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Solar Glossary Solar Electricity

Solar Glossary Solar Electricity Solar Glossary Solar Hot Water


In a solar photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).

Absorption Coefficient
In a solar photovoltaic device, the factor by which photons are absorbed as they travel a unit distance through a material.

Active Power
The power (in Watts) used by a device to produce useful work. Also called input power.

A building material made from clay, straw, and water, formed into blocks, and dried; used traditionally in the southwestern U.S.

The ratio of light reflected by a surface to the light falling on it.

Alternating Current
A type of electrical current, the direction of which is reversed at regular intervals or cycles; in the U.S. the standard is 120 reversals or 60 cycles per second; typically abbreviated as AC.

A generator producing alternating current by the rotation of its rotor, and which is powered by a primary mover.

A unit of measure for an electrical current; the amount of current that flows in a circuit at an electromotive force of one Volt and at a resistance of one Ohm. Abbreviated as amp.

A measure of the flow of current (in amperes) over one hour.

An instrument for measuring the force or velocity of wind; a wind gauge.

Angle of Incidence
In reference to solar energy systems, 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
In reference to solar energy systems, the angle that a solar collector is positioned above horizontal.

Angstrom Unit
A unit of length named for A.J. Angstome, a Swedish spectroscopist, used in measuring electromagnetic radiation equal to 0.000,000,01 centimeters.

Annual Load Fraction
That fraction of annual energy demand supplied by a solar system.

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.

The positive pole or electrode of an electrolytic cell, vacuum tube, etc. (see also sacrificial anode).

An opening; in solar collectors, the area through which solar radiation is admitted and directed to the absorber.

Apparent Day
A solar day; an interval between successive transits of the sun's center across an observer's meridian; the time thus measured is not equal to clock time.

Apparent Power (kVA)
This is the voltage-ampere requirement of a device designed to convert electric energy to a non-electrical form.

A device for converting one form of energy or fuel into useful energy or work.

Appliance Energy Efficiency Ratings
The ratings under which specified appliances convert energy sources into useful energy, as determined by procedures established by the U.S. Department of Energy.

Appliance Standards
Standards established by the U.S. Congress for energy consuming appliances in the National Appliance Energy Conservation Act (NAECA) of 1987, and as amended in the National Appliance Energy Conservation Amendments of 1988, and the Energy Policy Act of 1992 (EPAct). NAECA established minimum standards of energy efficiency for refrigerators, refrigerator-freezers, freezers, room air conditioners, fluorescent lamp ballasts, incandescent reflector lamps, clothes dryers, clothes washers, dishwashers, kitchen ranges and ovens, pool heaters, television sets (withdrawn in 1995), and water heaters. The EPAct added standards for some fluorescent and incandescent reflector lamps, plumbing products, electric motors, and commercial water heaters and Heating, Ventilation, and Air Conditioning (HVAC) systems. It also allowed for the future development of standards for many other products. The U.S. Department of Energy (DOE) is responsible establishing the standards and the procedures that manufacturers must use to test their models. These procedures are published in the Code of Federal Regulations (10 CFR, Ch. II, Part 430), January 1, 1994 (Federal Register).

Array (Solar)
Any number of solar photovoltaic modules or solar thermal collectors or reflectors connected together to provide electrical or thermal energy.

Abbreviation for the American Society of Heating, Refrigeration, and Air-Conditioning Engineers.

Abbreviation for the American Society for Testing and Materials, which is responsible for the issue of many standard methods used in the energy industry.

Asynchronous Generator
A type of electric generator that produces alternating current that matches an existing power source.

Atmospheric Pressure
The pressure of the air at sea level; one standard atmosphere at zero degrees centigrade is equal to 14.695 pounds per square inch (1.033 kilograms per square centimeter).

An interior court to which rooms open.

The usually unfinished space above a ceiling and below a roof.

Attic Fan
A fan mounted on an attic wall used to exhaust warm attic air to the outside.

Attic Vent
A passive or mechanical device used to ventilate an attic space, primarily to reduce heat buildup and moisture condensation.

Audit (Energy)
The process of determining energy consumption, by various techniques, of a building or facility.

Automatic (or Remote) Meter Reading System
A system that records the consumption of electricity, gas, water, etc, and sends the data to a central data accumulation device.

Describes the reliability of power plants. It refers to the number of hours that a power plant is available to produce power divided by the total hours in a set time period, usually a year.

Average Demand
The demand on, or the power output of, an electrical system or any of its parts over an interval of time, as determined by the total number of kilowatt-hours divided by the units of time in the interval.

Average Cost
The total cost of production divided by the total quantity produced.

Average Wind Speed (or Velocity)
The mean wind speed over a specified period of time.

Avoided Cost
The incremental cost to an electric power producer to generate or purchase a unit of electricity or capacity or both.

Azimuth (Solar)
The angle between true south and the point on the horizon directly below the sun.

The abbreviation for American Wire Gauge; the standard for gauging the size of wires (electrical conductors).

An architectural element for shading windows and wall surfaces placed on the exterior of a building; can be fixed or movable.


Backup Energy System
A reserve appliance; for example, a stand-by generator for a home or commercial building.

In a renewable energy system, refers to all components other than the mechanism used to harvest the resource (such as photovoltaic panels or a wind turbine). Balance-of-system costs can include design, land, site preparation, system installation, support structures, power conditioning, operation and maintenance, and storage.

A device used to control the voltage in a fluorescent lamp.

Ballast Efficacy Factor
The measure of the efficiency of fluorescent lamp ballasts. It is the relative light output divided by the power input.

Ballast Factor
The ratio of light output of a fluorescent lamp operated on a ballast to the light output of a lamp operated on a standard or reference ballast.

Baseload Capacity
The power output of a power plant that can be continuously produced.

Baseload Demand
The minimum demand experienced by a power plant.

