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Solar energy Glossary

A list of common terms in the Solar Energy Industry.

A

absorbed glass mat (AGM): A fibrous silica glass mat to suspend the electrolyte in batteries. This mat provides pockets that assist in the recombination gasses generated during charging back into water.

alternating current (AC): Electric current in which the direction of flow is reversed at frequent intervals, usually 100 or 120 times per second (50 or 60 cycles per second or 50/60 Hertz).

altitude: The angle between the horizon (a horizontal plane) and the sun's position in the sky, measured in degrees.

amorphous silicon: A non-crystalline semiconductor material that has no long-range order, often used in thin film photovoltaic modules.

ampere or amp: The unit for the electric current; the flow of electrons. One amp is 1 coulumb passing in one second. One amp is produced by an electric force of1 volt acting across a resistance of 1 ohm. The symbol for amp used in calculations is I.

Amp-hour Ah): Quality of electrical energy equal to the flow of one ampere of current for one hour. Typically, used to quantify battery bank capacity.

angle of incidence: Angle which references the sun's radiation striking a surface at a 90 degree angle.

array: Any number of photovoltaic modules connected together to provide a single electrical output at a specified voltage. Arrays are often designed to produce significant amounts of electricity.

autonomous system: A stand-alone PV system that has no back-up generating source. May or may not include storage batteries.

B

balance of system (BOS): All systems components and costs other than PV modules. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

barrier energy: The energy given up by an electron in penetrating the cell barrier, a measure of the electrostatic potential of the barrier.

base power: Power generated by a utility unit that operated at a very high capacity factor.

baseline performance value: Initial values of Isc, Voc, Pmp, and Imp measured by the accredited laboratory and corrected to Standard Test Conditions, used to validate the manufacturer's performance measurements provided with the qualification modules per IEEE 1262.

battery: Two or more cells electrically connected for storing electrical energy. Common usage permits this designation to be applied also to a single cell used independently,as in a flashlight battery.

battery capacity: The total number of ampere hours that can be withdrawn from a fully charged cell or battery.

battery cell: A galvanic cell for storage of electrical energy. This cell, after being discharged, may be restored to fully charged condition by an electric current.

battery cell 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 self-discharge: The rate at which a battery, without a load, will lose its charge.

battery state of charge: The percentage of full charge or 100 percent minus the depth of discharge.

building integrated photovoltaic (BIPV): A term for the design and integration of V 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 semi-conductor device connected in series with a PV module and a storage battery to prevent a reverse current discharge of the battery through the module when there is no output, or low output from the cells. When connected in series to a PV string, it protects it's modules from a reverse power flow preventing the risk of thermal destruction of solar cells.

boron: A chemical element, atomic number 5, semi-metallic in nature, used as a doping to make p-semiconductor layers.

British thermal unit: The amount of heat energy required to raise the temperature of one pound of water from 60 degrees F to 61 degrees F at one atmosphere pressure. Roughly equivalent to the amount of energy used released by burning one stick match.

bypass diode: A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cells become reverse biased. Alternately, a diode connected anti-parallel across a part of the solar cells of a PV module. 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.

C

cadmium (Cd): A chemical element, atomic number 48, used in making certain types of solar cells and batteries.

cadmium telluride (CdTe): A polycrystalline, thin film photovoltaic material.

capacity factor: The amount of energy that the system produces at a particular site as a percentage of the total amount that it would produces if it operated at rated capacity during the entire year. For example, the capacity factor for a wind farm ranges from 20% to 35%.

cathodic protection: A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.

cell: The basic unit of a photovoltaic module. This word is also commonly used to describe the basic unit of batteries.

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 another, so that higher-energy electrons from one side diffuse preferably through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone, cell junction, 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.

central power: The generation of electricity in large power plants with distribution through a network of transmission lines (grid) for sale to a number of users. Opposite of the distributed power.

charge controller: A device that controls the charging rate and/or state of charge for batteries.

charge rate: The current applied to a cell or battery to restore its capacity.

chemical vapour deposition (CVD): A method of depositing thin semiconductor films. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

cleavage of lateral epitaxial films for transfer (CLEFT): A process for making inexpensive cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

combined collector: A photovoltaic device or module that provides useful heat energy in addition to electricity.

compact fluorescent lights: Lights that use a lot less energy than regular incandescent light bulbs. There are two main parts in a CFL: the gas-filled tube (also called bulb or burner) and the magnetic or electronic ballast. An electrical current from the ballast flows through the gas (mercury vapour), causing it to emit ultraviolet light. The ultraviolet light then excites a phosphor coating on the inside of the tube. This coating emits visible light.

concentrator: A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use only the direct sunlight because the diffuse portion can not be focused onto the PV cells.

conversion efficiency: The ratio of the electric energy produced by a photovoltaic device (under full sun condition) to the energy from sunlight incident upon the cell.

copper indium diselenide (CuInSe2, or CIS)A polycrystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).

crystalline silicon: A type of PV cell made from a single crystal or polycrystalline slice of silicon.

current: The flow of electric charge in a conductor between two points having a difference in potential (voltage).

current at maximum power (lmp): The current at which maximum power is available from a module. [UL 1703]

cycle life: Number of discharge-charge cycles that a battery can tolerate under specified conditions before it fails to meet specified criteria as to performance (e.g., capacity decreases to 80% of the nominal capacity).

