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Glossary
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 Hz).
altitude: The angel between the horizon (a horizontal plane) and the sun’s position in the sky; Measured in degrees.
ampere (A) or amp: The unit for electric current; the flow of electrons. One amp is produced by an electric force of 1 volt acting across a resistance of 1 ohm.
ampere-hour (Ah) : Quantity of electrical energy equal to the flow of one ampere of current for one hour. Typically used to quantify battery bank capacity.
anemometer: instrument that measures the force or velocity of wind; wind gauge.
angle of incidence : The angle at which direct sunlight strikes a surface; the angle between the direction of the sun and the perpendicular to the surface. Sunlight with an incident angle of 90 degrees tends to be absorbed, while lower angles tend to be reflected.
angle of inclination: The angle that a solar collector is positioned above horizontal.
array: Any number of PV 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. Also called off-grid.
balance of system (BOS): All system components and costs other than the PV modules. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.
base power: Power generated by a utility unit that operates at a very high capacity factor.
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battery capacity: The total number of ampere-hours that can be withdrawn from a fully charged cell or battery.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 a 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: Percentage of full charge or 100 percent minus the depth of discharge.
building-integrated photovoltaics (BIPV): A term for the design and integration of PV into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other façade material: into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading “eyebrows” over windows; or other building envelope systems.
capability: The maximum load that a generating unit, power plant, or other electrical apparatus can carry under specified conditions for a given period of time, without exceeding its approved limits of temperature and stress.
capacity factor: The amount of energy that the system produces to a particular site as a percentage of the total amount that is would produce 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 (i.e. a 6-volt battery has 3 2-volt cells).
cell barrier: A very thin region of static electric charge along with 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 the 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) fro sale to a number of users. Opposite of 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 available capacity.
circuit: A device, or system of devices, that allows electrical current to flow through it and allows voltage to occur across positive and negative terminals.
circuit breaker: A device used to interrupt or break an electrical circuit when an overload condition exists; usually installed in the positive circuit; used to protect electrical equipment.
compact fluorescent lights: A smaller version of standard fluorescent lamps which can directly replace standard incandescent lights. These lights consist of a gas filled tube, and a magnetic or electronic ballast. They use a lot less energy than regular light bulbs.
connection charge: An amount paid by a customer for being connected to an electricity supplier’s transmission and distribution system.
conversion efficiency: The ratio of the electric energy produced by a photovoltaic device (under full sun conditions) to the energy from sunlight incident upon the cell.
current: The flow of electric charge in a conductor, measured in amperes.
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-percent of the normal capacity).
Darrius (wind) machine: A type of vertical-axis wind machine that has long, thin blades in the shape of loops connected to the top and bottom of the axle; often called an “eggbeater windmill.”
days of autonomy: The number of consecutive days a stand-alone system battery bank will meet a defined load without solar energy input.
DC to DC converter: Electric 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 percent or less of its full charge.
demand: The rate at which electricity is delivered to or by a system, part of a system, or piece of equipment expressed in kilowatts, kilovoltamperes, or other suitable unit, at a given instant or averaged over a specified period of time.
demand charge: A charge for the maximum rate at which energy is used during peak hours of a billing period. That part of a power provider service charged for on the basis of the possible demand as distinguished from the energy actually consumed.
deregulation: The process of changing regulatory policies and laws to increase competition among suppliers of commodities and services. The process of deregulating the electric power industry was initiated by the Energy Policy Act of 1992.
depth of discharge (DOD): 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 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.
diode: Electronic component that allows 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 diffuse insolation.
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. See ‘stand alone’; ‘remote site’
duty cycle: The ratio of active time to total time. Used to describe the operation regime of appliances or loads.
efficiency: The ratio of output power to input power. Expressed as a percent.
electric circuit: Path followed by electrons from a power source (generator or battery) through an external line (including devices that use the electricity) and returning through another line to the source.
electric current: A flow of electrons; electricity
electric grid: An integrated system of electricity distribution, usually covering a large area.
electrolyte: A liquid conductor of electricity in which flow of current takes place by migration of ions. The electrolyte for a lead-acid storage cell is an aqueous solution of sulfuric acid.
