Solar Energy Glossary

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Absorbers - Dark-coloured objects that soak up heat in thermal solar collectors.

Active solar heater - A solar water or space-heating system that moves heated air or water using pumps or fans.

AGM - Absorbed Glass Mat, a newer type of battery construction that uses saturated absorbent glass mats rather than gelled or liquid electrolyte. Somewhat more expensive than flooded (liquid), but offers very good reliability.

Alternating Current - Electric current in which the direction of flow is reversed at frequent intervals--usually 100 or 120 times per second (50 or 60 cycles per second or 50//60 Hz).

Amorphous semiconductor - A non-crystalline semiconductor material. Easier and cheaper to make than crystalline, but less efficient and slowly degrades over time. Also called thin film.

Ampere (A) - The unit for the electric current; the flow of electrons. One amp is 1 coulomb passing in one second. One amp is produced by an electric force of 1 volt acting across a resistance of 1 ohm.

Ampere-Hour (AH) - Quantity of electricity or measure of charge. How many amps flow or can be provided over a one hour period. Most batteries are rated in AH.

Angle of incidence - Angle between the normal to a surface and the direction of incident radiation; applies to the aperture plane of a solar collector. Most modern solar panels have only minor reductions in power output within plus/minus 15 degrees. The loss is a function of the cosine, so at 45 degree angle, output drops off by about 30%.

Anti reflection coating - A thin coating of a material, which reduces the light reflection and increases light transmission, applied to a photovoltaic cell surface.

Array - Any number of photovoltaic modules connected together to provide a single electrical output. Arrays are often designed to produce significant amounts of electricity.

Autonomous system - A stand-alone PV system that has no back-up generating source. May or may not include storage batteries. Most battery systems are designed for a certain minimum "days of autonomy" - which means that the batteries can supply sufficient power with no sunlight to charge the batteries. This varies from 3-5 days in the Sunbelt, to 5 to 10 days elsewhere.

AWG - American Wire Gauge, a standard system for designating the size of electrical wire. The higher the number, the smaller the wire. Most house wiring is #12 or 14. In most other countries, wire is specified by the size in millimeters.

Azimuth - Angle between the north direction and the projection of the surface normal into the horizontal plane; measured clockwise from north. As applied to the PV array, 180 degree azimuth means the array faces due south.


Back contact solar cell - 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) - A highly doped region at the rear surface of a solar cell that passivated the rear surface, thus minimizing the impact of rear surface recombination.

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

Band gap - 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 - The electricity generation resources on a given grid that operate continuously without downtime. Base load power plants only stop for maintenance or unexpected outages.

Battery capacity - The amount of electric charge a battery is able to deliver at its rated 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 cycle life — The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria. Battery energy capacity — 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 levelling at substations, and peak shaving on the customer side of the meter.

Battery Management System (BMS) - 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 unfavourable operating conditions such as deep discharge and overcharge.

Biracial solar cell - A double-sided solar cell.

Bi-directional inverter - 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 - 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) - Photovoltaic panels mounted on a building in place of conventional building materials, such as the façade, 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.

Bus bars - Strips of conductive metal that run along PV cells to carry electric charge.

Bypass diode - 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.


Cathodic protection - A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.

Cell - The basic unit of a photovoltaic panel or battery

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, cell junction, or space charge.

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

Charge controller - An electronic device which regulates the voltage applied to the battery system from the PV array. Essential for ensuring that batteries obtain maximum state of charge and longest life.

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 vapour 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.

Cloud enhancement — The increase in solar intensity caused by reflected radiance from nearby clouds.

Combined collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.

Concentrating photo voltaic (CPV) — A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.

Concentrating solar power (CSP) — A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.

Combined collector - A photovoltaic device or module that provides useful heat energy in addition to electricity.

Concentrator - A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Efficiency is increased, but lifespan is usually decreased due to the high heat.

Concentrator (module, array, or collector) - An arrangement of photovoltaic cells that includes a lens to concentrate sunlight onto small-area cells. Concentrators can increase the power flux of sunlight hundreds of times.

