystalline compound of silicon and carbon. It only ever occurs naturally on Earth as moissanite, which is found on meteorites, so it cannot be mined and must be manufactured in a furnace. Silicon carbide is used to make solar photovoltaic modules suited for high voltage, high temperature industrial applications.

Small-scale adj. Describes a solar photovoltaic system that is small in its rated power output. The maximum allowable size to classify a system as small-scale varies from market to market, although generally the maximum is somewhere between 2 and 10 kWp.

Solar thermal n. A technology that harnesses solar energy in the form of both heat and light to be used as thermal energy. See solar thermal collector.

Solar thermal collector n. A device that absorbs solar energy in the form of both heat and light to be used as thermal energy. The U.S. Energy Information Administration categorizes solar thermal collectors as being low, medium or high temperature collectors. Low and medium temperature collectors are generally used in heating applications for residential and small commercial applications. High temperature collectors are usually concentrated solar power plants that generate electricity.

Soldering n. The process of melting a filler metal and using it to join two or more metal items with higher melting points.

Specific power (W/kg) n. The ratio of the power output of a given solar photovoltaic module to its mass, measured in watts per kilogram.

Sputtering n. The process of ejecting atoms from a target by bombarding it with highly energized particles. Sputtering is used in photovoltaics manufacturing in both etching and film deposition.

Standard Test Conditions (STC) n. An industry standard for measuring the DC power output of solar modules. The module is placed in a flash tester with a temperature of 25⁰C, an air mass of 1.5 and the equivalent of 1,000 W/m2 of sunlight intensity.

State of charge (SOC) n. The present battery capacity expressed as a percentage of the maximum capacity.

Stand-alone system n. An off-grid system, usually in an area which is not services by any electrical grid. See off-grid.

String n. A group of solar modules connected in series in order to obtain a higher voltage. A number of strings are then connected in parallel in order to generate a greater current.

Substrate n. A material, usually an insulator, onto which a coating is applied in the production of thin film solar cells. The substrate can be flexible or rigid depending on the type of cell being manufactured.

Sulfation n. The reaction of lead and lead dioxide with sulfuric acid in a lead-acid battery, producing lead sulfate, which reduces the battery's capacity and performance. Sulfation is most common in batteries left discharged for long periods and can be mitigated by recharging a battery as soon as possible after a discharge cycle.

Surface recombination n. The loss of energy at the surface of a photovoltaic cell due to dangling silicon bonds. Surface recombination can be reduced by adding a passivating layer.

Temperature coefficient n. The rate of change of a solar module's power output as a function of its operating temperature. As a module's temperature rises, its efficiency diminishes due to the drop in open circuit voltage. This effect is more pronounced in silicon-based solar cells than thin film-based solar cells.

Temperature cycling n. See thermal cycling.

Thermocouple n. A device that measures temperature. It is usually lodged between the cell and its encapsulant to track the thermal gradient that is generated at this material transition.

Thermal cycling n. A testing process which involves subjecting a solar photovoltaic module to rotating hot and cold cycles in order to test its durability under normal climatic conditions. The current international standard test (IEC 61215) defines five major visual defects that may arise as a result of thermal cycling: broken, cracked or torn external surfaces; bent or misaligned external surfaces; a crack in a cell that could remove more than 10% of that cell's area from the electrical circuit of the module; bubbles or delaminations between the electrical circuit and the module edge; and loss of mechanical integrity.

Throughput n. The average rate of successful production of a given component in the manufacturing of solar photovoltaics. For example, a tabber & stringer system may have a throughput of 1,200 soldered solar cells per hour.

Tracker n. A device that tilts a solar array over the course of the day, so as to orient it toward the sun and maximize energy yield. Also see single-axis tracking and dual-axis tracking.

Transformer substation n. A large transformer for either stepping up or stepping down the voltage in an electric grid.

Tracking array n. See tracker.

Thin film n. A thin layer of photovoltaic material (for example, amorphous silicon or cadmium telluride) that is then deposited onto a substrate or wafer. In terms of market share, thin film solar cells are the main competitors to the much more dominant crystalline silicon solar cells.

Tilt n. The degree of the angle by which a module is tilted relative to a horizontal plane parallel to the earth's surface. In general, PV arrays should be tilted toward the average elevation of the sun for a given latitude. Fixed arrays are non-adjustable and must be installed at the optimum position for year-round production, whereas adjustable mounting systems allow system operators to adjust the tilt for various seasons as the sun's angle changes.

Transparent conductive oxide (TCO) n. A doped metal oxide used in the production of thin film solar cells.

Two-axis tracking n. See dual-axis tracking.

Utility-scale n. See large-scale.

Voltage drop n. The loss of electrical potential (i.e. voltage) that occurs across the conductive elements of an electrical system, resulting in loss of electrical energy. Voltage drop can also cause inverters to function less effectively or stop functioning altogether.

Wafer n. A thin slice of monocrystalline silicon used to manufacture solar cells.

Watt (W) n. A derived unit in the International System of Units (SI) for measuring power. One watt is the rate at which work is done when one ampere (A) of current flows through an electric potential difference of one volt (V). Also see gigawatt, megawatt, and kilowatt.

Watt hour (Wh) n. A unit of energy that is equal to one watt of power expended for one hour of time. Also see gigawatt hour, megawatt hour, and kilowatt hour.

Watt peak (Wp) n. A measure of the maximum power output of a photovoltaic device or system under standard test conditions (STC). In practice, these nameplate capacities are rarely reached due to less than ideal conditions, module degradation, imperfect alignment of the module in tilt or azimuth, and a less than ideal temperature. Also see nameplate capacity.

Wire saw n. A saw used in the sawing of polycrystalline or monocrystalline silicon from ingots into squared blocks and then wafers.

Zenith angle n. The angle between the centre of the sun and a line that is perpendicular to a horizontal plane at the earth's surface. Thus, if the zenith angle is 0⁰ then the sun is directly overhead and if it is 90⁰ then the sun will be on the horizon and soon after sunrise or soon before sunset.

Zinc oxide (ZnO) n. A wide bandgap semiconductor used in the manufacturing of hybrid organic photovoltaics (OPV). See organic solar cell.