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.
electron volt: An energy unit equal to the energy an electron acquires when it passes through a potential difference of one volt: it is equal to 1.602 x 10-19 volt.
energy: The ability to do work. Stored energy becomes working energy when we use it.
energy audit: A survey that shows how much energy you use in your house, apartment, or business. It can indicate your most intensive energy consuming appliances and even identify heating and cooling leaks that will help you find ways to use less energy.
energy density: The ratio of energy available from a battery to its volume (Wh/1) or mass (Wh/kg).
energy efficient mortgages (EEM): A type of home mortgage that takes into account the energy savings of a home that has cost-effective energy saving improvements that will reduce energy costs thereby allowing the homeowner to more income to the mortgage payment. A borrower can qualify for a larger loan amount than otherwise would be possible.
energy efficiency ratio (EER): The measure of the instantaneous energy efficiency of room air conditioners; the cooling capacity in Btu/hr divided by the watts of power consumed at a specific outdoor temperature (usually 95 degrees Fahrenheit).
energy pay back time: The time required for any energy producing system or device to produce as much energy as was required in its manufacture.
equalization: The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short period to “refresh” cell capacity.
fill factor: The ratio of a photovoltaic cell’s actual power to its power it both current and voltage were at their maxima. A key characteristic in evaluation cell performance.
flat-plate PV: Refers to 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: Number of years that a battery can keep its stated capacity when it is kept at float charge (see float charge).
fluorescent light: The conversion of electric power to visible light by using an electric charge to excite gaseous atoms in a glass tube. These atoms emit ultraviolet radiation that is absorbed by a phosphor coating on the walls of the lamp tube. The phosphor coating produces visible light.
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 examples.
frequency: The number of cycles through which an alternating current passes per second; in the U.S. the standard for electricity generation is 60 cycles per second (60 Hertz).
fuel: Any material that can be burned to make energy.
fuel cell: An electrochemical device that converts chemical energy directly into electricity.
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).
fuse: A safety device consisting of a short length of relatively fine wire, mounted in a holder or contained in a cartridge and connected as part of an electrical circuit. If the circuit source current exceeds a predetermined value, the fuse wire melts (i.e. the fuse ‘blows’) breaking the circuit and preventing damage to the circuit protected by the fuse.
gel-type battery: Lead-acid battery in which the electrolyte is composed of a silica gel matrix.
generator: A device for converting mechanical energy to electrical energy.
geothermal energy: Energy produced by the internal heat of the earth; geothermal heat sources include: hydrothermal convective systems; pressurized water reservoirs; hot dry rocks; manual gradients; and magma. Geothermal energy can be used directly for heating or to produce electric power.
gigawatt (GW): One billion watts; One million kilowatts; One thousand megawatts.
green certificates: Green certificates represent the environmental attributes of power produced from renewable resources. By separating the environmental attributes from the power, clean power generators are able to sell the electricity they produce to power providers at a competitive market value. The additional revenue generated by the sale of the green certificates covers the above-market costs associated with producing power made from renewable energy sources. Also known as green tags, renewable energy certificates, or tradable renewable certificates.
greenhouse effect: A popular term used to describe the heating effect due to the trapping of long wave (length) radiation by greenhouse gases produced from natural and human sources.
greenhouse gases: Those gases, such as water vapor, carbon dioxide, tropospheric ozone, methane, and low level ozone that are transparent to solar radiation, but opaque to long wave radiation, and which contribute to the greenhouse effect.
green power: A popular term for energy produced from clean, renewable energy resources.
green pricing: A practice engaged in by some regulated utilities (i.e. power providers) where electricity produced from clean, renewable resources is sold at a higher cost than that produced from fossil or nuclear power plants, supposedly because some buyers are willing to pay a premium for clean power.
grid: See electrical grid.
grid-connected: Independent power systems that are connected to an electricity transmission and distribution system (referred to as the electricity grid) such that the systems can draw on the grid’s reserve capacity in times of need, and feed electricity back into the grid during times of excess production.
ground: A device used to protect the user of any electrical system or appliance from shock.
horizontal-axis wind turbines: Turbines in which the axis of the rotor’s rotation is parallel to the wind stream and the ground.