Conversion efficiency (cell or module) - The ratio of the electric energy produced by a photovoltaic device (under one-sun conditions) to the energy from sunlight incident upon the cell.

current at maximum power (Imp) - The current at which maximum power is available from a module. [UL 1703]

Cycle life - Number of discharge-charge cycles that a battery can tolerate under specified conditions before it fails to meet specified criteria as to performance (e.g., capacity decreases to 80-percent of the nominal capacity).

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.


DC to DC converter - Electronic circuit to convert DC voltages (e.g., PV module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker (MPPT).

Deep discharge - Discharging a battery to 20-percent or less of its full charge.

Diffuse isolation - Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct isolation.

direct current(dc) - Electric current in which electrons flow in one direction only. Opposite of alternating current.

Diode — An electronic device that allows current to flow in one direction only. See also blocking diode and bypass diode

Direct beam radiation — Radiation received by direct solar rays. Measured by a ellipsometer 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.

Direct isolation - Sunlight falling directly upon a collector. Opposite of diffuse isolation.

Discharge rate - The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

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.

DOD - 'Depth of Discharge,' from 100-percent state of charge (SOC), in a battery or battery 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 unopened semiconductor.

Donor level — The level that donates conduction electrons to the system.

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 — The addition of dopant to a semiconductor.

Downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.

Dry cell — A cell (battery) with a captive electrolyte. A primary battery that cannot be recharged.

Duty cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.

Duty rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.


Edge-defined film-fed growth (EFG) — A method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upward by capillary action through a mold.

Electric circuit - Path followed by electrons from a power source (generator or battery) through an external line (including devices that use the electricity) and returning through another line to the source.

Electric current - A flow of electrons; electricity, amps.

electrical grid - An integrated system of electricity distribution, usually covering a large area. As in "off the grid".

Electrolyte - A liquid conductor of electricity. In batteries, usually H2SO4, sulphuric acid, but may be any number of things. Seawater is the most common electrolyte in the world - and by suspending a zinc and a steel sheet in it, you can get a little electricity.

Energy - The ability to do work. Stored energy becomes working energy when we use it.

Energy density - The ratio of energy available from a battery to its volume (Wh/1) or mass (Wh/kg). "watts to weight" ratio.

Energy payback time - The time required for any energy producing system or device to produce as much energy as was required in its manufacture. For solar electric panels, this is about 16-20 months.

EVA (ETHYLENE VINYL ACETATE) - An encapsulate used between the glass cover and the solar cells in PV modules. It is durable, transparent, resistant to corrosion, and flame retardant.

Energy audit — A survey that shows how much energy used in a home, which helps find ways to use less energy.

Energy contribution potential — Recombination occurring in the emitter region of photovoltaic cell

Energy density — The ratio of available energy per pound; usually used to compare storage batteries.

Energy imbalance service — A market service that provides for the management of unscheduled deviations in individual generator output or load consumption.

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

Equalization charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

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

Equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox).

Exciton — A quasi-particle created in a semiconductor that is composed of an electron hole pair in a bound state. An exciton can be generated by and converted back into a photon

External quantum efficiency (external QE or EQE) — Quantum efficiency that includes the effect of optical losses, such as transmission through the cell and reflection of light away from the cell.

Extrinsic semiconductor — The product of doping a pure semiconductor.


Feed-in tariff (FIT) - 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.

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.

Fill factor — The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.

Float service - 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.

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

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

Float life - Number of years that a battery can keep its stated capacity when it is kept at float charge (see float charge).

Fuel cell - A device that converts the energy of a fuel directly to electricity and heat, without combustion. Because there is no combustion, fuel cells give off few emissions; because there are no moving parts, fuel cells are quiet.

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 single crystal.

Frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

Frequency regulation — This indicates the variability in the output frequency. Some load swill switch off or not operate properly if frequency variations exceed 1%.

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).


