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Glossary of Energy Terms (GET)

Please email us if you have energy-related terms that you would like to see included in the glossary. If you run across acronyms you don't know, use our SAD-tool: the Secret Acronym Decoder.


A-D | E-G | H-L | M-O | P-S | T-Z

Active Solar

Active solar technologies are employed to convert solar energy into another more useful form of energy. This would normally be a conversion to heat or electrical energy

Alternating Current (AC)

An electric current that reverses its direction at regularly recurring intervals, usually 50 or 60 times per second.

Amorphous Silicon

An alloy of silica and hydrogen, with a disordered, noncrystalline internal atomic arrangement, that can be deposited in thin-layers (a few micrometers in thickness) by a number of deposition methods to produce thin-film photovoltaic cells on glass, metal, or plastic substrates.

Artificial Photosynthesis

Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel. Splitting water, or photoelectrolysis, is one form of artificial photosynthesis that converts water into hydrogen and oxygen using sunlight.

Availability Factor

A percentage representing the number of hours a generating unit is available to produce power (regardless of the amount of power) in a given period, compared to the number of hours in the period.

Barrel of oil equivalent

A unit of energy equal to the amount of energy contained in a barrel of crude oil. Approximately 5.78 million Btu or 1,700 kWh. A barrel is a liquid measure equal to 42 gallons.

Binary Cycle

A type of geothermal power plant. Binary cycle power plants operate on water at lower temperatures of about 225°–360°F (107°–182°C). Binary cycle plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Biochemical conversion process

The use of living organisms or their products to convert organic material to fuels, chemicals or other products.


Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, propyl or ethyl) esters


Useful, renewable energy produced from organic matter, which may either be used directly as a fuel or processed into liquids and gases.


An alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn or sugarcane. Ethanol can be used as a fuel for vehicles.


A type of fuel whose energy is derived from biological carbon fixation


Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen


A renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel.


Butanol may be used as a fuel in an internal combustion engine. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification. It can be produced from biomass as well as fossil fuels.

Capacity Factor

The ratio of the electrical energy produced by a generating unit for the period of time considered to the electrical energy that could have been produced at continuous full-power operation during the same period.

Carbon Offsets

A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere

Cast Silicon

Crystalline silicon obtained by pouring pure molten silicon into a vertical mold and adjusting the temperature gradient along the mold volume during cooling to obtain slow, vertically-advancing crystallization of the silicon. The polycrystalline ingot thus formed is composed of large, relatively parallel, interlocking crystals. The cast ingots are sawed into wafers for further fabrication into photovoltaic cells. Cast-silicon wafers and ribbon-silicon sheets fabricated into cells are usually referred to as polycrystalline photovoltaic cells.


A protocol for charging electric vehicles, which is an abbreviation of "CHArge de MOve", equivalent to "charge for moving", and is a pun for "O cha demo ikaga desuka" in Japanese, meaning "Let's have a tea while charging" in English. 


A unit that converts AC supply power to DC, to charge vehicle batteries, either on or off the vehicle.


A device, or system of devices, that allows electrical current to flow through and voltage to occur across positive and negative terminals.


The sequential production of electricity and useful thermal energy from a common fuel source. Rejected heat from industrial processes can be used to power an electric generator (bottoming cycle). Conversely, surplus heat from an electric generating plant can be used for industrial processes, or space and water heating purposes (topping cycle).

Combined Cycle

An electric generating technology in which electricity is produced from otherwise lost waste heat exiting from one or more gas (combustion) turbines. The exiting heat is routed to a conventional boiler or to a heat recovery steam generator for utilization by a steam turbine in the production of electricity. Such designs increase the efficiency of the electric generating unit.

Concentrated Solar Power

Concentrated solar power (CSP) systems use mirrors or lenses to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator.

DC Quick Charger

An offboard charger that connects directly to an electric vehicle's high-voltage battery bus.


Ethanol that has had a substance added to make it unfit for human consumption.

Digester Gas

Biogas that is produced using a digester which is an airtight vessel or enclosure in which bacteria decomposes biomass in water to produce biogas.