Czochralski process: A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

D

days of autonomy: The number if consecutive days a stand-alone system battery bank will meet a defined load without solar energy input.

DC to DC converter: Electronic circuit to convert DC voltages (e.g.,PV module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker (MPPT).

deep cycle battery: Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.

deep discharge: Discharging a battery to 50% or less of it's full charge.

depth of discharge: The amount of ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity.

design month: The month having the combination of insolation and load that requires the maximum energy from the array.

diffuse isolation: Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

diode: Electronic component that allow current flow in one direction only.

direct current (DC): Electric current in which electrons flow in one direction only. Opposite of alternating current.

direct insolation: Full sunlight falling directly upon a collector. Opposite of the alternating current.

discharge rate: The rate, usually expressed in amperes over time, at which electrical current is taken from the battery.

disconnect: Switch gear used to connect or disconnect components of a PV system for safety or service.

distributed power: Generic term for any power supply located near the point where the power is used. Opposite of central power.

dopant: A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant introduces more electron vacancies.

doping: The addition of dopants to a semiconductor.

duty cycle:The ratio of active time to total time. Used to describe the operating regime of appliances or loads.

E

days of autonomy: The number if consecutive days a stand-alone system battery bank will meet a defined load without solar energy input.

DC to DC converter: Electronic circuit to convert DC voltages (e.g.,PV module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker (MPPT).

deep cycle battery: Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.

deep discharge: Discharging a battery to 50% or less of it's full charge.

depth of discharge: The amount of ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity.

design month: The month having the combination of insolation and load that requires the maximum energy from the array.

diffuse isolation: Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

diode: Electronic component that allow current flow in one direction only.

direct current (DC): Electric current in which electrons flow in one direction only. Opposite of alternating current.

direct insolation: Full sunlight falling directly upon a collector. Opposite of the alternating current.

discharge rate: The rate, usually expressed in amperes over time, at which electrical current is taken from the battery.

disconnect: Switch gear used to connect or disconnect components of a PV system for safety or service.

distributed power: Generic term for any power supply located near the point where the power is used. Opposite of central power.

dopant: A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant introduces more electron vacancies.

doping: The addition of dopants to a semiconductor.

duty cycle:The ratio of active time to total time. Used to describe the operating regime of appliances or loads.

F - G

fill factor: The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.

flat-plate PV: Refers to a PV array of module that consists of non-concentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is in a fixed position, some portion of the direct sunlight is lost because of oblique sun angles in relation to the array.

float charge: Float charge is the voltage required to counteract the self-discharge of the battery at a certain temperature.

float life:Number of years that a battery can keep its stated capacity when it is kept at float charge.

fossil fuels: Fuels formed in the ground from the remains of dead plants and animals. It takes millions of years to form fossil fuels. Oil, natural gas, and coal are fossil fuels.

fuel: Any material that can be burned to make energy.

gassing current: Portion of charge current that goes into electrolytic production of hydrogen and oxygen from the electrolytic liquid in the battery. This current increases with increasing voltage and temperature.

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

gigawatt: One billion watts. One million kilowatts. One thousand megawatts.

glazing: Clear materials (such as glass or plastic) that allow sunlight to pass into solar collectors and solar buildings, trapping heat inside.

grain boundaries: The boundaries where crystallites in a multi-crystalline material meet.

grid: See 'Electrical grid'.

grid-connected: A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

grid-interactive: See 'grid-connected (PV system).'

H - K

hybrid system: A PV system that includes other sources of electricity generation, such as wind, propane, or gas generators.

incident light: Light that shines onto the surface of a solar cell or module.

infrared radiation: Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers.

insolation: Sunlight, direct or diffuse; from 'incident solar radiation.' Usually expressed in watts per square meter. Not to be confused with insulation.

insulation: Materials that reduce the rate of heat loss or slow down the movement of heat.

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

inverters: Devices that convert DC electricity into AC electricity (single or multiphase), either for stand-alone systems (not connected to the grid) or for utility-interactive systems.

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 condition. Typically measured at 1000 watts per square meter of solar insolation at a specific cell temperature. The shape of the curve characterizes cell performance.

junction box:An electrical box designed to be a safe enclosure in which to make proper electrical connections. On PV modules this is where PV strings are electrically connected.

kilowatt (kW): 1000 watts.

kilowatt-hour: One thousand watt hours. The kWh is a unit of energy. 1kWh = 3600 kilo Joules (kJ).