Gel-type battery - Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Grid-connected (PV system) - A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

Ground loop - An undesirable feedback condition caused by two or more circuits sharing a common electrical line, usually a grounded conductor.

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

Generation capacity - The maximum rated output of all the inverters in a power plant.

German renewable energy act (EEG) - 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 photovoltaic 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) - A unit that is equal to one billion watts. See watt.

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

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

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

Greenhouse effect - The trapping of solar infra-red radiation in the lower portions of the Earth’s atmosphere.

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

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


Hermetic seal - An enclosure that is airtight.

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

Hot spot - 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 - A system that combines more than one type of energy generation (e.g. solar and wind).

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

Indium tin oxide (ITO) - 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 - A metal cast into a given shape that is suited to further processing.

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

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

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

Incident light - Light that shines onto the face of a solar cell or module.

Interconnect - A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells. [UL 1703]

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 characterized cell performance.

I-V data - The relationship between current and voltage of a photovoltaic device in the power-producing quadrant, as a set of ordered pairs of current and voltage readings in a table, or as a curve plotted in a suitable coordinate system

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

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


Junction box - A 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.

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

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


Kilowatt (kW) - A unit that is equal to one thousand watts.

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

Kilowatt peak (kWp) - A unit that is equal to one billion watts peak.


light-induced defects - Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

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 DC to AC) is controlled by the power line, so that, if there is a failure in the power grid, the PV system cannot feed power into the line.

Load - Anything in an electrical circuit that, when the circuit is turned on, draws power from that circuit.

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

Load simulation - 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 - 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 - 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) - 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 - 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.

Megawatt - A unit that is equal to one million watts.

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

Megawatt peak (MWp) - A unit that is equal to one billion watts peak.

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

Metallization wrap through (MWT) - 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.

Module - A packaged, interconnected assembly of solar cells.

Module efficiency - 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.

Microgroove - A small groove scribed into the surface of a cell which is filled with metal for contacts.

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

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

Mono-like - Describes a type of crystalline silicon ingot used to manufacture solar cells, which is largely mono crystalline, 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 mono crystalline silicon.

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

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

Multi crystalline - Material that is solidified at such as rate that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged, whereas crystallites are jumbled together. These numerous grain boundaries reduce the device efficiency. A material composed of variously oriented, small individual crystals. (Sometimes referred to as polycrystalline or semi crystalline).

Multifunction device - A photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum, to achieve greater overall efficiency.


N-type cell - 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 - 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.

Net metering - 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 - Amount of electrical charge that can be stored without taking into account that it should not be discharged to 100%.

Nominal voltage - The reference voltage of a battery.

NOCT - Nominal Operating Cell Temperature. The solar cell temperature at a reference environment defined as 800 W/m2 radiance, 20°C ambient air temperature, and 1 m/s wind speed with the cell or module in an electrically open circuit state


Ohm - The unit of resistance to the flow of an electric current.

One-axis tracking - A system capable of rotating about one axis, usually following the sun from East to West.

Open-circuit voltage (Voc) - The maximum possible voltage across a photovoltaic cell or module; the voltage across the cell in sunlight when no current is flowing.

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

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

Open circuit voltage (VOC) - 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 - Any solar cell that is made from organic (i.e. carbon-based) polymers.


P-N junction - 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 - 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.

Passivation - The process of adding a passivating surface layer to a solar cell, such as silicon nitride, silicon dioxide, aluminium 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 - This term is used in the context of solar photovoltaic to refer to either the time taken for the energy payback or the time taken for the economic payback of a given solar system.

Peak load - 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 watts (Wp) - See 'Photovoltaic peak watt.'

Photon - A particle of light that acts as an individual unit of energy.

Peak shaving - 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 - 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.

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

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) efficiency - The ratio of electric power produced by a cell at any instant to the power of the sunlight striking the cell. This is typically about 9% to 14% for commercially available cells.