Horsepower (hp): A unit of rate of operation. Electrical hp: a measure of time rate of mechanical energy output; usually applied to electric motors as the maximum output; 1 electrical hp is equal to 0.746 kilowatts or 2,545 Btu per hour. Shaft hp: a measure of the actual mechanical energy per unit time delivered to a turning shaft; 1 shaft Hp is equal to 1 electrical Hp or 550 foot pounds per second. Boiler Hp: a measure to the maximum rate to heat output of a steam generator; 1 boiler Hp is equal to 33,480 Btu per hour steam output.
hybrid system: A PV system that includes other sources of electricity generation, such as wind or fossil fuel generators.
hydroelectric power plant: A power plant that produces electricity by the force of water falling through a hydro turbine that spins a generator.
impulse turbine: A turbine that is driven by high velocity jets of water or steam from a nozzle directed to vanes or buckets attached to a wheel. (A pelton wheel is an impulse hydro turbine).
incandescent: These lights use an electrically heated filament to produce light in a vacuum or inert gas-filled bulb.
incident light: Light that shines onto the surface of a solar cell or module.
insolation: Sunlight, direct or diffuse; from “incident solar radiation.’ Usually expressed in watts per square meter or Btu per square foot per hour. Not to be confused with ‘insulation’.
insulation: Materials that reduce the rate or slow down the movement of heat.
insulation blanket: A pre-cut layer of insulation applied around a water heater storage tank to reduce stand-by heat loss from the tank.
insulator: A device or material with a high resistance to electricity flow.
interconnect: A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells.
interconnection: A connection or link between power systems that enables them to draw on each other’s reserve capacity in time of need.
inverters: Devices that convert DC energy into AC electricity (single or multiphase), either for stand-alone systems (not connected to the grid) or for utility-interactive systems.
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.
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 (kWh) 
ne thousand watts hour is a unit of energy.
life cycle cost: An estimate of the cost of owning and operating a system for a period of its useful life; usually expressed in terms of the present value of all lifetime costs.
line-connected 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, the PV system cannot feed power into the line.
load: Anything in an electrical circuit that, when the circuit is turned on, draws power from the circuit.
mean power output (of a Wind Turbine): The average power output of a wind energy conversion system at a given mean wind speed based on a Raleigh frequency distribution.
mean wind speed: The arithmetic wind speed over a specified time period and height above the ground (the majority of U.S. National Weather Service anemometers are at 20 feet (6.1 meters).
megawatt (MW): One million watts; 1000 kilowatts
module: See ‘photovoltaic module’
NEC: An abbreviation for the National Electrical Code which contains safety guidelines and required practices for all types of electrical installations. Article 690 pertains to solar photovoltaic systems.
natural gas: A hydrocarbon gas obtained from underground sources, often in association with petroleum and coal deposits. It generally contains a high percentage of methane, varying amounts of ethane, and inert gases; used as a heating fuel.
net metering: The practice of using a single meter to measure consumption and generation of electricity by a small generation facility (such as a house with a wind or solar photovoltaic system). The net energy produced or consumed is purchased from or sold to the power provider, respectively.
nominal operating cell temperature (NOCT): The reference cell (module) operating temperature presented on manufacturer’s literature. Generally the NOCT is referenced at
25 oC, 77o F.
nominal voltage: A reference voltage used to describe batteries, modules, or systems (i.e. A 12-, 24-, or 48-volt battery, module, or system).
nonrenewable fuels: Fuels that cannot be easily made or “renewed”. We can use up nonrenewable fuels. Oil, natural gas, and coal are examples.
off-peak: The period of low energy demand, as opposed to maximum, or peak, demand.
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 as a single axis. These tracking systems usually follow the sun from east to west throughout the day.
open-circuit voltage (VOC): The maximum possible voltage across a photovoltaic cell or module; the voltage across the cell in sunlight when no current is flowing.
orientation: The alignment of a building along a given axis to face a specific geographical direction. The alignment of a solar collector, in number of degrees east or west of true south
panel: See ‘photovoltaic module’.
panemone: A drag-type wind machine that can react to wind from any direction.