Baseload Power Plant
A power plant that is normally operated to generate a base load, and that usually operates at a constant load; examples include coal fired and nuclear fueled power plants.

Base Power
Power generated by a power generator that operates at a very high capacity factor.

An energy storage device composed of one or more electrolyte cells.

Battery Energy Storage
Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

Building Orientation
The relationship of a building to true south, as specified by the direction of its longest axis.

The transparent or opaque sphere in an electric light that the electric light transmits through.

Bus (electrical)
An electrical conductor that serves as a common connection for two or more electrical circuits; may be in the form of rigid bars or stranded conductors or cables.

The power conduit of an electric power plant; the starting point of the electric transmission system.

Busbar Cost
The cost of producing electricity up to the point of the power plant busbar.


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.

Capability Margin
The difference between net electrical system capability and system maximum load requirements (peak load); the margin of capability available to provide for scheduled maintenance, emergency outages, system operating requirements and unforeseen loads.

A measure of the electrical charge of a capacitor consisting of two plates separated by an insulating material.

An electrical device that adjusts the leading current of an applied alternating current to balance the lag of the circuit to provide a high power factor.

The load that a power generation unit or other electrical apparatus or heating unit is rated by the manufacture to be able to meet or supply.

Capacity Factor
The ratio of the average load on (or power output of) a generating unit or system to the capacity rating of the unit or system over a specified period of time.

Carbon Zinc Cell Battery
A cell produces electric energy by the galvanic oxidation of carbon; commonly used in household appliances.

The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

Cathode Disconnect Ballast
An electromagnetic ballast that disconnects a lamp's electrode heating circuit once is has started; often called "low frequency electronic" ballasts.

Cathodic Protection
A method of preventing oxidation of the exposed metal in structures by imposing between the structure and the ground a small electrical voltage.

A component of a electrochemical battery. A 'primary' cell consists of two dissimilar elements, known as 'electrodes,' immersed in a liquid or paste known as the 'electrolyte.' A direct current of 1-1.5 volts will be produced by this cell. A 'secondary' cell or accumulator is a similar design but is made useful by passing a direct current of correct strength through it in a certain direction. Each of these cells will produce 2 volts; a 12 volt car battery contains six cells.

Central Power Plant
A large power plant that generates power for distribution to multiple customers.  

Charge Controller
An electronic device that regulates the electrical charge stored in batteries so that unsafe, overcharge conditions for the batteries are avoided.

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.

Circuit Lag
As time increases from zero at the terminals of an inductor, the voltage comes to a particular value on the sine function curve ahead of the current. The voltage reaches its negative peak exactly 90 degrees before the current reaches its negative peak thus the current lags behind by 90 degrees.

Legal documents that regulate construction to protect the health, safety, and welfare of people. Codes establish minimum standards but do not guarantee efficiency or quality.

The process by which a power plant, apparatus, or building is approved for operation based on observed or measured operation that meets design specifications.

Compact Fluorescent
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.

Concentrating (Solar) Collector
A solar collector that uses reflective surfaces to concentrate sunlight onto a small area, where it is absorbed and converted to heat or, in the case of solar photovoltaic (PV) devices, into electricity. Concentrators can increase the power flux of sunlight hundreds of times. The principal types of concentrating collectors include: compound parabolic, parabolic trough, fixed reflector moving receiver, fixed receiver moving reflector, Fresnel lense, and central receiver. A PV concentrating module uses optical elements (Fresnel lense) to increase the amount of sunlight incident onto a PV cell. Concentrating PV modules/arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Concentrating collectors for home or small business solar water heating applications are usually parabolic troughs that concentrate the sun's energy on an absorber tube (called a receiver), which contains a heat-transfer fluid.

Conduction Band
An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

A tubular material used to encase and protect one or more electrical conductors.

 Connected Load
The sum of the ratings of the electricity consuming apparatus connected to a generating system.

Connection Charge
An amount paid by a customer for being connected to an electricity supplier's transmission and distribution system.

Conservation Cost Adjustment
A means of billing electric power consumers to pay for the costs of demand side management/energy conservation measures and programs. (See also Benefits Charge.)

Constant Dollars
The value or purchasing power of a dollar in a specified year carried forward or backward.

Consumption Charge
The part of a power provider's charge based on actual energy consumed by the customer; the product of the kilowatt-hour rate and the total kilowatt-hours consumed.

Contact Resistance
The resistance between metallic contacts and the semiconductor.

Conversion Efficiency
The amount of energy produced as a percentage of the amount of energy consumed.

A device for transforming the quality and quantity of electrical energy; also an inverter.

A unit for the quantity of electricity transported in 1 second by a current of 1 ampere.

Crystalline Silicon Photovoltaic Cell
A type of photovoltaic cell made from a single crystal or a polycrystalline slice of silicon. Crystalline silicon cells can be joined together to form a module (or panel).

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

Current Dollars
The value or purchasing power of a dollar that has not been reduced to a common basis of constant purchasing power, but instead reflects anticipated future inflation; when used in computations the assumed inflation rate must be stated.

Customer Charge
An amount to be paid for energy periodically by a customer without regard to demand or energy consumption.

Customer Class
Categories of energy consumers, as defined by consumption or demand levels, patterns, and conditions, and generally included residential, commercial, industrial, agricultural.

In alternating current, the current goes from zero potential or voltage to a maximum in one direction, back to zero, and then to a maximum potential or voltage in the other direction. The number of complete cycles per second determines the current frequency; in the U.S. the standard for alternating current is 60 cycles.


The angular position of the sun at solar noon with respect to the plane of the equator.

Declining Block Rate
An electricity supplier rate structure in which the per unit price of electricity decreases as the amount of energy increases. Normally only available to very large consumers.

The process of removing a power plant, apparatus, equipment, building, or facility from operation.

To disconnect a transmission and/or distribution line; a power line that is not carrying a current; to open a circuit.