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 DC power under unconsecrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstate) or the back layer (substrate). [UL 1703]

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 (PV) peak watt - Maximum "rated" output of a cell, module, or system. Typical rating conditions are 0.645 watts per square inch (1000 watts per square meter) of sunlight, 68 degrees F (20 degrees C) ambient air temperature and 6.2 x 10-3 mi/s (1 m/s) wind speed.

Photovoltaic (PV) system - A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

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.

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

Polymer solar cell - 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.

Power Purchase Agreement (PPA) - 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) - 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) - 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 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 factor - The ratio of the average power and the apparent volt-amperes. Affected by the inductance and capacitance of the load. A pure resistance, such as an electric heater would have a power factor of 1.00.

Pulse-width-modulated (PWM) - A function of many of the newer charge controllers and battery chargers which instead of applying a steady DC voltage to the battery, sends out short pulses. The width of the pulses varies with the battery state of charge.

Pump storage - 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.

Pyrometer - An instrument used to measure hemispherical solar radiation on a given plane.

Perihelion meter - An instrument used to measure solar direct normal radiance (DNI) on a given plane.


Qualification test (PV) - A procedure applied to a selected set of PV 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.


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

Reflected radiation - 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) - A tax credit that incentives 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 - A solar photovoltaic system installed on a private residence. This term is also often used interchangeably with “small-scale system.”

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

Rectifier - A device that converts AC to DC, as in a battery charger or converter. See inverter and diode.

Remote systems - Systems off of the utility grid. Out in the boon docks.

Resistive voltage drop - The voltage developed across a cell by the current flow through the resistance of the cell.

Reverse bias - Condition where the current producing capability of a PV cell is significantly less than that of other cells in its series string. This can occur when a cell is shaded, cracked, or otherwise degraded or when it is electrically poorly matched with other cells in its string.


Schottky barrier - A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

Schottky diode - A special diode with a very low voltage drop, usually in the .15 to .25 volt range. Often used as blocking diodes in solar panels and arrays to minimize power loss. A normal silicon diode drops at least 7 volts.

Self discharge - The rate at which a battery, without a load, will lose its charge. This can vary considerably depending on the type of battery and age. It can be as low as 3% a month for a new AGM battery, and as high as 10% a week for an older Lead-Antimony (industrial) battery.

Semiconductor - Any material that has a limited capacity for conducting an electric current. Generally falls between a metal and an insulator in conductivity. Certain semiconductors, including silicon, gallium arsenide, copper indium dissidence, and cadmium tellurium, are uniquely suited to the photovoltaic conversion process.

Semi crystalline - See 'Multi crystalline.'

Series connection - A way of joining photovoltaic cells or batteries by connecting positive leads to negative leads; such a configuration increases the voltage.

Series regulator - Type of battery charge regulator where the charging current is controlled by a switch, transistor, or FET connected in series with the PV module or array. As opposed to a shunt regulator, which gradually shorts out the panel output as the battery gets charged up.

Series resistance - Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

Shelf life of batteries - The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

Short-circuit current (ISC) - The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance; the maximum current possible.

Shunt regulator - Type of a battery charge regulator where the charging current is controlled by a switch or transistor connected in parallel with the PV panel. Overcharging of the battery is prevented by shorting the PV output. Shunt regulators are common in PV systems as they are relatively cheap to build and simple to design. Series regulators usually have better control and charge characteristics. Most newer controllers have gone to series regulation.

Silicon (Si) - A chemical element, atomic number 14, semi-metallic in nature, dark gray, an excellent semiconductor material. A common constituent of sand and quartz (as the oxide).Crystallizes in face-centred cubic lattice like a diamond.The most common semiconductor material used in making photovoltaic devices.

Single-axis tracking - 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) - A hard crystalline compound of silicon and carbon. It only ever occurs naturally on Earth as moistness, 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.

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.

Solar cell - See 'Photovoltaic cell.'

Solar constant - The strength of sunlight; 1353 watts per square meter in space and about 1000 watts per square meter at sea level at the equator at solar noon. It increases at higher altitudes.

Solar energy - Energy from the sun. The heat that builds up in your car when it is parked in the sun is an example of solar energy.