Direct Current (DC)

An electric current that flows in a constant direction. The magnitude of the current does not vary or has a slight variation.

Dry Steam

A type of geothermal power plant. Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant where it is directed into a turbine/generator unit.

Dynamic Tidal Power

Dynamic tidal power (or DTP) is a theoretical generation technology that would exploit an interaction between potential and kinetic energies in tidal flows.

Electric Power

Can refer imprecisely to a quantity of electrical potential energy or more correctly to electrical energy per time, that is provided commercially by the electrical power industry.

Electric Power Transmissions

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers

Electric Utility

A corporation, person, agency, authority, or other legal entity or instrumentality aligned with distribution facilities for delivery of electric energy for use primarily by the public. Included are investor-owned electric utilities, municipal and State utilities, Federal electric utilities, and rural electric cooperatives. A few entities that are tariff based and corporately aligned with companies that own distribution facilities are also included.

Electric Vehicle Supply Equipment (EVSE)

Any off-board equipment used to supply charging energy to the vehicle. EVSE can take the form of a cord, a box mounted to a wall, pedestal or pole, and even the different outlets and plugs that make up the circuit. This equipment should prevent energizing of the charge plug until it is seated in a vehicle port. It should monitor for safety hazards. It communicates to the vehicle the amount of current that can be provided by the circuit and gets information about area ventilation requirements. (Thanks to


A general term encompassing a variety of phenomena resulting from the presence and flow of electric charge.

Emission Offset

A reduction in the air pollution emissions of existing sources to compensate for emissions from new sources

Energy crops

Crops grown specifically for their fuel value. These include food crops such as corn and sugarcane and nonfood crops such as poplar trees and switchgrass. Currently, two energy crops are under development in the United States: short-rotation woody crops, which are fast-growing hardwood trees harvested in 5 to 8 years, and herbaceous energy crops, such as perennial grasses, which are harvested annually after taking 2 to 3 years to reach full productivity.

Enhanced Geothermal System

Enhanced Geothermal System (EGS) is a new type of geothermal power technology that does not require natural convective hydrothermal resources.


A clear, colorless flammable oxygenated hydrocarbon with a boiling point of 173.5 degrees Fahrenheit in the anhydrous state. However it readily forms a binary azetrope with water, with a boiling point of 172.67 degrees Fahrenheit at a composition of 95.57 percent by weight ethanol. It is used in the United States as a gasoline octane enhancer and oxygenate (maximum 10 percent concentration). Ethanol can be used in higher concentrations (E85) in vehicles designed for its use. Ethanol is typically produced chemically from ethylene, or biologically from fermentation of various sugars from carbohydrates found in agricultural crops and cellulosic residues from crops or wood. The lower heating value, equal to 76,000 Btu per gallon, is assumed for estimates in this report.

Evacuated Tube

In a solar thermal collector, an absorber tube, which is contained in an evacuated glass cylinder, through which collector fluids flows


Any material that can be converted to another form of fuel or energy product


The biological conversion of biomass by yeast or sugar. The products of fermentation are carbon dioxide and alcohol

Fish Ladders

Also known as a fishway, fish pass or fish steps, is a structure on or around artificial barriers (such as dams and locks) to facilitate diadromous fishes' natural migration.

Flash Steam

A type of geothermal power plant. Flash steam power plants are the most common and use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Flexible-fuel vehicle

A vehicle with a single fuel tank designed to run on varying blends of unleaded gasoline with either ethanol or methanol.

Francis turbine

A water-powered turbine used to transform water falling vertically to mechanical (rotating) energy.


The number of cycles through which an alternating current passes per second, measured in hertz.


Any material that can be converted to energy

Fuel Cell

A device that converts the chemical energy of a fuel directly to electricity and heat, without combustion.

Generation (Electricity)

The process of producing electric energy from other forms of energy; also, the amount of electric energy produced, expressed in watthours (Wh).


In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy.

Geothermal Electricity

Eelectricity generated from geothermal energy

Geothermal Energy

As used at electric power plants, hot water or steam extracted from geothermal reservoirs in the Earth's crust that is supplied to steam turbines at electric power plants that drive generators to produce electricity.

