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Solar Energy Glossary

Glossary: Solar Energy and Photovoltaics


One of the most significant "going green ideas" is to adapt the use of solar power into your everyday life. As a resource for going green the following "basic" glossary is provided. The source, the DOE Office of Energy Efficiency and Renewable Energy, is probably the most reliable you will find on the web. You can find a more comprehensive glossary, for those with significant expertise, in the source information at the end of this solar energy glossary.

Absorber -- In a photovoltaic device, the material that readily absorbs photons to generate charge carriers.

Alternating Current (AC) -- A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

Ambient Temperature -- The temperature of the surrounding area.

Ampere (amp) -- A unit of electrical current or rate of flow of electrons.

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.

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.

Anode -- The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.
Antireflection Coating -- A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

Array -- see photovoltaic (PV) array.

Array Operating Voltage -- The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

Autonomous System -- See stand-alone system.

Balance of System -- Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

Base Load -- The average amount of electric power that a utility must supply in any period.

Battery Capacity -- The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

Battery Energy Capacity -- The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery.

Battery Life -- The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

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.

Blocking Diode -- A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

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.

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.

Cadmium Telluride (CdTe) -- A polycrystalline thin-film photovoltaic material.

Cell (battery) -- A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). Also see photovoltaic (PV) cell.

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.

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.

Converter -- A unit that converts a direct current (dc) voltage to another dc voltage.

Crystalline Silicon -- A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

Cutoff Voltage -- The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.

Cycle -- The discharge and subsequent charge of a battery.

Days of Storage -- The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.

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.

Dendrite -- A slender threadlike spike of pure crystalline material, such as silicon.

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.

Direct Beam Radiation -- Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7 to transcribe the solar disc.

Direct Current (DC) -- A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.

Direct Insolation -- Sunlight falling directly upon a collector. Opposite of diffuse insolation.

Discharge -- The withdrawal of electrical energy from a battery.

Distributed Generation -- A popular term for localized or on-site power generation.

Distributed Power -- Generic term for any power supply located near the point where the power is used. Opposite of central power.

Distributed Systems -- Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

Downtime -- Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.

Duty Cycle -- The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.

Duty Rating -- The amount of time an inverter (power conditioning unit) can produce at full rated power.

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.

Electric Circuit -- The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.

Electric Current -- The flow of electrical energy (electricity) in a conductor, measured in amperes.

Electrical grid -- An integrated system of electricity distribution, usually covering a large area.

Electrode -- A conductor that is brought in conducting contact with a ground.

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.

Energy Audit -- A survey that shows how much energy used in a home, which helps find ways to use less energy.

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.

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).

Gassing -- The evolution of gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from local action self-discharge or from the electrolysis of water in the electrolyte during charging.

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.

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.

Incident Light -- Light that shines onto the face of a solar cell or module.

Input Voltage -- This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

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.

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.

Life -- The period during which a system is capable of operating above a specified performance level.

Life-Cycle Cost -- The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

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 electric_current required by the electrical device.

Load Resistance -- The resistance presented by the load.

Low Voltage Cutoff (LVC) -- The voltage level at which a charge controller will disconnect the load from the battery.

Low Voltage Disconnect -- The voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.

Maintenance-Free Battery -- A sealed battery to which water cannot be added to maintain electrolyte level.

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.

Microgroove -- A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

Multijunction Device -- A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.
Multi-Stage Controller -- A charging controller unit that allows different charging currents as the battery nears full state_of_charge.

National Electrical Code (NEC) -- Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, “Solar Photovoltaic Systems” which should be followed when installing a PV system.

Nickel Cadmium Battery -- A battery containing nickel and cadmium plates and an alkaline electrolyte.

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.

N-Type -- Negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons.

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.

Open-Circuit Voltage (Voc) -- The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

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.

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.

Peak Demand/Load -- The maximum energy demand or load in a specified time period.

Peak Power Point -- Operating point of the I-V (current-voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.

Peak 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.

Photoelectrochemical Cell -- A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

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.

Pocket Plate -- A plate for a battery in which active materials are held in a perforated metal pocket.

Power Conditioning -- The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating 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.

Projected Area -- The net south-facing glazing area projected on a vertical plane.

PV -- See photovoltaic(s).

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.

Rated Power -- Rated power of the inverter. However, some units can not produce rated power continuously. See duty rating.

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.

Reverse Current Protection -- Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See blocking diode.

Satellite Power System (SPS) -- Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

Secondary Battery -- A battery that can be recharged.

Self-Discharge -- The rate at which a battery, without a load, will lose its charge.

Semiconductor -- Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

Series Connection -- A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

Series Controller -- A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

Series Regulator -- Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

Short-Circuit Current (Isc) -- The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

Silicon (Si) -- A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It’s commonly found in sand and quartz (as the oxide).

Single-Stage Controller -- A charge controller that redirects all charging current as the battery nears full state-of-charge.

Solar Cooling -- The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers (”swamp” coolers), heat-pumps, and air conditioners.

Solar Energy -- Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

Solar Panel -- See photovoltaic (PV) panel.

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.

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.

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.

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).

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.

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 5C 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.

Thermophotovoltaic Cell (TPV) -- A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.

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.

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.

Trickle Charge -- A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

Underground Feeder (UF) -- May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

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.

Varistor -- A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

Vented Cell -- A battery designed with a vent mechanism to expel gases generated during charging.

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 Regulation -- This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

Wafer -- A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

Watt -- The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

Wet Shelf Life -- The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

Zenith Angle -- the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

Going Green Resources offers this link for a full glossary of terms associated with solar energy. Please visit

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