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 constant.
passive solar building: A building that utilizes non-mechanical, non-electrical methods for heating, cooling and/or lighting.
payback period: The amount of time required before the savings resulting from your system equal the system cost.
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.
peak sun hours: The equivalent number of hours per day when solar irradiance averages 1000 w/m2 (full sun). 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 kW/m2.
peak wind speed: The maximum instantaneous wind speed (or velocity) that occurs within a specific period of time or interval.
phase: Alternating current is carried by conductors and a ground to residential, commercial, or industrial consumers. The waveform of the phase power appears as a single continuous sine wave at the system frequency whose amplitude is the rated voltage of the power.
photon: A particle of light that acts as an individual unit of energy
photovoltaic (PV): Pertaining to the direct conversion of photons of sunlight to 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 or panel: 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 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 10-3 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.
power coefficient: The ratio of power produced by a wind energy conversion device to the power in a reference area of the free wind stream.
power provider: A company or other organizational unit that sells and distributes electrical power (e.g., private or public electrical utility), either to other distribution and wholesale businesses or to end-users. Sometimes power providers also generate the power they sell.
propeller (hydro) turbine: A turbine that has a runner with attached blades similar to a propeller used to drive a ship. As water passes over the curved propeller blades, it causes rotation of the shaft.
quad: A measure of energy equal to One quadrillion Btu. (1,000,000,000,000,000 Btu); an energy equivalent to approximately 172 million barrels of oil.
rate schedule: A mechanism used by electric utilities to determine prices for electricity; typically defines rates according to amounts of power demanded/consumed during specific time periods.
renewable energy: Energy derived from resources that are regenerative or for all practical purposes can not be depleted. Types of renewable energy resources include moving water (hydro, tidal and wave power), thermal gradients in ocean water, biomass, geothermal energy, solar energy, and wind energy. Municipal solid waste (MSW) is also considered to be a renewable energy resource.
resistance (R): The inherent characteristic of a material to inhibit the transfer of energy. In electrical conductors, electrical resistance results in the generation of heat. Electrical resistance is measured in Ohms. The heat transfer resistance properties of insulation products are quantified as the R-value.
R-factor/R-valve: A measure of the capacity of a material to resist heat transfer. The R-Value is the reciprocal of the conductivity of a material (U-Value). The larger the R-Value of a material, the greater its insulating properties.
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.
series: A configuration of an electrical circuit in which the positive lead is connected to the negative lead of another energy producing, conducting, or consuming device. The voltages of each device are additive, whereas the current is not.
series connection: A way of joining electrical equipment by connecting positive leads to negative leads; such a configuration increases the voltage while current remains the same.
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.
series resistance: Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.
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: An electric current taking a shorter or different path than intended
short-circuit current (Isc): The current flowing freely from a photovoltaic cell through an external circuit that has no load or 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 the battery is prevented by shorting the PV generator.
single-phase: A generator with a single armature coil, which may have many turns and the alternating current output consists of a succession of cycles.
solar cell: See photovoltaic
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 in solar energy.
solar gain: The amount of energy that a building absorbs due to solar energy striking its exterior and conducting to the interior or passing through windows and being absorbed by materials in the building.
solar noon: That movement 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.
power purchasing agreement (SPPA): A creative way to finance a solar system. An investor or solar service provider pays all the upfront costs associated with installing a solar powered system. They are responsible for everything associated with the production, output, and maintenance of the system for a specified length of time. These agreements normaly last between 15 to 20 years. At the end of that time, the property owner, where the system is located, my purchase the system or renegotiate another agreement. These are more commonly seen in large coperations, schools, etc. During the agreement time, the property owner purchases the solar power from the investor for a pre-determined rate.
