The National Academies: What You Need To Know About Energy

The National Academies

What You Need To Know About Energy


Algal Biodiesel

A type of diesel fuel made by chemically processing oils from algae.

All-Electric Vehicle (EV)

A vehicle powered entirely by electricity stored in on-board batteries and without access to the use of gasoline or diesel fuel as an alternative energy source. Batteries are recharged by plugging them into an electricity source while the vehicle is parked.

America's Energy Future (AEF)

A project undertaken by the National Academy of Sciences, the National Academy of Engineering, and the National Research Council to evaluate current contributions and likely future impacts of existing and new energy technologies. More information about the project, including a roster of committee members, can be found on the AEF website.


A unit of measure for electric current that refers to the amount of electric charge passing a point per unit of time. Ampere is frequently abbreviated to “amp.”


Biologically-derived butanol, a four-carbon-atom alcohol (as opposed to ethanol, which is a two-carbon-atom alcohol), intended for use as an automotive transportation fuel. It is currently produced from the sugars and starches found in commodity crops using genetically engineered microorganisms.


Liquid fuels typically derived from harvested plant material, used primarily for transportation. These are different from fossil fuels, which are derived from transformed organic material residing in the Earth’s crust for millions of years.


A renewable energy source consisting of non-fossil biological material. This includes wood and wood byproducts, municipal waste, methane from landfills, and fuel from agricultural crops.

British Thermal Unit

A unit of measure for the energy content of fuels. One Btu is the amount of energy needed to raise a pound of water by one degree Fahrenheit.

Cap and Trade

An approach to control emissions of a given pollutant by setting a limit, or "cap," on the total amount permitted to be released. Industries, utilities, and other groups are required to purchase, or may be given, "credits" that establish allowable quantities of emissions. Unused credits may be sold in a resulting market for trading emissions allowances.

Carbon Dioxide (CO2)

A colorless, odorless, non-poisonous gas consisting of one carbon and two oxygen atoms. A by-product of fossil fuel combustion and other industrial processes, it is considered a greenhouse gas because it traps heat (infrared energy) radiated from Earth within the atmosphere. For this reason, CO2 is believed to be a major contributor to human-induced climate change.

Carbon Sequestration

The act of capturing gaseous atmospheric carbon, usually in the form of CO2, and placing it into a carbon sink through either biological or physical processes.

Carbon Tax

An approach to limiting emissions of carbon dioxide, a major contributor to human-induced climate change, by establishing a tax on goods and services that is commensurate with the amount of CO2 released in their creation and delivery.

Cellulosic Ethanol

An advanced type of biofuel that is produced by breaking down and using the cellulose compound found in trees and grasses.

Chemical Energy

Energy that is stored in chemical bonds between atoms within molecules. When a chemical reaction occurs, this can either increase the chemical energy within a molecule or release that energy into its surroundings as another form of energy (e.g., heat or light).

Clean Coal

Coal burned in power plants equipped with technologies that greatly reduce the emission of pollutants such as sulfur, nitrogen, mercury, and, potentially, carbon dioxide.

Climate Change

The process of shifting from one prevailing state in regional or global climate to another. Often used interchangeably with “global warming,” scientists sometimes also use the term to refer to periods of climatic cooling. Climate change is typically the preferred term over “global warming” because it helps convey that there are climate changes in addition to rising temperatures.

Coal Gasification

The process of converting coal into a gas before it is burned. The gas, called syngas, makes it easier to separate CO2 as a relatively pure gas before power is generated.

Corporate Average Fuel Economy (CAFE)

Federal standards that stipulate a target average fuel economy rating (typically expressed in miles per gallon, or mpg) to be met by passenger vehicles by a certain date. The most recent version of the CAFE law, pending passage by Congress, requires new cars, SUVs, and light trucks to average 35.5 mpg by 2016.

Distributed Generation

The generation of electricity from a number of small power plants located close to consumers as an alternative to concentrating power generation in large, centralized facilities located in remote areas. This places less stress on transmission lines and reduces the chance of widespread blackouts.


