• A


    In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).


    see alternating current.


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

    Activated Shelf Life

    The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

    Activation Voltage(s)

    The voltage(s) at which a charge controller will take action to protect the batteries.

    Adjustable Set Point

    A feature allowing the user to adjust the voltage levels at which a charge controller will become active.


    See amperage interrupt capability.

    Air mass (sometimes called air mass ratio)

    Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

    Alternating Current (AC)

    A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

    Ambient Temperature

    The temperature of the surrounding area.

    Amorphous Semiconductor

    A non-crystalline semiconductor material that has no long-range order.

    Amorphous Silicon

    A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

    Amperage Interrupt Capability (AIC)

    direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.

    Ampere (amp)

    A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

    Ampere Hour Meter

    An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

    Ampere-Hour (Ah/AH)

    A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

    Angle of Incidence

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

    Annual Solar Savings

    The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.


    The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.

    Antireflection Coating

    A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.


    See photovoltaic (PV) array.

    Array Current

    The electrical current produced by a photovoltaic array when it is exposed to sunlight.

    Array Operating Voltage

    The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

    Autonomous System

    See stand-alone system.


    The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

    Azimuth Angle

    The angle between true south and the point on the horizon directly below the sun.


    Balance of System

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

    Band Gap

    In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

    Band Gap Energy (Eg)

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

    Base Load

    The average amount of electric power that a utility must supply in any period.


    Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

    Battery Available Capacity

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

    Battery Capacity

    The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

    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 leveling at substations, and peak shaving on the customer side of the meter.

    Battery Life

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

    BIPV (Building-Integrated Photovoltaics)

    A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

    Blocking Diode

    A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

    Boron (B)

    The chemical element commonly used as the dopant in photovoltaic device or cell material.


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

    Btu (British Thermal Unit)

    The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

    Bypass Diode

    A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.


    Cadmium (Cd)

    A chemical element used in making certain types of solar cells and batteries.

    Cadmium Telluride (CdTe)

    A polycrystalline thin-film photovoltaic material.

    Capacity (C)

    See battery capacity.


    Centimeter-Gram-Second (abbreviated cm-gm-sec or cm-g-s). The system of measurement that uses these units for distance, mass, and time.


    The layer of the solar atmosphere that is located above the photosphere and beneath the transition region and the corona. The chromosphere is hotter than the photosphere but not as hot as the corona.

    Contour Map

    A map showing the intensity of radiation as a function of position. Each contour line corresponds to a specific intensity of radiation, with inner contours corresponding to higher intensities than outer contours. Therefore, a closed contour encircles a region where the intensity of the emitted radiation is greater than or equal to the intensity on the contour line. The contours outline the shape of the emitting source.


    The physical upwelling of hot matter, thus transporting energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. When it has cooled by passing on its extra heat to its surroundings, the bubble sinks again. Convection can occur when there is a substantial decrease in temperature with height, such as in the Sun's convection zone.

    Convection Zone

    A layer in a star in which convection currents are the main mechanism by which energy is transported outward. In the Sun, a convection zone extends from just below the photosphere to about seventy percent of the solar radius.


    The outermost layer of the solar atmosphere. The corona consists of a highly rarefied gas with a low density and a temperature greater than one million degrees Kelvin. It is visible to the naked eye during a solar eclipse.



    The amount of mass or number of particles per unit volume. In cgs units mass density has units of gm cm-3. Number density has units cm-3 (particles per cubic centimeter).


    Electromagnetic Radiation

    Radiation that travels through vacuous space at the speed of light and propagates by the interplay of oscillating electric and magnetic fields. This radiation has a wavelength and a frequency.

    Electromagnetic Spectrum

    The entire range of all the various kinds or wavelengths of electromagnetic radiation, including (from short to long wavelengths) gamma rays, x-rays, ultraviolet, optical (visible), infrared, and radio waves.


    A negatively charged elementary particle that normally resides outside (but is bound to) the nucleus of an atom.