Deep Discharge
Discharging a battery to 20 percent or less of its full charge capacity.

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.

Demand(ed) Factor
The ratio of the maximum demand on an electricity generating and distribution system to the total connected load on the system; usually expressed as a percentage.

Demand Power
see Peak Power

Demand-Side Management (DSM)
The process of managing the consumption of energy, generally to optimize available and planned generation resources.

Department of Energy (DOE)
A federal government agency created in 1977, that is entrusted to contribute to the welfare of the United States by providing technical information, and a scientific and educational foundation for technology, policy and institutional leadership to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense.

Dependable Capacity
The load-carrying ability of an electric power plant during a specific time interval and period when related to the characteristics of the load to be/being supplied; determined by capability, operating power factor, and the portion of the load the station is to supply.

The production of energy by a system or appliance at a level less than its design or nominal capacity.  

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. (See also Restructuring)

Design Voltage
The nominal voltage for which a conductor or electrical appliance is designed; the reference voltage for identification and not necessarily the precise voltage at which it operates.

Diffuse Solar Radiation
Sunlight scattered by atmospheric particles and gases so that it arrives at the earth's surface from all directions and cannot be focused.

An electronic device that allows current to flow in one direction only.

Direct Access
The ability of an electric power consumer to purchase electricity from a supplier of their choice without being physically inhibited by the owner of the electric distribution and transmission system to which the consumer is connected to. (See also Open Access.)

Direct Beam Radiation
Solar radiation that arrives in a straight line from the sun.

Direct Current
A type of electricity transmission and distribution by which electricity flows in one direction through the conductor; usually relatively low voltage and high current; typically abbreviated as dc.

Distributed Generation
A term used by the power industry to describe localized or on-site power generation.

The process of distributing electricity; usually defines that portion of a power provider's power lines between a power provider's power pole and transformer and a customer's point of connection/meter.

Distribution Feeder
(See Feeder)

Distribution Line
One or more circuits of a distribution system on the same line or poles or supporting structures' usually operating at a lower voltage relative to the transmission line.

Distribution System
That portion of an electricity supply system used to deliver electricity from points on the transmission system to consumers.


Electrical Energy
The energy of moving electrons.

Electrical Charge
A condition that results from an imbalance between the number of protons and the number of electrons in a substance.

Electrical System
All the conductors and electricity using devices that are connected to a source of electromotive force (or generator).

Electrical System Energy Losses
A measure of the amount of energy lost during the generation, transmission, and distribution of electricity.

Electric Circuit
The path followed by electrons from a generation source, through an electrical system, and returning to the source.

Electricity Generation
The process of producing electricity by transforming other forms or sources of energy into electrical energy; measured in kilowatt-hours.

Electricity Grid
A common term referring to an electricity transmission and distribution system.

Electric Rate
The unit price and quantity to which it applies as specified in a rate schedule or contract.

Electric Rate Schedule
A statement of the electric rate(s), terms, and conditions for electricity sale or supply.

Electric System
The physically connected generation, transmission, and distribution facilities and components operated as a unit.

Electric System Loss(es)
The total amount of electric energy loss in an electric system between the generation source and points of delivery.

Electric Power Transmission
The transmission of electricity through power lines.

Electric Utility
A corporation, person, agency, authority or other legal entity that owns and/or operates facilities for the generation, transmission, distribution or sale of electricity primarily for use by the public. Also known as a power provider.

Electrochemical Cell
A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.

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

Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.

A chemical change in a substance that results from the passage of an electric current through an electrolyte. The production of commercial hydrogen by separating the elements of water, hydrogen, and oxygen, by charging the water with an electrical current.

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.

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.

Electronic Ballast
A device that uses electronic components to regulate the voltage of fluorescent lamps.

Electron Volt
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^-12; a unit of energy or work; abbreviated as eV.

Ellipsoidal Reflector Lamp
A lamp where the light beam is focused 2 inches ahead of the lamp reducing the amount of light trapped in the fixture.

To send electricity through a electricity transmission and distribution network; a conductor or power line that is carrying current.

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 you use in your house or apartment. It will help you find ways to use less energy.

Energy Charge
That part of an electricity bill that is based on the amount of electrical energy consumed or supplied.

Energy Contribution Potential
Recombination occurring in the emitter region of a photovoltaic cell.

Energy Efficient Mortgages
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 Density
The ratio of available energy per pound; usually used to compare storage batteries.

Energy Intensity
The relative extent that energy is required for a process.

Energy Storage
The process of storing, or converting energy from one form to another, for later use; storage devices and systems include batteries, conventional and pumped storage hydroelectric, flywheels, compressed gas, and thermal mass.

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).


A unit of electrical capacitance; the capacitance of a capacitor between the plates of which there appears a difference of 1 Volt when it is charged by one coulomb of electricity.

Federal Energy Regulatory Commission (FERC)
This is an independent regulatory agency within the U.S. DOE that has jurisdiction over interstate electricity sales, wholesale electric rates, natural gas pricing, oil pipeline rates, and gas pipeline certification. It also licenses and inspects private, municipal, and state hydroelectric projects and oversees related environmental matters.

Federal Power Marketing Administrations (PMA)
These are separate and distinct organizational agencies within the U.S. DOE that market power at federal multipurpose water projects at lowest possible rates to consumers consistent with sound business principles. There are five PMA's: Alaska Power Administration, Bonneville Power Administration, Southeastern Power Administration, Southwestern Power Administration, Western Area Power Administration.

A power line for supplying electricity within a specified area.

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.

Metal, usually galvanized sheet metal, used to provide protection against infiltration of precipitation into a roof or exterior wall; usually placed around roof penetrations such as chimneys.

Flat Plate Solar Photovoltaic Module
An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.

Flat Roof
A slightly sloped roof, usually with a tar and gravel cover. Most commercial buildings use this kind of roof.