Geothermal Plant

A plant in which a turbine is driven either from hot water or by natural steam that derives its energy from heat found in rocks or fluids at various depths beneath the surface of the earth. The fluids are extracted by drilling and/or pumping.

Geothermal Heat Pump

A geothermal heat pump, ground source heat pump (GSHP), or ground heat pump is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer).

Gigawatt (GW)

The gigawatt is equal to one billion (10^9) watts or 1 gigawatt = 1000 megawatts.

Green Pricing/Marketing

In the case of renewable electricity, green pricing represents a market solution to the various problems associated with regulatory valuation of the nonmarket benefits of renewables. Green pricing programs allow electricity customers to express their willingness to pay for renewable energy development through direct payments on their monthly utility bills.


An electric utility´s system for distributing power.

Grid Connection

Joining a plant that generates electric power to a utility system so that electricity can flow in either direction between the utility system and the plant.

Grid Energy Storage

Also called large-scale energy storage, refers to the methods used to store electricity on a large scale within an electrical power grid.

Heat Pump (Geothermal)

A heat pump in which the refrigerant exchanges heat (in a heat exchanger) with a fluid circulating through an earth connection medium (ground or ground water). The fluid is contained in a variety of loop (pipe) configurations depending on the temperature of the ground and the ground area available. Loops may be installed horizontally or vertically in the ground or submersed in a body of water.

Heat Pump (efficiency)

The efficiency of a heat pump, that is, the electrical energy to operate it, is directly related to temperatures between which it operates. Geothermal heat pumps are more efficient than conventional heat pumps or air conditioners that use the outdoor air since the ground or ground water a few feet below the earth's surface remains relatively constant throughout the year. It is more efficient in the winter to draw heat from the relatively warm ground than from the atmosphere where the air temperature is much colder, and in summer transfer waste heat to the relatively cool ground than to hotter air. Geothermal heat pumps are generally more expensive ($2,000 $5,000) to install than outside air heat pumps. However, depending on the location geothermal heat pumps can reduce energy consumption (operating cost) and correspondingly, emissions by more than 20 percent compared to high efficiency outside air heat pumps. Geothermal heat pumps also use the waste heat from air-conditioning to provide free hot water heating in the summer.

Hertz (Hz)

A measure of the number of cycles or wavelengths of electrical energy per second. The United States electricity supply has a standard frequency of 60 hertz.

Hydroelectric Dam

A dam in a river or stream used to produce hydroelectricity or hydroelectric power.


Electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator.

Hydrogen Fuel Cell

A device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel.

Independent Power Producer (IPP)

A corporation, person, agency, authority, or other legal entity or instrumentality that owns or operates facilities for the generation of electricity for use primarily by the public, and that is not an electric utility.

Internal Collector Storage (ICS)

A solar thermal collector in which incident solar radiation is absorbed by the storage medium.


An SAE standard that covers AC Level 1 and 2 charging for electric vehicles: the standard in the United States.

Kilowatt (kW)

One kilowatt is equal to one thousand (10^3) watts.

Kilowatt Hour (kWh)

A measure of energy equivalent to the expenditure of one kilowatt for one hour. For example, 1 kWh will light a 100-watt light bulb for 10 hours. 1 kWh = 3,413 Btu.

Kinetic Energy

The kinetic energy of an object is the energy which it possesses due to its motion.

Landfill Gas

Gas that is generated by decomposition of organic material at landfill disposal sites. Landfill gas is approximately 50 percent methane.

Low-Temperature Collectors

Metallic or nonmetallic solar thermal collectors that generally operate at temperatures below 110 degrees Fahrenheit and use pumped liquid or air as the heat transfer medium. They usually contain no glazing and no insulation, and they are often made of plastic or rubber, although some are made of metal.

Medium-Temperature Collectors

Solar thermal col-lectors designed to operate in the temperature range of 140 degrees to 180 degrees Fahrenheit, but that can also operate at a temperature as low as 110 degrees Fahrenheit. The collector typically consists of a metal frame, metal absorption panels with integral flow channels (attached tubing for liquid collectors or integral ducting for air collectors), and glazing and insulation on the sides and back.