    Electron Flux

    The rate of flow of electrons through a reference surface. In cgs units, measured in electrons s-1, or simply s-1.

    Electron Volt

    Abbreviated eV. A unit of energy used to describe the total energy carried by a particle or photon. The energy acquired by an electron when it accelerates through a potential difference of 1 volt in a vacuum. 1 eV = 1.6 x 10-12 erg.

    Energy Flux

    The rate of flow of energy through a reference surface. In cgs units, measured in erg s-1. Also measured in watts, where 1 watt = 1 x 107 erg s-1. Flux density, the flux measured per unit area, is also often referred to as "flux".


    A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance of 1 cm. 107 (ten million) erg s-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 x 1010 (42 billion) ergs.


    Flare (Solar)

    Rapid release of energy from a localized region on the Sun in the form of electromagnetic radiation, energetic particles, and mass motions.

    Flare Star

    A member of a class of stars that show occasional, sudden, unpredicted increases in light. The total energy released in a flare on a flare star can be much greater that the energy released in a solar flare.


    The intersection of magnetic loops with the photosphere.

    Free Electron

    An electron that has broken free of it's atomic bond and is therefore not bound to an atom.


    The number of repetitions per unit time of the oscillations of an electromagnetic wave (or other wave). The higher the frequency, the greater the energy of the radiation and the smaller the wavelength. Frequency is measured in Hertz.



    See gallium.


    See gallium arsenide.

    Gallium (Ga)

    A chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.

    Gallium Arsenide (GaAs)

    A crystalline, high-efficiency compound used to make certain types of solar cells and semiconductor material.


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

    Gel-Type Battery

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

    Gigawatt (GW)

    A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.


    See electrical grid.

    Grid Lines

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

    Grid-Connected System

    A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

    Grid-Interactive System

    Same as grid-connected system.


    Harmonic Content

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


    A region of electrical contact between two different materials.

    High Voltage Disconnect

    The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.

    High Voltage Disconnect Hysteresis

    The voltage difference between the high voltage disconnect set point and the voltage at which the full photovoltaic array current will be reapplied.


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

    Hybrid System

    A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

    Hydrogenated Amorphous Silicon

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


    I-Type Semiconductor

    Semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

    I-V Curve

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

    Incident Light

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

    Indium Oxide

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

    Infrared Radiation

    Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

    Input Voltage

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


    The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. See diffuse insolation and direct insolation.

    Intrinsic Layer

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

    Intrinsic Semiconductor

    An undoped semiconductor.


    A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.


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


    The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

    ISPRA Guidelines

    Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.



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


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

    Junction Box

    A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

    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.


    Kilowatt (kW)

    A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

    Kilowatt-Hour (kWh)

    1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.


    Langley (L)

    Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.


    The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

    Lead-Acid Battery

    A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.


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

    Life-Cycle Cost

    The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

    Light Trapping

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

    Light-Induced Defects

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

    Line-Commutated Inverter

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

    Liquid Electrolyte Battery

    A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.


    The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

    Load Circuit

    The wire, switches, fuses, etc. that connect the load to the power source.

    Load Current (A)

    The current required by the electrical device.

    Load Resistance

    The resistance presented by the load. See resistance.

    Low Voltage Cutoff (LVC)

    The voltage level at which a charge controller will disconnect the load from the battery.

    Low Voltage Disconnect

    The voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.

    Low Voltage Disconnect Hysteresis

    The voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.

    Low Voltage Warning

    A warning buzzer or light that indicates the low battery voltage set point has been reached.


    Maintenance-Free Battery

    A sealed battery to which water cannot be added to maintain electrolyte level.

    Majority Carrier

    Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

    Maximum Power Point (MPP)

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

    Maximum Power Point Tracker (MPPT)

    Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

    Maximum Power Tracking

    Operating a photovoltaic array at the peak power point of the array's I-V curve where maximum power is obtained. Also called peak power tracking.

