A Solar Energy Glossary
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.
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 (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.
Azimuth Angle — The angle between true south and the point on the horizon directly below the sun.
Balance of System - all the parts of a solar electric system excluding the solar panels and the inverter. Balance of system usually includes items like racking, wires, conduit, and safety shut-offs.
Cloud Enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.
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.
Disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.
Downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
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.
Electricity — Energy resulting from the flow of charge particles, such as electrons or ions.
Electrochemical Cell — A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.
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).
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.
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.
Inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.
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.
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 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.
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.
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.
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.
Micro-inverter - A device that converts direct current (DC) from a single solar module (panel) to alternating current (AC).
Net Metering - An agreement between the utility company and the system owner allowing for ‘banking’ excess power produced by a solar system in the electric grid and ‘drawing’ from the electric grid when more power is needed.
Peak Demand/Load — The maximum energy demand or load in a specified time period.
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.
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) 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.
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.
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.
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).
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