Glossary of Energy Terms

Watt (W)

The unit of electrical power equal to one ampere under a pressure of one volt. A Watt is equal to 1/746 horse power.

Kilowatt (kW)

One thousand watts.

Kilowatt hour (kWh)

A measure of electricity defined as a unit of work or energy, measured as 1 kilowatt (1,1,000 watts) of power expended for 1 hour. One kWh is equivalent to 3,412 Btu.

Megawatt (MW)

One million watts of electricity.

Gigawatt (GW)

One billion watts or one thousand megawatts.


The capacity for doing work as measured by the capability of doing work (potential energy) or the conversion of this capability to motion (kinetic energy). Energy has several forms, some of which are easily convertible and can be changed to another form useful for work. Most of the world's convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks. Electrical energy is usually measured in kilowatt hours, while heat energy is usually measured in British thermal units (Btu).


The rate at which energy is converted or utilized.  The most common unit of power is the Watt, which is equal to one Joule per second or one one-thousandth of a Btu per second.

Heat content

The amount of heat energy available to be released by the transformation or use of a specified physical unit of an energy form (e.g., a ton of coal, a barrel of oil, a kilowatt hour of electricity, a cubic foot of natural gas, or a pound of steam). The amount of heat energy is commonly expressed in British thermal units (Btu). Note: Heat content of combustible energy forms can be expressed in terms of either gross heat content (higher or upper heating value) or net heat content (lower heating value), depending upon whether or not the available heat energy includes or excludes the energy used to vaporize water (contained in the original energy form or created during the combustion process). The Energy Information Administration typically uses gross heat content values.

British thermal unit

The quantity of heat required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at the temperature at which water has its greatest density (approximately 39 degrees Fahrenheit).

Btu conversion factor

A factor for converting energy data between one unit of measurement and British thermal units (Btu). Btu conversion factors are generally used to convert energy data from physical units of measure (such as barrels, cubic feet, or short tons) into the energy-equivalent measure of Btu. (See British Thermal Unit Conversion Factors for further information on Btu conversion factors.)

The following table contains some Btu conversion factors for several common energy sources.



Heat Content (Btu)


1 barrela

5.8 × 106

Natural Gas

1 cubic foot



1 ton

2.07 × 107


1 gallon


Residual Oil

1 barrel

6.29 × 106


1 kWh


Diesel or Heating Oil

1 gallon



1 gallon



Btu per cubic foot

The total heating value, expressed in Btu, produced by the combustion, at constant pressure, of the amount of the gas that would occupy a volume of 1 cubic foot at a temperature of 60 degrees F if saturated with water vapor and under a pressure equivalent to that of 30 inches of mercury at 32 degrees F and under standard gravitational force (980.665 cm. per sec. squared) with air of the same temperature and pressure as the gas, when the products of combustion are cooled to the initial temperature of gas and air when the water formed by combustion is condensed to the liquid state. (Sometimes called gross heating value or total heating value.)


Quadrillion Btu, or 1015 Btu.


The quantity 1,000,000,000,000,000 (10 to the 15th power).

Joule (J)

The meter-kilogram-second unit of work or energy, equal to the work done by a force of one newton when its point of application moves through a distance of one meter in the direction of the force; equivalent to 107 ergs and one watt-second.  One Btu is approximately one thousand Joules.  The Joule is the preferred unit for measuring energy or heat content in much of the world where the metric system is used; the United States and the United Kingdom use the British Thermal Unit (Btu).

Primary energy

Energy in the form that it is first accounted for in a statistical energy balance, before any transformation to secondary or tertiary forms of energy. For example, coal can be converted to synthetic gas, which can be converted to electricity; in this example, coal is primary energy, synthetic gas is secondary energy, and electricity is tertiary energy.

Energy-use sectors

A group of major energy-consuming components of U.S. society developed to measure and analyze energy use. The sectors most commonly referred to in EIA are: residential, commercial, industrial, transportation, and electric power.


A term used to identify a type of electric generating station, capacity, capability, or output in which the source of energy for the prime mover is heat.

Thermal efficiency

A measure of the efficiency of converting a fuel to energy and useful work; useful work and energy output divided by higher heating value of input fuel times 100 (for percent).

*Heat rate

Specifically for electric power plants, the heat rate measures the efficiency with which fuel is converted to electric energy.  The heat rate takes units of Btu of primary input energy per kWh of electric energy produced.  A power plant with 100% thermal efficiency would have a heat rate of 3,412 (since the heat content of one kWh of electric energy is 3,412 Btu).  The given heat rate of a power plant can be converted to a thermal efficiency by dividing 3,412 by the heat rate.  For example, a power plant with a heat rate of 8,000 Btu per kWh would have a thermal efficiency of 4,412/8,000 = 0.43 (i.e., 43% thermal efficiency).

Capital cost

A term referring to the fixed costs of constructing an energy project. Capital costs may include materials, labor, land and financing costs.