Megawatt (MW)

The megawatt is equal to one million (10^6) watts.

Ocean Energy

The potential energy that may be found within and above the ocean, such as tidal, wave and wind power.


The ohm is the International System of Units (SI) unit of electrical resistance.

Parabolic Dish

A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator, generally bowl-shaped, with two-axis tracking.

Parabolic Trough

A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator with the capacity for tracking the sun using one axis of rotation

Passive Solar

In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design or climatic design because, unlike active solar heating systems, it doesn't involve the use of mechanical and electrical devices.

Peak Watt

A manufacturer's unit indicating the amount of power a photovoltaic cell or module will produce at standard test conditions (normally 1,000 watts per square meter and 25 degrees Celsius).

Photovoltaic (PV) Module

An integrated assembly of interconnected photovoltaic cells designed to deliver a selected level of working voltage and current at its output terminals, packaged for protection against environment degradation, and suited for incorporation in photovoltaic power systems.

Photovoltaic Power

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect.

Production Tax Credit (PTC)

An inflation - adjusted payment per kilowatt hour for electricity produced using qualifying renewable energy sources.


A clear colorless liquid used as a solvent and antiseptic. Also called propyl alcohol.

Radiant Energy Transfer

A patented process technology that provides a means of separating hydrogen and oxygen efficiently, so that recombination back to water is avoided (up to 90% efficiency).

Renewable Energy

Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally replenish).

Renewable Energy Credits (REC)

Also called Green tags or Tradable Renewable Certificates, are certificates issued by a government agency to a power company which utilizes environmentally friendly methods to generate electricity. The Renewable Energy Credits can in turn be traded and sold on the open market, providing an incentive to companies which produce “green” power.

Renewable Portfolio Standard (RPS)

A mandate requiring that renewable energy provide a certain percentage of total energy generation or consumption.


A semiconductor material made from silica, purified for photovoltaic applications.


Siltation is the pollution of water by fine particulate terrestrial clastic material, with a particle size dominated by silt or clay.

Solar Array

A grouping of multiple solar panels.

Solar Power

The conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP).

Solar Radiation

Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy.

Solar Thermal

Solar thermal energy (STE) is a technology for harnessing solar energy for thermal energy (heat).

Solar Thermal Collector

A device designed to receive solar radiation and convert it into thermal energy. Normally, a solar thermal collector includes a frame, glazing, and an absorber, together with the appropriate insulation. The heat collected by the solar thermal collector may be used immediately or stored for later use.


One Terrawatt is equal to one trillion (10^12) watts.

Thermal Mass

A concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect.

Thermosiphon System

A solar collector system for water heating in which circulation of the collection fluid through the storage loop is provided solely by the temperature and density difference between the hot and cold fluids.

Tidal Barrage

A system that makes use of the potential energy in the difference in height (or head) between high and low tides to generate power. Barrages are essentially dams across the full width of a tidal estuary.

Tidal Power

Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into useful forms of power - mainly electricity.

Tidal Stream Generator

Tidal stream generators (or TSGs) make use of the kinetic energy of moving water to power turbines, in a similar way to wind turbines that use wind to power turbines


In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R′ of an alcohol.


A device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils.

Volt (V)

The volt (symbol: V) is the SI derived unit for electric potential, electric potential difference, and electromotive force.

Watt (W)

A derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736–1819). The unit, defined as one joule per second, measures the rate of energy conversion. In terms of electromagnetism, one watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).

Wave Power

The transport of energy by ocean surface waves, and the capture of that energy to do useful work — for example, electricity generation, water desalination, or the pumping of water (into reservoirs).

Wind Farm

A piece of land on which wind turbines are sited for the purpose of electricity generation.

Wind Turbine

A device that converts kinetic energy from the wind into mechanical energy.

Wind Turbine Rated Capacity

The amount of power a wind turbine can produce at its rated wind speed.

Wind Power

The conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships.

Wind power plant

A group of wind turbines interconnected to a common utility 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. This is a term commonly used in the United States. In Europe, it is called a generating station.

Wind Power Profile

The change in the power available in the wind due to changes in the wind speed or velocity.