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 turbogenerator.
stand-alone: An autonomous or hybrid photovoltaic system not connected to the grid. Some stand-alone systems require batteries or some other form of storage.
stand-by power: For the consumer, this is the electricity that is used by your TVs, stereos, and other electronic devices that use remote controls. When you press “off” to turn off your device, minimal power (dormant mode) is still being used to maintain the internal electronics in a ready, quick-response mode. This way, your device can be turned on with your remote control and be immediately ready to operate.
stand-off mounting: Technique for mounting a PV array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle. This promotes air flow to cool the modules.
state of charge (SOC): 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 percent.
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.
superconductivity: The pairing of electrons in certain materials that, when cooled below a critical temperature, cause the material to lose all resistance to electricity flow. Superconductors can carry electric current without any energy loss.
superstrate: The covering on the sun side of the 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 electrical current.
surge capacity: The ability of an inverter or generator to deliver high currents momentarily required when starting a motor.
temperature compensation: An allowance made in charge controller set points for changing battery temperatures.
thermal electric: Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbogenerator. See also ‘solar thermal electric’.
thermal mass: Materials, typically masonry, that store heat in a passive solar home.
tilt angle: The angle at which a solar collector or module 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 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 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 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 a low 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 axis and following the sun’s orientation and height in the sky (e.g., vertical and horizontal).
true south: The direction, at any point on the earth that is geographically in the northern hemisphere, facing toward the South Pole of the earth. Essentially a line extending from the point on the horizon to the highest point that the sun reaches on any day (solar noon) in the sky.
turbine: A device for converting the flow of a fluid (air, steam, water, or hot gases) into mechanical motion.
uninterruptible power supply (UPS): The designation of a power supply providing continuous uninterruptible service when a main power source is lost.
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 PV systems’ output is fully synchronized with the utility.
Vac: Volts AC
Vds: Volts DC
vertical-axis wind turbine (VAWT): A type of wind turbine in which the axis of rotation is perpendicular to the wind stream and the ground.
Voc: Open-circuit voltage
volt (V): 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: The amount of electromotive force, measured in volts, that exists between two points.
volt-ampere: A unit of electrical measurement equal to the product of a volt and an ampere.
voltage at maximum power (Vmp): The voltage at which maximum power is available from a module.
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 (Wh): A quantity of electrical energy when one watt is used for one hour.
wind energy: Energy available from the movement of the wind across a landscape caused by the heating of the atmosphere, earth, and oceans by the sun.
wind energy conversion system (WECS) or device: An apparatus for converting the energy available in the wind to mechanical energy that can be used to power machinery (grain mills, water pumps) and to operate an electrical generator.
wind generator: A WECS designed to produce electricity.
windmill: A WECS that is used to grind grain, and that typically has a high-solidity rotor; commonly used to refer to all types of WECS.
wind power plant: A group of wind turbines interconnected to a common power provider system through a system of transformers, distribution lines, and (usually) one substation. Operation, control, and maintenance functions are often centralized through a network of computerized monitoring systems, supplemented by visual inspection.
wind power profile: The change in the power available in the wind due to changes in the wind speed or velocity profile; the wind power profile is proportional to the cube of the wind speed profile.
wind resource assessment: The process of characterizing the wind resource, and its energy potential, for a specific site or geographical area.
wind rose: A diagram that indicates the average percentage of time that the wind blows from different directions, on a monthly or annual basis.
wind speed: The rate of flow of the wind undisturbed by obstacles.
wind speed duration curve: A graph that indicates the distribution of wind speeds as a function of the cumulative number of hours that the wind speed exceeds a given wind speed in a year.
wind speed frequency curve: A curve that indicates the number of hours per year that specific wind speeds occur.
wind speed profile: A profile of how the wind speed changes with height above the surface of the ground or water.
wind turbine: A term used for a wind energy conversion device that produces electricity; typically having one, two, or three blades.
wind turbine rated capacity: The amount of power a wind turbine can produce at its rated wind speed, e.g., 100 kW at 20 mph. The rated wind speed generally corresponds to the point at which the conversion efficiency is near its maximum. Because of the variability of the wind, the amount of energy a wind turbine actually produces is a function of the capacity factor (e.g., a wind turbine produces 20% to 35% of its rated capacity over a year).
wind velocity: The wind speed and direction in an undisturbed flow.