Solar Energy Glossary I-R


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

independent system operator (ISO) — The entity responsible for maintaining system balance, reliability, and electricity market operation.

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.

ingot — A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell.

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

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

interconnect — A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells.

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.

inverted metamorphic multijunction (IMM) cell — A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency.

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

ion — 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.

irradiance — 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.

i-type semiconductorSemiconductor 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.

Back to Top


joule — 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.

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

junction box — A junction box is any electrical enclosure where connections are made. 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.

Back to Top


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.

Back to Top


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

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

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

levelized cost of energy (LCOE) — The cost of energy of a solar system that is based on the system's installed price, its total lifetime cost, and its lifetime electricity production.

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

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

light-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 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.

lithium ion battery – A secondary battery where lithium ions move between the positive and negative electrodes.

load — 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 forecast — Predictions of future demand. For normal operations, daily and weekly forecasts of the hour-by-hour demand are used to help develop generation schedules to ensure that sufficient quantities and types of generation are available when needed.

load resistance — The resistance presented by the load. See also resistance.

load-side interconnection - A type of grid interconnection where a PV system is connected on the load side of a facility’s main service breaker. This type of interconnection is typically made with a PV circuit breaker in the main service panel or a subpanel or as a feeder tap.

locational marginal price (LMP) — The price of a unit of energy at a particular electrical location at a given time. LMPs are influenced by the nearby generation, load level, and transmission constraints and losses.

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.

Back to Top


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.

measurement and characterization — A field of research that involves assessing the characteristics of photovoltaic materials and devices.

megawatt (MW) — 1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

megawatt-hour — 1,000 kilowatt-hours or 1 million watt-hours.

metrology — The science of measurement.

microinverter– A small PV inverter that typically works with one or two PV modules. They are generally installed at the PV array, mounted to the PV racking or PV module frames.

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

micrometer (micron) — One millionth of a meter.

mid clamp - A clamp between two PV modules that secure them to the PV module racking.

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.

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

moduleSee 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.

Module-Level Power Electronics (MLPE) – Devices such as microinverters, DC optimizers and rapid shutdown switches that are connected to one or two PV modules.

monolithic — Fabricated as a single structure.

monocrystalline – A semiconductor (photovoltaic) material composed of single-crystalline material.

movistor — Short for metal oxide varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

Multi-Contact (MC) connector – A brand name of connector that is typically used on the positive and negative leads of a PV module.

multicrystalline — 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.

multi-stage controller — A charging controller unit that allows different charging currents as the battery nears full state_of_charge.

Back to Top


nanometer — One billionth of a meter.

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. Equipment rated for indoor use is usually rated NEMA 1. Typical NEMA ratings for outdoor equipment are NEMA 3, NEMA 3R and NEMA 4. 

NECSee National Electrical Code.

NEMASee National Electrical Manufacturers Association.

net metering – A electricity billing method that provides credits for energy produced on site allowing a utility customer with PV to use the power from their PV system at a time other than when it was generated.

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, 20°C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

n-type — Negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons.

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

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

Back to Top


off-grid – A term used to describe a stand-alone system.

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

one-axis tracking — A system capable of rotating about one axis.

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

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

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

outgasSee gassing.

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

Back to Top


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.

panelSee 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.

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 trackingSee 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.

photocurrent — An electric current induced by radiant energy.

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

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

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

photovoltaic(s) (PV) — Pertaining to the direct conversion of light into electricity.

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

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

photovoltaic (PV) conversion efficiency — The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

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

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

photovoltaic (PV) generator — The total of all PV strings of a PV power supply system, which are electrically interconnected.

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

photovoltaic (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-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 or solar thermal system.

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

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

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

plug-and-play PV system — A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation.

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

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.

polycrystallineSee multicrystalline.

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

polycrystalline thin film — A thin film made of multicrystalline material.

pool solar – A common term for solar thermal system that is specifically used to heat water for a swimming pool.

power — The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.

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.

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).

pulse-width-modulated (PWM) wave inverter — A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

PVSee photovoltaic(s).

PV wire – A conductor that is manufactured to meet the standards in UL 4703. It is a stranded conductor with thick insulation that is typically sunlight resistant and rated for a minimum of 600 Volts and operating temperatures of 90°C or higher.  

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.

Back to Top


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

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

quantum efficiency (QE) — The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight.

Back to Top


rapid shutdown– An NEC code requirement that the circuits within a PV system can be quickly reduced to a minimal voltage for safety.

ramp — A change in generation output.

ramp rate — The ability of a generating unit to change its output over some unit of time, often measured in MW/min.

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 also battery capacity.

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

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

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

rectifier — A device that converts alternating current to direct current. See also inverter.

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

remote systemsSee stand-alone systems.

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

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

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 also 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.

RMSSee 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.

Back to Top