Float-Zone Process
In reference to solar photovoltaic cell manufacture, 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 a single crystal.

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.

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). 

Fresnel Lens
An optical device for concentrating light that is made of concentric rings that are faced at different angles so that light falling on any ring is focused to the same point.

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).

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.


Gallium Arsenide
A compound used to make certain types of solar photovoltaic cells.

The process in which a solid fuel is converted into a gas; also known as pyrolitic distillation or pyrolysis. Production of a clean fuel gas makes a wide variety of power options available.

A device for converting mechanical energy to electrical energy.

Gigawatt (GW)
A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.

The excessive brightness from a direct light source that makes it difficult to see what one wishes to see. A bright object in front of a dark background usually will cause glare. Bright lights reflecting off a television or computer screen or even a printed page produces glare. Intense light sources—such as bright incandescent lamps—are likely to produce more direct glare than large fluorescent lamps. However, glare is primarily the result of relative placement of light sources and the objects being viewed.

Global Insolation (or Solar Radiation)
The total diffuse and direct insolation on a horizontal surface, averaged over a specified period of time.

Global Warming
A popular term used to describe the increase in average global temperatures due to the greenhouse effect.

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.

A common term referring to an electricity transmission and distribution system.

Grid-Connected System
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.

Gross Generation
The total amount of electricity produced by a power plant.

A device used to protect the user of any electrical system or appliance from shock.

Ground Loop
In geothermal heat pump systems, a series of fluid-filled plastic pipes buried in the shallow ground, or placed in a body of water, near a building. The fluid within the pipes is used to transfer heat between the building and the shallow ground (or water) in order to heat and cool the building.

Ground Reflection
Solar radiation reflected from the ground onto a solar collector.


A sinusoidal quantity having a frequency that is an integral multiple of the frequency of a periodic quantity to which it is related.

A device used to simulate the angle of the sun for assessing shading potentials of building structures or landscape features.

A device that tracks the movement of the sun; used to orient solar concentrating systems.

A measure of the number of cycles or wavelengths of electrical energy per second; U.S. electricity supply has a standard frequency of 60 hertz.

A region of electrical contact between two different materials.

High-Intensity Discharge Lamp
A lamp that consists of a sealed arc tube inside a glass envelope, or outer jacket. The inner arc tube is filled with elements that emit light when ionized by electric current. A ballast is required to provide the proper starting voltage and to regulate current during operation.

High-Pressure Sodium Lamp
A type of High-Intensity Discharge (HID) lamp that uses sodium under high pressure as the primary light-producing element. These high efficiency lights produce a golden white color and are used for interior industrial applications, such as in warehouses and manufacturing, and for security, street, and area lighting.

The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

Home Energy Rating Systems (HERS)
A nationally recognized energy rating program that gives builders, mortgage lenders, secondary lending markets, homeowners, sellers, and buyers a precise evaluation of energy losing deficiencies in homes. Builders can use this system to gauge the energy quality in their home and also to have a star rating on their home to compare to other similarly built homes.

The region between an n-layer and a p-layer in a single material, photovoltaic cell.

Hybrid System
A renewable energy system that includes two different types of technologies that produce the same type of energy; for e.g., a wind turbine and a solar photovoltaic array combined to meet a power demand.

Hydroelectric Power Plant
A power plant that produces electricity by the force of water falling through a hydro turbine that spins a generator.

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.


A measure of the amount of light incident on a surface; measured in foot-candles or Lux.

The distribution of light on a horizontal surface. The purpose of all lighting is to produce illumination.

These lights use an electrically heated filament to produce light in a vacuum or inert gas-filled bulb.

Incident Solar Radiation
The amount of solar radiation striking a surface per unit of time and area.

Independent Power Producer
A company or individual that is not directly regulated as a power provider. These entities produce power for their own use and/or sell it to regulated power providers. 

The production of an electric current in a conductor by the variation of a magnetic field in its vicinity.

Induction Generator
A device that converts the mechanical energy of rotation into electricity based on electromagnetic induction. An electric voltage (electromotive force) is induced in a conducting loop (or coil) when there is a change in the number of magnetic field lines (or magnetic flux) passing through the loop. When the loop is closed by connecting the ends through an external load, the induced voltage will cause an electric current to flow through the loop and load. Thus rotational energy is converted into electrical energy.

Induction Motor
A motor in which a three phase (or any multiphase) alternating current (i.e. the working current) is supplied to iron-cored coils (or windings) within the stator. As a result, a rotating magnetic field is set up, which induces a magnetizing current in the rotor coils (or windings). Interaction of the magnetic field produced in this manner with the rotating field causes rotational motion to occur.

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.

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.

Installed Capacity
The total capacity of electrical generation devices in a power station or system.

Instantaneous Efficiency (of a Solar Collector)
The amount of energy absorbed (or converted) by a solar collector (or photovoltaic cell or module) over a 15 minute period.

A connection or link between power systems that enables them to draw on each other's reserve capacity in time of need.

Intermittent Generators
Power plants, whose output depends on a factor(s) that cannot be controlled by the power generator because they utilize intermittent resources such as solar energy or the wind.

Internal Combustion Electric Power Plant
The generation of electric power by a heat engine which converts part of the heat generated by combustion of the fuel into mechanical motion to operate an electric generator.

Internal Rate of Return
A widely used rate of return for performing economic analysis. This method solves for the interest rate that equates the equivalent worth of an alternative's cash receipts or savings to the equivalent worth of cash expenditures, including investments. The resultant interest rate is termed the internal rate of return (IRR).

Interruptible Load
Energy loads that can be shut off or disconnected at the supplier's discretion or as determined by a contractual agreement between the supplier and the customer.

Intrinsic Layer
A layer of semiconductor material (as used in a solar photovoltaic device) whose properties are essentially those of the pure, undoped, material.

