Principal Units and Conversion Factors
This report uses a variety of units to describe the supply and consumption of energy. Although these units are in common usage throughout the energy industry, they are generally not well understood by nonexperts. This appendix describes the principal supply and consumption units and provides some useful conversion factors. The Department of Energy–Energy Information Administration’s website (see www.eia.doe.gov/basics/conversion_basics.html) provides additional information about energy units and conversion factors, including easy-to-use energy conversion calculators.
Electrical generating capacity is expressed in units of kilowatts (kW), megawatts (MW = 103 kW), and gigawatts (GW = 106 kW). It is defined as the maximum electrical output that can be supplied by a generating facility operating at ambient conditions. Coal power plants typically have generation capacities of about 500 MW; nuclear plants about 1000 MW (1 GW); intermittent sources (e.g., natural gas peaking plants and wind plants) about one to a few megawatts; and residential roof-top installations of solar photovoltaics about a few kilowatts.
Electricity supply and consumption are expressed in units of kilowatt-hours (kWh), megawatt-hours (MWh = 103 kWh), gigawatt-hours (GWh = 106 kWh), and terawatt-hours (TWh = 109 kWh). One kilowatt-hour is equal to the energy of 1000 watts (the typical amount of elec-
tricity that is consumed by a handheld hair dryer) supplied or consumed over a period of 1 hour. Annual total delivered electricity in the United States is about 4,000 TWh, and the average annual electricity consumption per U.S. household is about 11,000 kWh.
FOSSIL FUELS AND OTHER LIQUID FUELS
Coal supply and consumption are usually expressed in units of metric tons (tonnes); 1 metric ton is equal to about 2205 pounds. A typical coal-fired power plant consumes about 2 million tonnes of coal per year, and annual coal consumption in the United States is about 1 billion tonnes per year. Coal prices are expressed in units of dollars per gigajoule ($/GJ). A tonne of coal contains about 23.5 GJ of energy.
Petroleum and gasoline supply and consumption are expressed in units of barrels (bbl); a barrel contains 42 U.S. gallons or 159 liters. Units of barrels of gasoline equivalent (bbl gasoline eq) represent the energy content of other liquid fuels (e.g., ethanol) in terms of the energy content of a barrel of motor gasoline. The United States consumes about 9 million barrels of motor gasoline per day and over 7 billion barrels of liquid fuels (crude oil, finished products, and other liquid fuels) per year.
Natural gas supply and consumption are expressed in units of trillion cubic feet (Tcf). The United States consumes about 23 Tcf of natural gas each year.
Biomass supply for liquid fuels production is expressed in units of dry tonnes; 1 dry tonne is equal to about 2205 pounds. The dry ton equivalent is 2000 pounds.
ENERGY CONVERSION FACTORS
Total energy supply and consumption are expressed in British thermal units (Btu) and quads (quadrillion Btu = 1015 Btu). A Btu is defined as the amount of energy (in the form of heat) needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit. The energy content of electricity and natural gas, liquid, and coal fuels can be quantified in terms of Btu using the following approximate factors:
1 kilowatt-hour electricity = 3,412 Btu
1 barrel crude oil = 5,800,000 Btu
1 barrel gasoline = 5,200,000 Btu
1 barrel fuel ethanol = 3,500,000 Btu
1 cubic foot of natural gas = 1,028 Btu
1 tonne coal = 22,230,000 Btu
Carbon dioxide (CO2) emissions from energy production and use are expressed in tonnes. The term tonnes of CO2equivalent (CO2 eq) indicates the global warming potential of other greenhouse gases (e.g., methane) in terms of CO2 quantities. The United States emits about 7 billion tonnes of CO2 eq per year, about 6 billion of which is CO2 arising primarily from energy production and use (see Figure 1.3 in Chapter 1). Average CO2 emissions in the United States are about 20 tonnes per person.