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A
Absorber
In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).
Adjustable Set Point
A feature allowing the user to adjust the voltage levels at which a charge controller will become active.
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.
Acceptor
A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.
Air mass (sometimes called air mass ratio)
Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).
Amorphous Silicon
A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.
Amperage Interrupt Capability (AIC)
direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.
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 Meter
An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).
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.
Antireflection Coating
A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.
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.
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.
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B
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.
Battery
Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.
Battery Cycle 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.
Boron (B)
The chemical element commonly used as the dopant in photovoltaic device or cell material.
Bypass Diode
A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. 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.
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C
Cadmium (Cd)
A chemical element used in making certain types of solar cells and batteries.
Cadmium Telluride (CdTe)
A polycrystalline thin-film photovoltaic material.
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.
Charge
The process of adding electrical energy to a battery.
Charge Controller
A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.
Charge Factor
A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.
Charge Rate
The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.
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 (Fresnel lens) 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.
Contact Resistance
The resistance between metallic contacts and the semiconductor.
Converter
A unit that converts a direct current (dc) voltage to another dc voltage.
Copper Indium Diselenide (CuInSe2, or CIS)
A polycrystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).
Crystalline Silicon
A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.
Current at Maximum Power (Imp)
The current at which maximum power is available from a module.
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D
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.
Deep-Cycle Battery
A battery with large plates that can withstand many discharges to a low state-of-charge.
Deep Discharge
Discharging a battery to 20% or less of its full charge capacity.
Depth of Discharge (DOD)
The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.
Depletion Zone
Same as cell barrier. The term derives from the fact that this microscopically thin region is depleted of charge carriers (free electrons and hole).
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.
Diode
An electronic device that allows current to flow in one direction only. See blocking diode and bypass diode.
Direct Insolation
Sunlight falling directly upon a collector. Opposite of diffuse insolation.
Discharge
The withdrawal of electrical energy from a battery.
Discharge Factor
A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours. Related to discharge rate.
Disconnect
Switchgear used to connect or disconnect components in a photovoltaic system.
Distributed Power
Generic term for any power supply located near the point where the power is used. Opposite of central power.
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.
Donor
In a photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.
Donor Level
The level that donates conduction electrons to the system.
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 creates electron vacancies (holes).
Doping
The addition of dopants to a semiconductor.
Downtime
Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
Duty Rating
The amount of time an inverter (power conditioning unit) can produce at full rated power.
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E
Edge-Defined Film-Fed Growth (EFG)
A method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upward by capillary action through a mold.
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.
Electrode
A conductor that is brought in conducting contact with a ground.
Electro deposition
Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.
Electrolyte
A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.
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.
Electron Volt (eV)
The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-19; a unit of energy or work.
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.
Energy Levels
The energy represented by an electron in the band model of a substance.
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).
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F
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.
Fixed Tilt Array
A photovoltaic array set in at a fixed angle with respect to horizontal.
Flat-Plate Array
A photovoltaic (PV) array that consists of non-concentrating PV modules.
Flat-Plate Module
An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.
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.
Float Charge
The voltage required to counteract the self-discharge of the battery at a certain temperature.
Float Life
The number of years that a battery can keep its stated capacity when it is kept at float charge.
Float-Zone Process
A method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes single crystal.
Frequency
The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).
Frequency Regulation
This indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed 1%.
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).
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G
Gallium (Ga)
Gel-Type Battery
Lead-acid battery in which the electrolyte is composed of a silica gel matrix.
Gigawatt (GW)
A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
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.
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H
Harmonic Content
The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.
High Voltage Disconnect
The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.
Hybrid System
A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.
Hydrogenated Amorphous Silicon
Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.
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I
Incident Light
Light that shines onto the face of a solar cell or module.
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.
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.
Intrinsic Semiconductor An undoped semiconductor.
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.
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.
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J
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 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.
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K
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.
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L
Langley (L)
Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.
Life
The period during which a system is capable of operating above a specified performance level.
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.
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.
Low Voltage Disconnect
The voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.
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M
Maintenance-Free Battery
sealed battery to which water cannot be added to maintain electrolyte level.
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.
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.
Modularity
The use of multiple inverters connected in parallel to service different loads.
Module Derate Factor
A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.
Multicrystalline
A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.
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N
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.
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O
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.
Overcharge
Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.
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P
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.
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.
Passivation
A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell's surface.
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 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, or reach 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.
Polycrystalline Silicon
A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon.
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.
Pulse-Width-Modulated (PWM) Wave Inverter
A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.
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.
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Q
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.
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R
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 duty rating.
Reactive Power
The sine of the phase angle between the current and voltage waveforms in an alternating current system. See power factor.
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.
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 blocking diode.
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S
Sealed Battery
A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.
Seasonal Depth of Discharge
An adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.
Secondary Battery
A battery that can be recharged.
Self-Discharge
The rate at which a battery, without a load, will lose its charge.
Series Connection
A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.
Shallow-Cycle Battery
A battery with small plates that cannot withstand many discharges to a low state-of-charge.
Shelf Life of Batteries
The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.
Short-Circuit Current (Isc)
The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.
Shunt Controller
A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.
Shunt Regulator
Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.
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).
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.
Single-Crystal Silicon
Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.
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 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-Grade Silicon
Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.
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 Resource
The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.
Solar Spectrum
The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.
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.
Specific Gravity
The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.
Square Wave
A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.
Square Wave Inverter
A type of inverter that produces square wave output. It consists of a direct current 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.
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.
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.
Starved Electrolyte Cell
A battery containing little or no free fluid electrolyte.
State-of-Charge (SOC)
The available capacity remaining in the battery, expressed as a percentage of the rated capacity.
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).
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 extremely difficult to recharge.
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.
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T
Tare Loss
Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.
Temperature Compensation
A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.
Temperature Factors
It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.
Thick-Crystalline Materials
Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.
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.
Thin Film Photovoltaic Module
A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See amorphous silicon.
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.
Total Harmonic Distortion
The measure of closeness in shape between a waveform and it's fundamental component.
Total Internal Reflection
The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.
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).
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U
Ultraviolet
Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.
Uninterruptible Power Supply (UPS)
The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.
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.
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V
Vacuum Evaporation
The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.
Vacuum Zero
The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.
Valence Band
The highest energy band in a semiconductor that can be filled with electrons.
Varistor
A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.
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 Regulation
This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.
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W
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).
Waveform
The shape of the phase power at a certain frequency and amplitude.
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Z
Zenith Angle
The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).
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