A device that converts direct current electricity (from for example a solar photovoltaic module or array) to alternating current for use directly to operate appliances or to supply power to a electricity grid.

Investment Tax Credit
A tax credit granted for specific types of investments.

Investor Owned Utility (IOU)
A power provider owned by stockholders or other investors; sometimes referred to as a private power provider, in contrast to a public power provider that is owned by a government agency or cooperative.

The direct, diffuse, and reflected solar radiation that strikes a surface.

Isolated Solar Gain System
A type of passive solar heating system where heat is collected in one area for use in another.

I-Type Semiconductor
A semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V Curve
A graphical plot or representation the current and voltage output of a solar photovoltaic cell or module as a load on the device is increased from short circuit (no load) condition to the open circuit condition; used to characterize cell/module performance.


A structural, load-carrying building member with an open web system that supports floors and roofs utilizing wood or specific steels and is designed as a simple span member.

A metric unit of energy or work; the energy produced by a force of one Newton operating through a distance of one meter; 1 Joule per second equals 1 Watt or 0.737 foot-pounds; 1 Btu equals 1,055 Joules.

Joule's Law
The rate of heat production by a steady current in any part of an electrical circuit that is proportional to the resistance and to the square of the current, or, the internal energy of an ideal gas depends only on its temperature.

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).


Kilovolt-Ampere (kVa)
A unit of apparent power, equal to 1,000 volt-amperes; the mathematical product of the volts and amperes in an electrical circuit.

Kilowatt (kW)
A standard unit of electrical power equal to one thousand watts, or to the energy consumption at a rate of 1000 Joules per second.

A unit or measure of electricity supply or consumption of 1,000 Watts over the period of one hour; equivalent to 3,412 Btu.

Kinetic Energy
Energy available as a result of motion that varies directly in proportion to an object's mass and the square of its velocity.

A wall usually about 3 to 4 feet high located that is placed in the attic of a home, anchored with plates between the attic floor joists and the roof joist. Sheathing can be attached to these walls to enclose an attic space.


A unit or measure of solar radiation; 1 calorie per square centimeter or 3.69 Btu per square foot.

Lead Acid Battery
An electrochemical battery that uses lead and lead oxide for electrodes and sulfuric acid for the electrolyte.

Leaking Electricity
Related to stand-by power, leaking electricity is the power needed for electrical equipment to remain ready for use while in a dormant mode or operation. Electricity is still used by many electrical devices, such as TVs, stereos, and computers, even when you think they are turned "off."

Levelized Life Cycle Cost
A total life cycle cost divided into equal amounts.

Life Cycle Cost
The sum of all the costs both recurring and nonrecurring, related to a product, structure, system, or service during its life span or specified time period.

Line Loss (or Drop)
Electrical energy lost due to inherent inefficiencies in an electrical transmission and distribution system under specific conditions.

The power required to run a defined circuit or system, such as a refrigerator, building, or an entire electricity distribution system.

Load Analysis
Assessing and quantifying the discrete components that comprise a load. This analysis often includes time of day or season as a variable.

Load Duration Curve
A curve that displays load values on the horizontal axis in descending order of magnitude against the percent of time (on the vertical axis) that the load values are exceeded.

Load Factor
The ratio of average energy demand (load) to maximum demand (peak load) during a specific period.

Load Forecast
An estimate of power demand at some future period.

Load Leveling
The deferment of certain loads to limit electrical power demand, or the production of energy during off-peak periods for storage and use during peak demand periods.

Load Management
To influence the demand on a power source.

Load Profile or Shape
A curve on a chart showing power (kW) supplied (on the horizontal axis) plotted against time of occurrence (on the vertical axis) to illustrate the variance in a load in a specified time period.

Load Shedding
Turning off or disconnecting loads to limit peak demand.

Load Shifting
A load management objective that moves loads from on-peak periods to off-peak periods.

Local Solar Time
A system of astronomical time in which the sun crosses the true north-south meridian at 12 noon, and which differs from local time according to longitude, time zone, and equation of time.

Long-Wave Radiation
Infrared or radiant heat.

An empirical measure of the quantity of light. It is based upon the spectral sensitivity of the photosensors in the human eye under high (daytime) light levels. Photometrically it is the luminous flux emitted with a solid angle (1 steradian) by a point source having a uniform luminous intensity of 1 candela. As reference, a 100-watt incandescent lamp emits about 1600 lumens.

Lumens/Watt (lpw)
A measure of the efficacy (efficiency) of lamps. It indicates the amount of light (lumens) emitted by the lamp for each unit of electrical power (Watts) used.

A complete lighting unit consisting of a lamp(s), housing, and connection to the power circuit.

The physical measure of the subjective sensation of brightness; measured in lumens.

The unit of illuminance equivalent to 1lumen per square meter.


Magnetic Ballast
A type of florescent light ballast that uses a magnetic core to regulate the voltage of a florescent lamp.

One thousand kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

One thousand kilowatt-hours or 1 million watt-hours.

Mercury Vapor Lamp
A high-intensity discharge lamp that uses mercury as the primary light-producing element. Includes clear, phosphor coated, and self-ballasted lamps.

Metal Halide Lamp
A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. These lights have the best Color Rendition Index (CRI) of the High-Intensity Discharge lamps. They can be used for commercial interior lighting or for stadium lights.

Minority Carrier
A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

Minority Carrier Lifetime
The average time a minority carrier exists before recombination.

The smallest self-contained, environmentally protected structure housing interconnected photovoltaic cells and providing a single dc electrical output; sometimes also called a panel.

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.


Name Plate
A metal tag attached to a machine or appliance that contains information such as brand name, serial number, voltage, power ratings under specified conditions, and other manufacturer supplied data. A sticker or label on back of the solar module listing the various parameters of the module like power, different voltages and currents etc.