The capacity for doing work; usable power (as heat or electricity); the resources for producing such power.

Energy Content

The total amount of energy stored within a given quantity of fuel.

Energy Conversion

The transformation of energy from one form to another. For example, when coal (chemical energy) is burned, it produces heat (thermal energy) that is then captured and used to turn a generator (mechanical energy), which transforms the energy into electricity (electrical energy).

Energy Density

The amount of energy stored within a system, typically measured either as energy per unit volume or energy per unit mass.

Energy Efficiency

The achievement of using less energy without reducing the benefit provided by the end-use service. Energy efficiency is exemplified in a wide variety of applications—from improved lightbulbs and refrigeration to less energy-intensive industrial and manufacturing processes.

Energy Independence and Security Act of 2007

An act of Congress regarding the energy policy of the United States that was primarily focused on automobile fuel economy, development of biofuels, and energy efficiency in public buildings and lighting.

Energy Information Administration (EIA)

An agency within the Department of Energy (DOE) that provides policy-neutral data, forecasts, and analyses to promote sound policymaking, efficient markets, and public understanding regarding energy and its interaction with the economy and the environment.

Energy Intensity

A measure of a nation’s energy efficiency, typically represented through the amount of energy used per unit of gross domestic product (GDP).

Ethanol (or Ethyl Alcohol)

Best known as the type of alcohol found in alcoholic beverages, it is produced in large quantities through the fermentation of the sugars and carbohydrates in agricultural crops and blended with gasoline as an alternative to conventional oil-based fuels for motor vehicles.

Fossil Fuels

Fuels formed in the Earth’s crust over millions of years from decomposed organic matter. The most widely known fossil fuels are petroleum (oil), coal, and natural gas.


One billion watts, a watt being a unit of measure of power, or how fast energy is used. Gigawatts are typically used to describe very large quantities of power, such as the power carried by a major section of the U.S. electrical grid.

Global Warming

A term used to describe the phenomenon of Earth’s rising average near-surface temperature. Although such fluctuations have occurred in the past due to natural causes, the term is most often used today to refer to current warming trends. Most scientists have concluded that this is very likely due to the observed increase in human-generated greenhouse gas concentrations in the atmosphere.

Greenhouse Gas

A gas which, like a greenhouse window, allows sunlight to enter and then prevents heat from escaping—in this case, from Earth’s atmosphere. The most common greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons, and ozone (O3).

Hydrogen Fuel Cell

An emerging technology which uses a controlled combination of hydrogen and oxygen to generate electrical current, giving off only water vapor as a by-product.

Integrated Gasification Combined Cycle (IGCC)

A technology that converts coal into synthetic gas, or “syngas.” See Coal Gasification.

Intermittent Energy Source

An energy source characterized by output that is dependent on the natural variability of the source rather than the requirements of consumers. Solar energy is an example of an intermittent energy source since it is only available when the sun is shining. Wind is also an intermittent energy source.


One thousand watts, a watt being a unit of measure of power, or how fast energy is used. Kilowatts are typically used to describe intermediate quantities of power, such as power usage in a home.

Kilowatt Hour (kWh)

A unit of measure for energy, typically applied to electricity usage. It is equal to the amount of energy used at a rate of 1000 watts over the course of one hour. One kWh is roughly equal to 3,412 British Thermal Units (Btu).

Kinetic Energy

Energy that an object possesses as a result of its motion.

Mechanical Energy

The sum of kinetic energy and potential energy, or the energy stored within a system.


One million watts, a watt being a unit of measure of power, or how fast energy is used. Megawatts are typically used to describe large quantities of power, such as the power output of an electrical generating plant.

Metric Ton

Also referred to as a metric tonne, it is a measurement of mass equal to 1,000 kilograms, or the mass of one cubic meter of water. This is different from the short ton, a unit of measure commonly used in the United States, which is equal to 2,000 lbs.