    Megawatt (MW)

    1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.


    1,000 kilowatt-hours or 1 million watt-hours.


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

    Minority Carrier

    A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

    Minority Carrier Lifetime

    The average time a minority carrier exists before recombination.

    Modified Sine Wave

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


    The use of multiple inverters connected in parallel to service different loads.


    See photovoltaic (PV) module.

    Module Derate Factor

    A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.


    Fabricated as a single structure.


    Metal Oxide Varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

    Multi-Stage Controller

    A charging controller unit that allows different charging currents as the battery nears full state_of_charge.


    A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

    Multijunction Device

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


    N-Type Semiconductor

    A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity (e.g., phosphorus in silicon).

    N-Type Silicon

    Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.

    National Electrical Code (NEC)

    Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV system.

    National Electrical Manufacturers Association (NEMA)

    This organization sets standards for some non-electronic products like junction boxes.


    See National Electrical Code.


    See National Electrical Manufacturers Association.

    Nickel Cadmium Battery

    A battery containing nickel and cadmium plates and an alkaline electrolyte.

    Nominal Voltage

    A reference voltage used to describe batteries, modules, or systems (i.e., a 12-volt or 24-volt battery, module, or system).

    Normal Operating Cell Temperature (NOCT)

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




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

    One-Axis Tracking

    A system capable of rotating about one axis.

    Open-Circuit Voltage (Voc)

    The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

    Operating Point

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


    Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.


    See gassing.


    Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.



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

    P-Type Semiconductor

    A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).


    A semiconductor photovoltaic device structure in which the junction is formed between a p-type layer and an n-type layer.

    Packing Factor

    The ratio of array area to actual land area or building envelope area for a system; or, the ratio of total solar cell area to the total module area, for a module.


    See photovoltaic (PV) panel.

    Parallel Connection

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


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

    Peak Demand/Load

    The maximum energy demand or load in a specified time period.

    Peak Power Current

    Amperes produced by a photovoltaic module or array operating at the voltage of the I-V curve that will produce maximum power from the module.

    Peak Power Point

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

    Peak Power Tracking

    See maximum power tracking.

    Peak Sun Hours

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

    Peak Watt

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

    Phosphorous (P)

    A chemical element used as a dopant in making n-type semiconductor layers.


    An electric current induced by radiant energy.

    Photoelectric Cell

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

    Photoelectrochemical Cell

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


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

    Photovoltaic (PV) Array

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

    Photovoltaic (PV) Cell

    The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.

    Photovoltaic (PV) Conversion Efficiency

    The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

    Photovoltaic (PV) Device

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

    Photovoltaic (PV) Effect

    The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

    Photovoltaic (PV) Generator

    The total of all PV strings of a PV power supply system, which are electrically interconnected.

    Photovoltaic (PV) Module

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

    Photovoltaic (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) System

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

    Photovoltaic(s) (PV)

    Pertaining to the direct conversion of light into electricity.

    Photovoltaic-Thermal (PV/T) System

    A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system.

    Physical Vapor Deposition

    A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.


    A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.

    Pocket Plate

    A plate for a battery in which active materials are held in a perforated metal pocket.

    Point-Contact Cell

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


    See Multicrystalline.

    Polycrystalline Silicon

    A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon.

    Power Conditioning

    The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating current).

    Power Conditioning Equipment

    Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.

    Power Conversion Efficiency

    The ratio of output power to input power of the inverter.

    Power Density

    The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).

    Power Factor (PF)

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

    Primary Battery

    A battery whose initial capacity cannot be restored by charging.

    Projected Area

    The net south-facing glazing area projected on a vertical plane.

    Pulse-Width-Modulated (PWM) Wave Inverter

    A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.


    See photovoltaic(s).


    An instrument used for measuring global solar irradiance.


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



    One quadrillion Btu (1,000,000,000,000,000 Btu).