National Electrical Code (NEC)
The NEC is a set of regulations that have contributed to making the electrical systems in the United States one of the safest in the world. The intent of the NEC is to ensure safe electrical systems are designed and installed. The National Fire Protection Association has sponsored the NEC since 1911. The NEC changes as technology evolves and component sophistication increases. The NEC is updated every three years. Following the NEC is required in most locations.

National Rural Electric Cooperative Association (NRECA)
This is a national organization dedicated to representing the interests of cooperative electric power providers and the consumers they serve. Members come from the 46 states that have an electric distribution cooperative.

Net Energy Production (or Balance)
The amount of useful energy produced by a system less the amount of energy required to produce the fuel.

Net Generation
Equal to gross generation less electricity consumption of a power plant.

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.

N-Type Semiconductor
A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity (e.g., phosphorous in silicon).


The period of low energy demand, as opposed to maximum, or peak, demand.

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.

Ohm's Law
In a given electrical circuit, the amount of current in amperes (i) is equal to the pressure in volts (V) divided by the resistance, in ohms (R).

One-Axis Tracking
A system capable of rotating about one axis.

One Sun
The maximum value of natural solar insolation.

On-Peak Energy
Energy supplied during periods of relatively high system demands as specified by the supplier.

On-Site Generation
Generation of energy at the location where all or most of it will be used.

Open Access
The ability to send or wheel electric power to a customer over a transmission and distribution system that is not owned by the power generator (seller).

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

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.

A discontinuance of electric power supply.

To exceed the design capacity of a device.


Panel (Solar)
A term generally applied to individual solar collectors, and typically to solar photovoltaic collectors or modules.

A configuration of an electrical circuit in which the voltage is the same across the terminals. The positive reference direction for each resistor current is down through the resistor with the same voltage across each resistor.

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

Peaking Capacity
Power generation equipment or system capacity to meet peak power demands.

Peak Power
Power generated that operates at a very low capacity factor; generally used to meet short-lived and variable high demand periods.

Peak Shifting
The process of moving existing loads to off-peak periods.

Peak Sun Hours
The equivalent number of hours per day when solar irradiance averages 1 kW/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 kW/m2.

Peak Watt
A unit used to rate the performance of a solar photovoltaic (PV) cells, modules, or arrays; the maximum nominal output of a PV device, in Watts (Wp) under standardized test conditions, usually 1000 Watts per square meter of sunlight with other conditions, such as temperature specified.

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.

An electric current induced by radiant energy.

Photovoltaic / Photogalvanic Processes
The production of electrical current from light.

A particle of light that acts as an individual unit of energy.

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 (PV; Solar) Array
A group of solar photovoltaic modules connected together.

Photovoltaic (Solar) Cell
Treated semiconductor material that converts solar irradiance to electricity.

Photovoltaic 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 semi-conductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multi-crystalline, or amorphous forms.

Photovoltaic (Solar) Module or Panel
A solar photovoltaic product that generally consists of groups of PV cells electrically connected together to produce a specified power output under standard test conditions, mounted on a substrate, sealed with an encapsulant, and covered with a protective glazing. Maybe further mounted on an aluminum frame. A junction box, on the back or underside of the module is used to allow for connecting the module circuit conductors to external conductors.

Photovoltaic Peak Watt
see Peak Watt.

Photovoltaic (Solar) System
A complete PV power system composed of the module (or array), and balance-of-system (BOS) components including the array supports, electrical conductors/wiring, fuses, safety disconnects, and grounds, charge controllers, inverters, battery storage, etc.

Photovoltaic-Thermal (PV/T) Systems
A solar energy system that produces electricity with a PV module, and collects thermal energy from the module for heating. There are no commercially available systems available (as of 11/97).

A semiconductor (photovoltaic) device structure in which the junction is formed between a p-type layer and an n-type layer.

The space between a hanging ceiling and the floor above or roof; usually contains HVAC ducts, electrical wiring, fire suppression system piping, etc.

A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals.

Energy that is capable or available for doing work; the time rate at which work is performed, measured in horsepower, Watts, or Btu per hour. Electric power is the product of electric current and electromotive force.

Power Conditioning
The process of modifying the characteristics of electrical power (for e.g., inverting dc to ac).

Power (Output) Curve
A plot of a wind energy conversion device's power output versus wind speed.

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.

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.

Power (Solar) Tower
A term used to describe solar thermal, central receiver, power systems, where an array of reflectors focus sunlight onto a central receiver and absorber mounted on a tower.

Power Transmission Line
An electrical conductor/cable that carries electricity from a generator to other locations for distribution.

P-Type Semiconductor
A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).

Public Utilities Regulatory Policy Act (PURPA) of 1978
A law that requires electric utilities to purchase electricity produced from qualifying power producers that use renewable energy resources or are cogenerators. Power providers are required to purchase power at a rate equal to the avoided cost of generating the power themselves. (See Avoided Costs and Qualifying Facility)

Pulse-Width-Modulated (PWM) Wave Inverter
A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

A device used to measure total incident solar radiation (direct beam, diffuse, and reflected radiation) per unit time per unit area.

A device that measures the intensity of direct beam solar radiation.


One quadrillion Btu. (1,000,000,000,000,000 Btu)


Radiant Energy
Energy that transmits away from its source in all directions.

The transfer of heat through matter or space by means of electromagnetic waves.

A construction element used for ceiling support.

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.

Reactive Power
The electrical power that oscillates between the magnetic field of an inductor and the electrical field of a capacitor. Reactive power is never converted to non-electrical power. Calculated as the square root of the difference between the square of the kilovolt-amperes and the square of the kilowatts. Expressed as reactive volt-amperes.

An electrical device for converting alternating current to direct current. The chamber in a cooling device where water is separated from the working fluid (for example ammonia).

The amount (percent) of light that is reflected by a surface relative to the amount that strikes it.