*Operating cost

The cost of operating a plant once it has been constructed.  Components of operating cost include fuel, labor and maintenance costs.

Levelized Cost of Energy (LCOE)

The average sales price (per energy unit, such as $ per MWh of electricity or $ per million Btu of natural gas) from an energy project over its lifetime that results in the project breaking even, i.e., recovering all capital and operating costs.

Electric power grid

A system of synchronized power providers and consumers connected by transmission and distribution lines and operated by one or more control centers. In the continental United States, the electric power grid consists of three systems the Eastern Interconnect, the Western Interconnect, and the Texas Interconnect. In Alaska and Hawaii, several systems encompass areas smaller than the State (e.g., the interconnect serving Anchorage, Fairbanks, and the Kenai Peninsula; individual islands)

Fossil fuel steam-electric power plant

An electricity generation plant in which the prime mover is a turbine rotated by high-pressure steam produced in a boiler by heat from burning fossil fuels.

Combined cycle unit

An electric generating unit that consists of one or more combustion turbines and one or more boilers with a portion of the required energy input to the boiler(s) provided by the exhaust gas of the combustion turbine(s).

Combined heat and power (CHP) plant

A plant designed to produce both heat and electricity from a single heat source.

Solar Photovoltaic Cell

An electronic device consisting of layers of semiconductor materials fabricated to form a junction (adjacent layers of materials with different electronic characteristics) and electrical contacts and being capable of converting incident light directly into electricity (direct current). An integrated assembly of photovoltaic cells is referred to as a photovoltaic module or panel.

Solar thermal power generation

A solar thermal technology that uses a modular mirror system that approximates a parabola and incorporates two-axis tracking to focus the sunlight onto receivers located at the focal point of each dish. The mirror system typically is made from a number of mirror facets, either glass or polymer mirror, or can consist of a single stretched membrane using a polymer mirror. The concentrated sunlight may be used directly by a Stirling, Rankine, or Brayton cycle heat engine at the focal point of the receiver or to heat a working fluid that is piped to a central engine. The engine then acts as a conventional steam-powered turbine to generate electricity.

Nitrogen oxides (NOx)

Compounds of nitrogen and oxygen produced by the burning of fossil fuels. NOx compounds are among the “criteria pollutants” regulated under the Clean Air Act. These compounds have been associated with acid rain and the formation of ground-level ozone (smog) in urban areas.

Sulfur oxides (SOx)

Compounds containing sulfur and oxygen, such as sulfur dioxide (SO2) and sulfur trioxide (SO3). Sulfur dioxide is one of the “criteria pollutants” covered under the Clean Air Act; it historically has been associated with acid rain.

Greenhouse gases

Those gases, such as water vapor, carbon dioxide, nitrous oxide, methane, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride, that are transparent to solar (short-wave) radiation but opaque to long-wave (infrared) radiation, thus preventing long-wave radiant energy from leaving Earth's atmosphere. The net effect is a trapping of absorbed radiation and a tendency to warm the planet's surface.

*Cushing, Oklahoma

An oil pipeline hub and major storage area for crude oil, located in central Oklahoma.  It is the settlement point for the West Texas Intermediate crude oil futures contract on the New York Mercantile Exchange (NYMEX).

Henry Hub

A natural gas pipeline hub on the Louisiana Gulf coast. It is the delivery point for the natural gas futures contract on the New York Mercantile Exchange (NYMEX).

Wood conversion to Btu

Converting cords of wood into a Btu equivalent is an imprecise procedure. The number of cords each household reports having burned is inexact, even with the more precise drawings provided, because the estimate requires the respondent to add up the use of wood over a 12-month period during which wood may have been added to the supply as well as removed. Besides errors of memory inherent in this task, the estimates are subject to problems in definition and perception of what a cord is. The nominal cord as delivered to a suburban residential buyer may differ from the dimensions of the standard cord. This difference is possible because wood is most often cut in lengths that are longer than what makes a third of a cord(16 inches) and shorter than what makes a half cord (24 inches). In other cases, wood is bought or cut in unusual units (for example, pickup-truck load, or trunk load). Finally, volume estimates are difficult to make when the wood is left in a pile instead of being stacked. Other factors that make it difficult to estimate the Btu value of the wood burned is that the amount of empty space between the stacked logs may vary from 12 to 40 percent of the volume. Moisture content may vary from 20 percent in dried wood to 50 percent in green wood. (Moisture reduces the useful Btu output because energy is used in driving off the moisture). Finally, some tree species contain twice the Btu content of species with the lowest Btu value. Generally, hard woods have greater Btu value than soft woods. Wood is converted to Btu at the rate of 20 million Btu per cord, which is a rough average that takes all these factors into account.


U. S. Energy Information Administration

*Seth Blumsack, Department of Energy and Mineral Engineering, Penn State