Solar-grade silicon - Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

Solar thermal - 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 - 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 - 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) - The ratio of the power output of a given solar photovoltaic module to its mass, measured in watts per kilogram.

Sputtering - The process of ejecting atoms from a target by bombarding it with highly energized particles. Sputtering is used in photovoltaic manufacturing in both etching and film deposition. solar noon--That moment of the day that divides the daylight hours for that day exactly in half. To determine solar noon, calculate the length of the day from the time of sunset and sunrise and divide by two. Solar noon may be quite a bit different from 'clock' noon.

Solar spectrum - The total distribution of electromagnetic radiation emanating from the sun.

Solar thermal electric - Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbo generator.

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 'multifunction device.'

Square wave inverter - The inverter consists of a DC 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. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.

Stand-alone (PV system) - 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.

Stand-off mounting- - Technique for mounting a PV array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

Standard test conditions (STC) - Conditions under which a module is typically tested in a laboratory: (1) Radiance intensity of 1000 W/square meter (0.645 watts per square inch), AM1.5 solar reference spectrum, and (3) a cell (module) temperature of 25 degrees C, plus or minus 2 degrees C (77 degrees F, plus or minus 3.6 degrees F). [IEC 1215]

State of charge (SOC) - The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

Sulfation - The physical material upon which a photovoltaic cell is made.

Insulation - A condition that afflicts unused and discharged batteries; large crystals of lead sulphate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

Superstrate - The covering on the sun side of a 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.

Substrate - 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 - The reaction of lead and lead dioxide with sulphuric acid in a lead-acid battery, producing lead sulphate, 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 - 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.


Thermal electric - Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbo generator. See 'solar thermal electric.'

Thermos photovoltaic (TPV) device - A device that converts secondary thermal radiation, re-emitted by an absorber or heat source, into electricity; The device is designed for maximum efficiency at the wavelength of the secondary radiation.

Thick-crystalline materials - Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

Temperature coefficient - 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.

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

Thermal cycling - 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 de laminations between the electrical circuit and the module edge; and loss of mechanical integrity.

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

Tracker - 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.

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

Thin film - A thin layer of photovoltaic material (for example, amorphous silicon or cadmium tellurium) 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 - 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) - A doped metal oxide used in the production of thin film solar cells.

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 eventually be absorbed by the semiconductor.

Tracking array - 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. Typically, a single axis tracker will give you 15% to 25% more power per day, and dual axis tracking will add about 5% to that. Depends somewhat on latitude and season.

transformer - Steps AC voltage up or down, depending on the application.

Trickle charge - A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

Two-axis tracking - A system capable of rotating independently about two axes (e.g., vertical and horizontal) and following the sun for maximum efficiency of the solar array.


Ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometres. 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 PV system's output is fully synchronized with the utility power.


VAC - Volts AC

VDC - Volts DC

Vmp - Voltage at maximum power

Voc - Open-circuit voltage

Vacuum evaporation - The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

Vacuum zero — The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

Valence band — The highest energy band in a semiconductor that can be filled with electrons.

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 multi junction (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 also multi-junction device and split-spectrum cell.

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

Voltage — The amount of electromotive force, measured in volts, that exists between two points.

Voltage at maximum power (Vmp) — The voltage at which maximum power is available from a photovoltaic module.

Voltage protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

Voltage regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.


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 hour (Wh) - A unit of energy that is equal to one watt of power expanded for one hour of time.

Watt peak (Wp) - 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 of ideal conditions, module degradation, imperfect alignment of the module in tilt or azimuth, and a less than ideal temperature.

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

Waveform — The shape of the phase power at a certain frequency and amplitude.

Wet shelf life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

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.

Work function — The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.


Zenith angle - 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 there is zenith angle then the sun is directly overhead and if it is 90⁰ then the sun will be on the horizon soon after sunrise or soon before sunset.

Zinc oxide (ZnO) - A wide band gap semiconductor used in the manufacturing of hybrid organic photovoltaic (OPV).

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