Photovoltaic Glossary

In this section terms most often used in solar energy engineering and photovoltaics are explained. This glossary was compiled from several mostly internet sources. For sources, details and additional information please see section Literature and additional information at the bottom of the page.


Absorption coefficient - the factor by which photons are absorbed as they travel a unit distance through a material.

Air mass index - length of a pat through the earth's atmosphere traversed by the direct solar beam, expressed as a multiple of the path traversed to a point at sea level with the sun directly overhead. Air mass index is defined at standard barometric pressure. The reference spectrum for standard test conditions (STC) was defined to be AM1.5

Alternating current - electric current in which the direction of flow is reversed at frequent intervals.

Amorphous semiconductor - a non-crystalline semiconductor material manufactured by depositing layers of doped silicon on a substrate.

Amorphous silicon - silicon without crystalline structure.

Ampere (A) - unit of electric current. The rate of flow of electrons in a conductor equal to one coulomb per second.

Ampere hour (Ah) - the quantity of electrical energy equal to the flow of current of one ampere for one hour.

Anode - the positive electrode in an electrochemical cell (battery).

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

Array - electrically connected photovoltaic (PV) modules.

Array current - The electrical current produced by a PV array.

Azimuth - horizontal angle measured clockwise from true north (180° is true south). In solar energy engineering azimuth is measured from south toward W or E (0° is true south, toward west negative)


Base load - the average amount of electric power that a utility must supply in any period.

Battery - a device that converts the chemical energy directly into electrical energy by means of an electrochemical reaction.

Battery capacity - the total number of ampere-hours that can be withdrawn from a fully charged battery at a specified discharge rate and temperature.

BIPV - Building Integrated Photovoltaics; A term for the design and integration of PV into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass or other facade material; into roofing systems, over windows; or other building envelope systems.

Blocking diode - a diode used to restrict or block reverse current from flowing backward through a module. It protects modules against the risk of thermal destruction of solar cells due reverse power flow.

Bypass diode - a diode connected anti-parallel across a part of the solar cells of a PV module. It protects solar cells from destruction in case of total or partial shading of individual solar cells.


Cadmium Telluride (CdTe) - A polycrystalline thin-film photovoltaic material.

Capacity - see battery capacity.

Cathode - the negative electrode in an electrochemical cell. Also, the negative terminal of a diode.

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.

Charge Controller - a device that controls the charging rate and/or state of charge for batteries.

Chemical vapour deposition - a method of depositing thin semiconductor films. A substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents.

CIS - Copper-Indium-Diselelide.

Converter - electrical device that converts a DC or AC voltage to another DC or AC voltage. For DC/AC conversion see inverter, for DC/DC convertion see DC/DC converter.

Crystalline silicon - silicon made from a single crystal or poly-crystalline slice of silicon.

Current - the flow of electric charge in a conductor between two points having a difference in potential (voltage).

Cutoff voltage - the voltage levels (activation) at which the charge controller disconnects the array from the battery or the load from the battery.

Czochralski Process - a method of growing high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions. Method was invented by polish engineer Jan Czohralski.

Cycle - the discharge and subsequent charge cycle of a battery.


DC/DC Converter - electronic cirquit that converts DC voltages into other levels. Usually it is part of a maximum power point trackers (MPPT) and charge regulators.

Deep cycle - discharged to a large fraction of capacity many times without damaging the battery.

Depth of discharge - the percent of the rated battery capacity that has been withdrawn. Sometimes also expressed as abbreviation DoD.

Diffuse radiation - radiation received from the sun after scattering and reflection by the atmosphere and ground.

Diode - electronic part that allows current flow only in one direction.

Direct radiation - radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc. Sometimes also expressed as "beam radition".

Direct Current (DC) - electric current flowing in only one direction.

Duty cycle - the ratio of active time to total time. Used to describe the operating regime of appliances or loads in PV systems.

DVM - digital Volt-meter.


Efficiency - the ratio of output power (or energy) to input power (or energy). Expressed in percent.