Reflective Coatings
Materials with various qualities that are applied to glass windows before installation. These coatings reduce radiant heat transfer through the window and also reflects outside heat and a portion of the incoming solar energy, thus reducing heat gain. The most common type has a sputtered coating on the inside of a window unit. The other type is a durable "hard-coat" glass with a coating, baked into the glass surface.

The change in direction of a ray of light when it passes through one media to another with differing optical densities.

The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and absorb heat.

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.

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.

Resistance Heating
A type of heating system that provides heat from the resistance of an electrical current flowing through a conductor.

Resistive Voltage Drop
The voltage developed across a cell by the current flow through the resistance of the cell.

An electrical device that resists electric current flow.

The process of modifying a building's structure.

Ribbon (Photovoltaic) Cells
A type of solar photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

A building element that provides protection against the sun, wind, and precipitation.

Rural Electrification Administration (REA)
An agency of the U.S. Dept. of Agriculture that makes loans to states and territories in the U.S. for rural electrification and the furnishing of electric energy to persons in rural areas who do not receive central station service. It also furnishes and improves electric and telephone service in rural areas, assists electric borrowers to implement energy conservation programs and on-grid and off-grid renewable energy systems, and studies the condition and progress of rural electrification.


Sacrificial Anode
A metal rod placed in a water heater tank to protect the tank from corrosion. Anodes of aluminum, magnesium, or zinc are the more frequently metals. The anode creates a galvanic cell in which magnesium or zinc will be corroded more quickly than the metal of the tank giving the tank a negative charge and preventing corrosion.

Safety Disconnect
An electronic (automatic or manual) switch that disconnects one circuit from another circuit. These are used to isolate power generation or storage equipment from conditions such as voltage spikes or surges, thus avoiding potential damage to equipment.

Seebeck Effect
The generation of an electric current, when two conductors of different metals are joined at their ends to form a circuit, with the two junctions kept at different temperatures.

Selective Surface Coating
A material with high absorbence and low emittance properties applied to or on solar absorber surfaces.

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.

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 photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

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.

Short Circuit
An electric current taking a shorter or different path than intended.

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

Shunt Load
An electrical load used to safely use excess generated power when not needed for its primary uses. A shunt load in a residential photovoltaic system might be domestic water heating, such that when power is not needed for typical building loads, such as operating lights or running HVAC system fans and pumps, it still provides value and is used in a constructive, safe manner.

A chemical element, of atomic number 14, that is semi-metallic, and an excellent semiconductor material used in solar photovoltaic devices; commonly found in sand.

Sine Wave
The type of alternative current generated by alternating current generators, rotary inverters, and solid-state inverters.

Single-Crystal Material
In reference to solar photovoltaic devices, a material that is composed of a single crystal or a few large crystals.

A generator with a single armature coil, which may have many turns and the alternating current output consists of a succession of cycles.

The process of designing a solar system to meet a specified load given the solar resource and the nominal or rated energy output of the solar energy collection or conversion device.

Sodium Lights
A type of high intensity discharge light that has the most lumens per watt of any light source.

Solar Access or Rights
The legal issues related to protecting or ensuring access to sunlight to operate a solar energy system, or use solar energy for heating and cooling.

Solar Altitude Angle
The angle between a line from a point on the earth's surface to the center of the solar disc, and a line extending horizontally from the point.

Solar Array
A group of solar collectors or solar modules connected together.

Solar Azimuth
The angle between the sun's apparent position in the sky and true south, as measured on a horizontal plane.

Solar Cell
A solar photovoltaic device with a specified area.

Solar Collector
A device used to collect, absorb, and transfer solar energy to a working fluid. Flat plate collectors are the most common type of collectors used for solar water or pool heating systems. In the case of a photovoltaics system, the solar collector could be crystalline silicon panels or thin-film roof shingles, for example.

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 Declination
The apparent angle of the sun north or south of the earth's equatorial plane. The earth's rotation on its axis causes a daily change in the declination.

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 Energy Industries Association (SEIA)
A national trade association of solar energy equipment manufacturers, retailers, suppliers, installers, and consultants.

Solar Energy Research Institute (SERI)
A federally funded institute, created by the Solar Energy Research, Development and Demonstration Act of 1974, that conducted research and development of solar energy technologies. Became the National Renewable Energy Laboratory (NREL) in 1991.

Solar Film
A window glazing coating, usually tinted bronze or gray, used to reduce building cooling loads, glare, and fabric fading.

Solar Fraction
The percentage of a building's seasonal energy requirements that can be met by a solar energy device(s) or system(s).

Solar Irradiation
The amount of solar radiation, both direct and diffuse, received at any location.

Solar Module (Panel)
A solar photovoltaic device that produces a specified power output under defined test conditions, usually composed of groups of solar cells connected in series, in parallel, or in series-parallel combinations.

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 Panel
See Photovoltaic Module.

Solar Radiation
A general term for the visible and near visible (ultraviolet and near-infrared) electromagnetic radiation that is emitted by the sun. It has a spectral, or wavelength, distribution that corresponds to different energy levels; short wavelength radiation has a higher energy than long-wavelength radiation.

Solar Time
The period marked by successive crossing of the earth's meridian by the sun; the hour angle of the sun at a point of observance (apparent time) is corrected to true (solar) time by taking into account the variation in the earth's orbit and rate of rotation. Solar time and local standard time are usually different for any specific location.

The two times of the year when the sun is apparently farthest north and south of the earth's equator; usually occurring on or around June 21 (summer solstice in northern hemisphere, winter solstice for southern hemisphere) and December 21 (winter solstice in northern hemisphere, summer solstice for the southern hemisphere).

Spectral Irradiance
The monochromatic irradiance of a surface per unit bandwidth at a particular wavelength, usually expressed in Watts per square meter-nanometer bandwidth.

