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Absorber — In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).
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.
Array — see photovoltaic (PV) array.
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.
B
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.
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.
C
Combined Collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.
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.
D
Discharge Rate — The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.
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.
Dry Cell — A cell (battery) with a captive electrolyte. A primary battery that cannot be recharged.
E Electric Circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.
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.
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.
F
Flat-Plate Photovoltaics (PV) — A PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.
Frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).
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).
G 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.
H
High Voltage Disconnect — The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.
I
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.
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.
J 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 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.
K
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.
L
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.
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 Resistance — The resistance presented by the load. See resistance.
M
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.
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.
N
National Electrical Manufacturers Association (NEMA) — This organization sets standards for some non-electronic products like junction boxes.
Nickel Cadmium Battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.
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O
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.
Overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.
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P
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 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.
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) 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) 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).
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.
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Q
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R
Rectifier — A device that converts alternating current to direct current. See inverter.
Regulator — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.
Remote Systems — See stand-alone systems.
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S Self-Discharge — The rate at which a battery, without a load, will lose its charge.
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.
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.
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.
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.
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.
T 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.
U 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.
V 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.
W 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).
Wire Types — See Article 300 of National Electric Code for more information.
Z Source: US Department of Energy, Energy Efficiency and Renewable Energy http://www1.eere.energy.gov/solar/solar_glossary.html
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