EFG - Edge-defined Film-fed Growth. A method for making sheets of polycrystalline silicon in which molten silicon is drawn upward by capillary action through a mold.

Electrolyte - The medium that provides the ion transport mechanism between the positive and negative electrodes of an electrochemical cell (battery).

Electric current - a flow of electrons; electricity.

Electrical grid - an integrated system of electricity distribution, usually covering a large area.

Electron-volt (eV) - an energy unit equal to the energy an electron acquires when it passes through a potential difference of one volt; it is equal to 1.602 x 10-19 V.

Equalisation charge - the process of mixing the electrolyte in batteries by short periodically overcharging of the batteries.

EVA - Ethylene-Vinyl-Acetate Foil. It is a copolymer of ethylene and vinyl acetate. It is used for solar cell encapsulation during module production.


Fill Factor (FF) - the ratio of the maximum power to the product of the open-circuit voltage and the short-circuit current (for an I-V curve, ).

Fixed Tilted Array - a photovoltaic array with fixed inclination angle with respect to horizontal.

Frequency - the number of cycles per unit time expressed in Hertz (Hz).


Gassing - gas by-products (hydrogen), produced during charging of a battery.

Gallium arsenide (GaAs) - a crystalline semiconductor/photovoltaic material.

Gel Type Battery - lead-acid battery in which the electrolyte is composed of a gel.

Grid - term used to describe an electrical utility distribution network.

Grid-connected PV system - a PV system which is connected to the grid.


Hybrid System - a PV system that includes other sources of electricity generation, such as wind, fuel cell or diesel generators.


I-U Characteristics - the current versus voltage characteristics of a solar cell, module, or array. Important points of the I-V characteristic are the open-circuit voltage, short-circuit current, maximum power point.

Incident radition - solar radiation incident on solar cell or module.

Irradiation - the solar radiation incident on an area over time. Equivalent to energy and expressed in J/m2. In technical practise kilowatt-hours per square meter is more often used.

Inverter - also named Power Conditioning Unit (PCU), or Power Conditioning System (PCS). An inverter converts DC power from the array (battery) to AC power compatible with the utility (loads).

Irradiance - the solar radiation incident on a surface. Usually expressed in kW/m2.


Joule (J) - Unit of energy 1 J = 1/3600 kWh.

Junction Box - a PV generator junction box is an enclosure on the module back side, where electrical contacts and protection devices (if used) are located.


Kilowatt (kW) - a unit of power.

Kilowatt Hour (kWh) - A unit of energy. Power multiplied by time equals energy.


Load - the amount of electric power used by any electrical appliance at any given time.

Load current - the current required by the electrical device (load).

Low voltage cutoff - the voltage level at which a controller will disconnect the load from the battery.


Maintanace free battery - a sealed battery which does not require maintance during lifetime.

Module - the smallest replaceable unit in a PV array.

MOS-FET - metal-oxid-silicon field effect transistor; used as semiconductor power switch in charge regulators, inverters etc.

MPP - maximum power point; the point on the current-voltage (I-V) characteristic of a PV device, where the product of current and voltage is maximal.

MPPT - maximum power point tracker; electrical cirquit that operates at generators maximum power point. MPP tracker is part of inverter and charge regulators.


N-type silicon - silicon material that has been doped with a material that has more electrons in its atomic structure than silicon.

NOCT - nominal operating cell temperature. The estimated temperature of a PV module when operating under 800 W/m2 irradiance, 20°C ambient temperature and wind speed of 1 m/s. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

Nominal Voltage - a reference voltage used to describe system, module or battery.


Ohm (Ω) - the unit of electrical resistance.

One axis tracking - a system capable of rotating about one axis.

Open cirquit voltage - the maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

Operating point - the current and voltage that a module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Overcharge - forced charging of a fully charged battery. The battery can be damaged if overcharged for a long period.


Panel - a designation for a number of PV modules assembled in a single mechanical frame.