National Research Council

A private, nonprofit institution that provides science, technology, and health policy advice under a U.S. congressional charter.

Natural Gas Combined Cycle (NGCC)

A process which helps improve the efficiency of natural gas power plants by capturing some of the waste heat given off by the combustion of natural gas in order to turn a steam turbine to generate electricity.

Nuclear Fusion

The union of two light atomic nuclei to form a heavier nucleus, resulting in the release of enormous amounts of energy.

Nuclear Regulatory Commission

An independent agency within the U.S. federal government that regulates commercial nuclear power plants and civilian uses of nuclear materials.

Particulate Matter

Extremely small particles of solid or liquid droplets suspended in either a liquid or gas. Particulate matter is a common emission from the combustion of fossil fuels and can increase the risk of health problems. Examples include dust, smoke, aerosols, and other fine particles.

Photovoltaic (PV) Cell

Sometimes referred to as a solar cell, a device that utilizes the photoelectric effect to convert incident sunlight directly into electricity. This can be distinguished from solar thermal energy, which is sometimes used to create electricity indirectly.

Plug-In Hybrid Electric Vehicle (PHEV)

A vehicle that contains a gasoline powered engine as well as batteries that can be charged when plugged into an electric power source. The vehicle typically runs on battery power until the charge has been depleted and then uses the gasoline engine for extended range.

Public-Private Sector Partnership (PPP)

A contractual agreement between a public agency (local, state, or federal) and a private-sector entity to deliver a service or product to the general public. For example, the FutureGen project is a collaboration of the U.S. Department of Energy and members of the coal industry to develop a near-zero emissions coal-fired power plant.

Pulverized Coal

Coal that has been crushed into a fine dust prior to combustion, allowing the coal to burn more rapidly and efficiently.


A unit of measure used to describe very large quantities of energy, such as the annual energy output of the United States. One quad is equal to one quadrillion—that is, one million billion, or 1015—Btu.

Renewable Energy Resource

An energy source that is naturally replenished. Examples include biomass, wind, geothermal, hydro, and solar energy.

Secondary Energy Resource (or Source)

A source of energy that is dependent on a primary source of energy for its power. Since the production of electricity, for example, is dependent on the use of fossil fuels, nuclear power, or renewable sources, it is referred to as a secondary energy source.


A photochemical haze that is produced when sunlight reacts with the emissions of hydrocarbons and oxides of nitrogen in the atmosphere. Primarily caused by excess automobile exhaust, it is a form of air pollution that can be threatening to human health.


Sustaining the supply of energy and materials needed to support current levels of consumption, making them available where most needed, and addressing the environmental problems resulting from their extraction, consumption, and disposal.

U.S. Department of Energy

A federal agency within the executive branch of the U.S. federal government responsible for the advancement of the national, economic, and energy security of the United States.

U.S. Department of Transportation

A federal agency within the executive branch of the U.S. federal government responsible for ensuring a fast, safe, efficient, accessible, and convenient transportation system.


A unit of measure for electric potential or electromotive force.


A unit of measure for power, or how fast energy is used. One watt of power is equal to one ampere (a measure of electric current) moving at one volt (a measure of electrical force).

Explore Other Topics

Energy Hands-on

What do you know about energy?

Which primary energy source did we depend on the most to generate electricity in the United States in 2008?

  • Sorry, that’s incorrect.

    In 2008, the United States depended most on coal to generate electricity. Coal provided approximately 49% of the electricity generated by the United States that year.

  • Sorry, that’s incorrect.

    In 2008, the United States depended most on coal to generate electricity. Coal provided approximately 49% of the electricity generated by the United States that year.

  • Correct!

    In 2008, the United States depended most on coal to generate electricity. Coal provided approximately 49% of the electricity generated by the United States that year.

  • Sorry, that’s incorrect.

    In 2008, the United States depended most on coal to generate electricity. Coal provided approximately 49% of the electricity generated by the United States that year.