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.
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.
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.
Array — 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.
Availability — 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.
Base Load — The average amount of electric power that a utility must supply in any period.
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.
BPU — Board of Public Utilities. The government agency that regulates utility companies, utility policies and utility rates in a state.
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.
Cadmium Telluride (CdTe) — A polycrystalline thin-film photovoltaic material.
Capacity Factor — The ratio of the average load on (or power output of) an electricity generating system to the capacity rating over a specified period of time.
Cell Barrier — A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge.
Cell Junction — The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.
Cloud Enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.
Combined Collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.
Concentrator — A photovoltaic module, which includes optical components such as lenses to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.
Conductor — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.
Conversion Efficiency — See photovoltaic (conversion) efficiency.
Converter — A unit that converts a direct current (dc) voltage to another dc voltage.
Crystalline Silicon — A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.
Current — See electric current.
DC — See direct current.
DC-to-DC Converter — Electronic circuit to convert direct current voltages (e.g., photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker.
Design Month — The month having the combination of insolation and load that requires the maximum energy from the photovoltaic array.
Diffuse Insolation — Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.
Diffuse Radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground..
Direct Current (DC) — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.
Direct Insolation — Sunlight falling directly upon a collector. Opposite of diffuse insolation.
Disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.
Distributed Generation — A popular term for localized or on-site power generation.
Distributed Power — Generic term for any power supply located near the point where the power is used. Opposite of central power. See stand-alone systems.
Distributed Systems — Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.
Downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
Duty Cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.
Duty Rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.
Electric Circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.
Electric Current — The flow of electrical energy (electricity) in a conductor, measured in amperes.
Electrical grid — An integrated system of electricity distribution, usually covering a large area.
Electrode — A conductor that is brought in conducting contact with a ground.
Electron — An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.
Energy — The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.
Energy Audit — A survey that shows how much energy used in a home, which helps find ways to use less energy.
Equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).
FITC — Federal Incentive Tax Credit. In solar, it refers to the tax credit of 30% of the total cost of the solar installation.
Fixed Tilt Array — A photovoltaic array set in at a fixed angle with respect to horizontal.
Flat-Plate Module — An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.
Fresnel Lens — An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.
Full Sun — The amount of power density in sunlight received at the earth’s surface at noon on a clear day (about 1,000 Watts/square meter).
Gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
Grid — See electrical grid.
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.
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.
Hybrid System — A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.
Incident Light — Light that shines onto the face of a solar cell or module.
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 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.
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-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.
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.
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.
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.
Load — The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment; in reference to electricity, usually expressed in terms of amperes or watts.
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.
MACRS — stands for Modified Accelerated Cost Recovery System and is a method required by the IRS in depreciating business fixed assets. Depreciation rates vary and should be checked by your accountant.
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.
Megawatt-Hour — 1,000 kilowatt-hours or 1 million watt-hours.
Modularity — The use of multiple inverters connected in parallel to service different loads.
Module — 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.
Multi-crystalline — A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as poly-crystalline or semi-crystalline.
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.
NEC — See National Electrical Code.
NEMA — See National Electrical Manufacturers Association.
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.
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.
Panel — 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.
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.
Photocurrent — An electric current induced by radiant energy.
Photoelectric Cell — A device for measuring light intensity that works by converting light falling on it, or reaching it, to electricity, and then measuring the current; used in photometers.
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.
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.
Polycrystalline — 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 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.
PV — See photovoltaic(s).
Rated Power — Rated power of the inverter. However, some units cannot produce rated power continuously. See duty rating.
Rectifier — A device that converts alternating current to direct current. See inverter.
Remote Systems — See 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.
RPS — Renewable Portfolio Standard. The amount of energy a power company must provide from a renewable source like solar or wind.
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.
Semiconductor — 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.
Semicrystalline — See Multicrystalline.
Series Connection — A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.
Short-Circuit Current (Isc) — The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.
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).
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 Cell — see photovoltaic (PV) cell.
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 Panel — See photovoltaic (PV) panel.
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.
Space Charge — See cell barrier.
Spinning Reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.
SREC — Solar Renewable Energy Credit. A solar installation produces one SREC for each 1,000 Kilowatts of energy produced. An SREC is a commodity not unlike copper or pork bellies and can be sold by the owner of a solar installation. Like any commodity, prices fluctuate depending on availability and demand.
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.
String — A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.
Substrate — The physical material upon which a photovoltaic cell is applied.
Subsystem — Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load).
Superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.
Superstrate — The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.
Surge Capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.
System Availability — The percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.
System Operating Voltage — The photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.
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 that can be printed onto a material.
Thin Film Photovoltaic Module — A photovoltaic module constructed with sequential layers of thin film semiconductor materials.
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.
Total AC Load Demand — The sum of the alternating current loads. This value is important when selecting an inverter.
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.
Transformer — An electromagnetic device that changes the voltage of alternating current electricity.
Tray Cable (TC) – may be used for interconnecting balance-of-systems.
Two-Axis Tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).
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.
Volt (V) — A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.
Voltage — The amount of electromotive force, measured in volts, that exists between two points.
Voltage at Maximum Power (Vmp) — The voltage at which maximum power is available from a photovoltaic module.
Voltage Protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.
Voltage Regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.
Wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.
Watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).
Window — A wide band gap material chosen for its transparency to light. Generally used as the top layer (superstrate) of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.
Zenith Angle — the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).