L - M

life cycle cost: An estimate of the cost of owning and operating a system for the period of its useful life; usually expressed in terms of the present value of all lifetime costs.

line-commutated inverter: An inverter that is tied to a power grid or line. The commutation of power (conversion from DC to AC) is controlled by the power line, so that, if there is a failure in the power grid, PV system can not feed power to the line.

load: Anything in an electrical circuit that, when the circuit is turned on, draws power from that circuit.

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 0.45V.

maximum power point tracker (MPPT): Means of a power conditioning unit that automatically operates the PV generator at its MPP under all conditions.

megawatt: One million watts; 1000 kilowatts.

module: See 'photovoltaic module'.

multicrystalline: Material that is solidified at such a rate that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged, whereas crystallites are jumbled together. These numerous grain boundaries reduce the device efficiency. A material composed of variously oriented, small individual crystals. (Sometimes referred to as polycrystalline or semi-crystalline.)

N - O

NEC:An abbreviation for National Electrical Code, which contains safety guidelines and required practices for all types of electrical installations. Article 690 pertains to solar photovoltaic systems.

nominal operating cell temperature (NOCT): The reference cell (module) operating temperature presented on the manufacturer's literature. Generally the NOCT is referenced as 25°C, 77°F.

nominal voltage: A reference voltage used to describe batteries, modules, or systems (ie. a 12-, 24-,or 48-volt battery, module or system.)

nonrenewable energy: Fuels that cannot be easily made or "renewed". We can use up nonrenewable energy. Oil, natural gas, and coal are nonrenewable fuels.

n-type semiconductor: A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity, for example phosphorous in silicon.

ohm: The unit of resistance to the flow of an electric current.

one-axis tracking: A system capable of rotating about one axis, also referred to a single axis. These tracking systems usually follow the sun from east to west throughout the day.

open-circuit voltage: The maximum possible voltage a photovoltaic cell or module; the voltage across the cell in sunlight when no current is flowing.

orientation:Placement according to the compass directions, north, south, east, west. The current position of a device in relation to a known reference point.

P

panel: See 'photovoltaic panel.'

parallel connection: A way of joining two or more electricity-producing devices such as PV cells or modules, or batteries by connecting positive leads together and negative leads together; such a configuration increases the current but the voltage is unchanged.

passive solar building: A building that utilizes non-mechanical, non-electrical methods for heating, cooling, and/or lighting.

peak load; peak demand: The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.

peak power: Power generated by a utility unit that operates at a very low capacity factor; generally used to meet short-lived and variable high demand periods (burst traffic).

peak sun hours: The equivalent number of hours per day when solar irradiance averages 1000 w/m2 (full sun).

phosphorous (P): A chemical element,atomic number 15, used as a dopant in making n-semiconductor layers.

photon: A particle of light that acts as an individual unit of energy. When a photon hits the surface of semiconductor material, it excites an electron from the valance band of the atom into the stream of electrons in a current flow, thereby producing electricity.

photovoltaic (PV): Pertaining to the direct conversion of photons of sunlight into electricity.

photovoltaic 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 cell:The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (DC voltage and current).

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

photovoltaic 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 DC 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 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 peak watt: Maximum "rated" output of a cell,module, or system.Typical rating conditions are 0.645 watts per square inch (1000 watts per square meter of sunlight, 68 degrees F (20 degrees C) ambient air temperature and 6.2 x 10 to the minus 3 mi/s (1 m/s) wind speed.

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

physical vapor deposition: A method of depositing thin semiconductor 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/n junction: The interface between p-doped and n-doped semiconductor material. A diode is a p/n junction.

polycrystalline: See multi-polycrystalline.

power conditioning equipment: Electrical devices 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 factor: The ratio of the average power and the apparent volt-amperes.

pulse-width-modulated wave inverter (PWM): inverters that are the most expensive, but produce a high quality of output signal at minimum current harmonics. The output voltage is very close to sinusoidal.

PV: Abbreviation for photovoltaic.

P-type silicon: Semiconductor grade silicon doped with the element boron giving it a positive bias.

R - O

quad: A measure of energy equal to one trillion BTUs; an energy equivalent to approximately 172 million barrels of oil.

qualification test: A procedure applied to a selected set of PV 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.

rectifier: A device that converts AC to DC. See "inverter".

remote site: A location which is not near the grid.

resistance (R): The property of a semiconductor which opposes the flow of electric current resulting in the generation of heat in the conducting material. The unit of resistance is called ohms.

S

satellite power systems (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 dislenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

semicrystalline: See 'multicrystalline'.

series connection: A way of joining electrical equipment by connecting positive leads to negative leads; such a configuration increases the voltage while maintaining the same current.

series regulator: Type of battery charge regulator where the charging current is controlled by a switch connected in series with the PV module or array.