Spectral Reflectance
The ratio of energy reflected from a surface in a given waveband to the energy incident in that waveband.

see Solar Spectrum above.

Spectrally Selective Coatings
A type of window glazing films used to block the infrared (heat) portion of the solar spectrum but admit a higher portion of visible light.

Split Spectrum Photovoltaic Cell
A photovoltaic device where incident sunlight is split into different spectral regions, with an optical apparatus, that are directed to individual photovoltaic cells that are optimized for converting that spectrum to electricity.

Stand-Alone Inverter
An inverter that operates independent of or is not connected to an electric transmission and distribution network.

Stand-Alone System
An system that operates independent of or is not connected to an electric transmission and distribution network.

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.

An electrical installation containing power conversion (and sometimes generation) equipment, such as transformers, compensators, and circuit breakers.

The physical material upon which a photovoltaic cell is applied.

Sun Path Diagram
A circular projection of the sky vault onto a flat diagram used to determine solar positions and shading effects of landscape features on a solar energy system.

A room that faces south (in the northern hemisphere), or a small structure attached to the south side of a house.

Sun Tempered Building
A building that is elongated in the east-west direction, with the majority of the windows on the south side. The area of the windows is generally limited to about 7% of the total floor area. A sun-tempered design has no added thermal mass beyond what is already in the framing, wall board, and so on. Insulation levels are generally high.

Superconducting Magnetic Energy Storage (SMES)
SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. SMES has been proposed as a storage option to support large-scale use of photovoltaics and wind as a means to smooth out fluctuations in power generation.

The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

Super Insulated Houses
A type of house that has massive amounts of insulation, airtight construction, and controlled ventilation without sacrificing comfort, health, or aesthetics.

Super Window
A popular term for highly insulating window with a heat loss so low it performs better than an insulated wall in winter, since the sunlight that it admits is greater than its heat loss over a 24 hour period.

Supplementary Heat
A heat source, such as a space heater, used to provide more heat than that provided by a primary heating source.

Supply Side
Technologies that pertain to the generation of electricity.

Synchronous Generator
An electrical generator that runs at a constant speed and draws its excitation from a power source external or independent of the load or transmission network it is supplying.

Synchronous Inverter
An electrical inverter that inverts direct current electricity to alternating current electricity, and that uses another alternating current source, such as an electric power transmission and distribution network (grid), for voltage and frequency reference to provide power in phase and at the same frequency as the external power source.


Temperature Coefficient (of a solar photovoltaic cell)
The amount that the voltage, current, and/or power output of a solar cell changes due to a change in the cell temperature.

A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make solar photovoltaic cells.

Three-phase Current
Alternating current in which three separate pulses are present, identical in frequency and voltage, but separated 120 degrees in phase.

Tilt Angle (of a Solar Collector or Module)
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.

Time-of-Use (TOU) Rates
The pricing of electricity based on the estimated cost of electricity during a particular time block. Time-of-use rates are usually divided into three or four time blocks per twenty-four hour period (on-peak, mid-peak, off-peak and sometimes super off-peak) and by seasons of the year (summer and winter). Real-time pricing differs from TOU rates in that it is based on actual (as opposed to forecasted) prices which may fluctuate many times a day and are weather-sensitive, rather than varying with a fixed schedule.

Tracking Solar Array
A solar energy 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.

An electromagnetic device that changes the voltage of alternating current electricity. It consists of an induction coil having a primary and secondary winding and a closed iron core.

The process of sending or moving electricity from one point to another; usually defines that part of an electric power provider's electric power lines from the power plant buss to the last transformer before the customer's connection.

Transmission and Distribution Losses
The losses that result from inherent resistance in electrical conductors and transformation inefficiencies in distribution transformers in a transmission and distribution network.

Transmission Lines
Transmit high-voltage electricity from the transformer to the electric distribution system.

True Power
The actual power rating that is developed by a motor before losses occur.

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.

Tube (Fluorescent Light)
A fluorescent lamp that has a tubular shape.

A type of solar thermal collector where the heat transfer fluid flows through tubes formed in the absorber plate.

Tube-Type Collector
A type of solar thermal collector that has tubes (pipes) that the heat transfer fluid flows through that are connected to a flat absorber plate.

Tungsten Halogen Lamp
A type of incandescent lamp that contains a halogen gas in the bulb, which reduces the filament evaporation rate increasing the lamp life. The high operating temperature and need for special fixtures limits their use to commercial applications and for use in projector lamps and spotlights.

Two-Axis Tracking
A solar array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).


Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

A regulated entity which exhibits the characteristics of a natural monopoly (also referred to as a power provider). For the purposes of electric industry restructuring, "utility" refers to the regulated, vertically-integrated electric company. "Transmission utility" refers to the regulated owner/operator of the transmission system only. "Distribution utility" refers to the regulated owner/operator of the distribution system which serves retail customers.


Valence Band
The highest energy band in a semiconductor that can be filled with electrons.

Vertical Ground Loop
In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are laid out in a plane perpendicular to the ground surface. For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building. Large commercial buildings and schools often use vertical systems because the land area required for horizontal ground loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, or for existing buildings, as they minimize the disturbance to landscaping. Also see closed-loop geothermal heat pump systems.

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.

The amount of electromotive force, measured in volts, that exists between two points.

A unit of electrical measurement equal to the product of a volt and an ampere.


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

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).

A unit of electricity consumption of one Watt over the period of one hour.

A device for measuring power consumption.

Wave Form
The shape of the phase power at a certain frequency and amplitude.

The distance between similar points on successive waves.

Caulking and weatherstripping to reduce air infiltration and exfiltration into/out of a building.

Wire (Electrical)
A generic term for an electrical conductor.


The rotation of a horizontal axis wind turbine around its tower or vertical axis.


An area within the interior space of a building, such as an individual room(s), to be cooled, heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.

The combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling, and ventilation equipment.


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