Peak load - the maximum load demand of a system.

Peak sun hours - the equivalent number of hours per day when solar irradiance averages 1,000 W/m2 - one peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for one hour been 1,000 W/m2.

Peak Watt (Wp) - the amount of power a photovoltaic module will produce at standard test conditions (normally 1,000 W/m2 and 25° cell temperature).

Photon - a particle of light that acts as an individual unit of energy. Its energy depends on wavelenght.

Photovoltaic system - an installation of PV modules and other components designed to produce power from sunlight.

Polycrystalline silicon - a material used to make PV cells which consist of many crystals as contrasted with single crystal silicon.

Power factor - the cosine of the phase angle between the voltage and the current waveforms in an AC circuit. Used as a designator for inverter performance.

Primary battery - a battery whose initial capacity cannot be restored by charging.

Pyranometer - an instrument used for measuring global solar irradiance.

Pyrheliometer - an instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.


Rated module current - the current output of a PV module measured at standard test conditions of 1,000 W/m2 and 25°C cell temperature.

Reactive power - the sine of the phase angle between the current and voltage waveforms in an AC system.

Resistance - the property of a conductor which opposes the flow of an electric current resulting in the generation of heat in the conducting material.


Secondary battery - a battery that can be recharged.

Self discharge - the rate at which a battery, without a load, will lose its charge.

Series regulator - type of battery charge regulator where the charging current is controlled by a switch connected in series with the PV array.

Short cirquit current - the current produced by module, or array when its output terminals are shorted.

Silicon (Si) - a chemical element, atomic number 14, semimetallic in nature, dark gray, an excellent semiconductor material. A common constituent of sand and quartz (as the oxide). The most common semiconductor material used in making photovoltaic devices.

Single Crystall silicon - material with a single crystalline formation.

Solar constant - the solar radiation at the edge of earth atmosphere, 1367 W/m2.

Stand alone system - A photovoltaic system that operates independent of the utility grid.

Standard test conditions - conditions under which a module is typically tested in a laboratory: Irradiance of 1000 W/m2, AM 1.5 solar reference spectrum, a cell (module) temperature of 25°C.

String - a number of modules or panels interconnected electrically in series.

Subsystem - any one of several components in a PV system (i.e. array, controller, batteries, inverter, load).

Sun path diagram - graphical representation of the Sun's height and azimuth.

System operating voltage - the array output voltage under load.


Tilt Angle - the angle of inclination of a solar array measured from the horizontal.

Thin-film PV module - a PV module constructed with layers of thin film semiconductor materials.

Tracking array - a PV array that follows the path of the sun. Tracking can be one-axis or two-axis tracking where the array follows the sun in azimuth and elevation.

Transformer - converts the AC generator's voltage to higher voltage levels suitable for transmission.

Trickle charge - a small charge current intended to maintain a battery in a fully charged condition.

Two axis tracking - a system capable of rotating independently about two axes (e.g., vertical and horizontal).


Uninterruptible power supply (UPS) - a power supply providing continuous uninterruptible service.

Utility interactive inverter - an inverter that can function only when tied to the utility grid.


Varistor - a voltage-dependent variable resistor. Normally used as protection device against 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.

Volt - the unit of electrical voltage force a current of one ampere through a resistance of one Ohm.

Voltage at Maximum Power - The voltage at which maximum power is available from a module.


Wafer - a thin sheet of semiconductor material made by mechanicall sawing it from a single-crystal or multicrystal ingot or casting.

Watt - The unit of electrical power. The power developed when a current of one ampere flows through a potential difference of one volt.

Watt hour (Wh) - a unit of energy equal to one watt of power connected for one hour.

Waveform - the characteristic shape of an AC current or voltage output.


Zenith angle - the angle between vertical line and the line intersecting the sun. (90°- zenith).

More Information

IEC/TS 61836, Solar photovoltaic energy systems - Terms, definitions and symbols.
Renewable Resource Data Center (RReDC), Glossary of Solar Radiation Resource Terms.