    Qualification Test

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


    Rated Battery Capacity

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

    Rated Module Current (A)

    The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25C cell temperature.

    Rated Power

    Rated power of the inverter. However, some units can not produce rated power continuously. See duty rating.

    Reactive Power

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


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


    A device that converts alternating current to direct current. See inverter.


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

    Remote Systems

    Remote Systems

    Reserve Capacity

    The amount of generating capacity a central power system must maintain to meet peak loads.

    Resistance (R)

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

    Resistive Voltage Drop

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

    Reverse Current Protection

    Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See blocking diode.

    Ribbon (Photovoltaic) Cells

    A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.


    See root mean square.

    Root Mean Square (RMS)

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


    Sacrificial Anode

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

    Satellite Power System (SPS)

    Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

    Schottky Barrier

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


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

    Sealed Battery

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

    Seasonal Depth of Discharge

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

    Secondary Battery

    A battery that can be recharged.


    The rate at which a battery, without a load, will lose its charge.


    Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

    Series Connection

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

    Series Controller

    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.

    Shallow-Cycle Battery

    A battery with small plates that cannot withstand many discharges to a low state-of-charge.

    Shelf Life of Batteries

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

    Short-Circuit Current (Isc)

    The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

    Shunt Controller

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

    Shunt Regulator

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

    Siemens Process

    A commercial method of making purified silicon.

    Silicon (Si)

    A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide).

    Sine Wave

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

    Sine Wave Inverter

    An inverter that produces utility-quality, sine wave power forms.

    Single-Crystal Material

    A material that is composed of a single crystal or a few large crystals.

    Single-Crystal Silicon

    Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.

    Solar Constant

    The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 Watts per square meter or 492 Btu per square foot.

    Solar Cooling

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

    Solar Energy

    Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

    Solar Noon

    The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky; equal to true or due, geographic south.

    Solar Resource

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

    Solar Spectrum

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

    Solar Thermal Electric Systems

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

    Solar-Grade Silicon

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

    Specific Gravity

    The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.

    Spinning Reserve

    Electric power plant or utility capacity on-line and running at low power in excess of actual load.

    Split-Spectrum Cell

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

    Square Wave

    A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.

    Square Wave Inverter

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

    Staebler-Wronski Effect

    The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.

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

    Standard Reporting Conditions (SRC)

    A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.

    Standard Test Conditions (STC)

    Conditions under which a module is typically tested in a laboratory.

    Standby Current

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

    Starved Electrolyte Cell

    A battery containing little or no free fluid electrolyte.

    State-of-Charge (SOC)

    The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

    Storage Battery

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


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


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


    Tare Loss

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

    Temperature Compensation

    A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.

    Temperature Factors

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

    Thermophotovoltaic Cell (TPV)

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

    Thick-Crystalline Materials

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

    Thin Film

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

    Thin Film Photovoltaic Module

    A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See amorphous silicon.

    Tilt Angle

    The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

    Tin Oxide

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

    Total AC Load Demand

    The sum of the alternating current loads. This value is important when selecting an inverter.

    Total Harmonic Distortion

    The measure of closeness in shape between a waveform and it's fundamental component.

    Total Internal Reflection

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

    Tracking Array

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


    An electromagnetic device that changes the voltage of alternating current electricity.

    Tray Cable (TC)

    may be used for interconnecting balance-of-systems.

    Trickle Charge

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


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

    Two-Axis Tracking

    A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).



    Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

    Underground Feeder (UF)

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

    Underground Service Entrance (USE)

    May be used within battery enclosures and for interconnecting balance-of-systems.

    Uninterruptible Power Supply (UPS)

    The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

    Utility-Interactive Inverter

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


    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.


    A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

    Vented Cell

    A battery designed with a vent mechanism to expel gases generated during charging.

    Vertical Multijunction (VMJ) Cell

    A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See multijunction device and split-spectrum cell.

    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.


    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.



    A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.


    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).


    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.


    Zenith Angle

    The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).