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

short circuit current (Isc): The current flowing freely from a photovoltaic cell through an external circuit that has no load resistance; the maximum current possible.

shunt regulator: Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the PV generator. Overcharging of the battery is prevented by shorting the PV generator.

silicon (Si): A chemical element, atomic number 14, semi-metallic in nature, dark gray, an excellent semiconductor material. A common constituent of sand and quartz (as the oxide). Crystallizes in face-centered cubic lattice-like a diamond. The most common semiconductor material used in making photovoltaic devices.

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.

solar cell: See 'photovoltaic' cell.

solar constant: The strength of sunlight; 1353 watts per square meter in space and about 1000 watts per square meter at sea level at the equator at solar noon.

solar energy: Energy from the sun. For example, the heat that builds up in your car when the windows are closed is solar energy.

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

solar noon: That moment of the day that divides the daylight hours for that day exactly in half. To determine solar noon, calculate the length of the day from the time of sunset and sunrise and divide by two. The moment the sun is highest in the sky.

solar spectrum: The total distribution of electromagnetic radiation emanating from the sun.

solar thermal electric: Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbo-generator.

square wave inverter: The inverter consists of a DC 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. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.

Staebler-Wronski effect: The tendency of amorphous silicon photovoltaic devices to lose efficiency upon initial exposure to light; named for Dr. David Staebler and Dr. Christopher Wronski; work performed at RCA.

stand-alone: An autonomous or hybrid photovoltaic system not connected to a grid. Some stand-alone systems require batteries or some other kind of storage. Also called "stand-alone PV systems".

Stand-off mounting:

standard reporting conditions (SRC): A fixed set of conditions (including meterological) to which the elctrical performance data of a photovoltaic module is translated from the set of actual conditions.

standard test conditions (STC)Conditions under which the module is typically tested in a laboratory: (1) Irradiance intensity of 1000 W/square meter (0.645 watts per square inch), (2) AM1.5 solar reference spectrum,and(3) a cell (module) temperature of 25°C, plus or minus 2°C, (77°F, plus or minus 3.6°F).

state of charge: The available capacity remaining in a cell or battery, expressed as a percentage of the rated capacity. For example, if 25 amp-hours have been removed from a fully charged 100 amp-hour cell, the state of charge is 75 per cent.

substrate: The physical material upon which a photovoltaic cell is made.

sulfation: A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery unchargeable.

superconductivity The pairing of electrons in certain materials that, when cooled below a critical temperature, cause the material to loose all resistance to electricity flow. Superconductors can carry electric current without any energy loss.

superstrate: The covering on the sun side of a 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: The momentary start-up condition of a motor requiring a large amount of of electrical current.

surge capacity: The ability of an inverter or generator to deliver high currents momentarily required when starting a motor.

T - Z

Temperature compensation: An allowance made in charge controller set points for charging battery temperatures.

thermal electric: Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbo-generator. See also 'solar thermal electric'.

thermal mass: Materials,typically masonry, that store heat in a passive solar home.

thin film: A layer of semiconductor material,such as copper indium diselnide, cadmium telluride, gallium arsenide, or amorphous silicon, a few microns or less in thickness, used to make photovoltaic cells.

tilt angle: Angle of inclination of collector as measured in degrees from the horizontal. For maximum performance solar collectors/modules should be set at a perpendicular to the sun.

total harmonic distortion:The measure of closeness in shape between a waveform and its fundamental component.

tracking PV array: 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 use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

transformer: An electromagnetic device used to convert AC electricity, either to increase or decrease the voltage.

transmission lines:Conductors used to transmit high-voltage electricity from the transformer to the electric distribution system.

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

two-axis tracking: A system capable of rotating independently about two axes and following the sun's orientation and height in the sky (e.g., vertical and horizontal).

ultraviolet (UV): Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

uninterpretable power supply: The designation of a power supply providing continuous service when a main power source is lost.

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

Vac: Volts alternating current.

Vdc:Volts direct current.

Voc:Volts open circuit.

vacuum deposition: Method of depositing thin coatings of a substance by heating it in vacuum system.

vacuum evaporation: The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

volt:A unit of measure of the force, or 'push,' given the electrons in an electric circuit. One volt produces one ampere of current when acting against a resistance of one ohm.

voltage at maximum power (Vmp): The voltage at which maximum power is available from a module.

wafer: A thin sheet of semiconductor material made by mechanically sawing it from a single-crystal or multi-crystal ingot or casting.

watt (W): The unit of electric power, or amount of work. One ampere of current flowing at a potential of one volt produces one watt of power.

watt-hour: A quantity of electrical energy when one watt is used for one hour.

waveform: The shape of the curve graphically representing the change in the AC signal voltage and current amplitude, with respect to time.

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