International System of Units (SI)

The SI unit of energy is the joule (J). The joule is a derived unit defined as the amount of energy expended in applying one Newton of force through a distance of one metre. It can be reformulated in electrical terms as the energy needed to pass an electric current of one ampere through a resistance of one ohm during one second.

Commonly used energy units

The term calorie is in fact used for two units of energy. The small calorie (cal) is approximately the amount of energy needed to raise the temperature of one gram water by one degree Celcius. The large calorie (Cal), which often is used when discussing food and nutrition, is 1000 times the small calorie and therefore equal to one kilocalorie (kcal). As these units are part of the metric system since they are derived from grams and degrees Celcius they are exchanged for the Joule in the SI system. The name comes from the latin calor meaning heat which was what Nicolas Clément was trying to define a unit for in 1824.

The unit erg is defined as the amount of work expended by a force of one dyne applied over one centimetre. The electronvolt is often used when working on the small scales of physics and is defined as the amount of energy lossed or gained by a charge of one electron when it moves across one volt of electric potential difference.

British and U.S. specific units

The quad unit is practical on the big scales and is used by the U.S. Department of Energy when describing national and world energy consumption. The global energy production was 446 quads in 2004.

The therm is a British unit of heat energy. One unit is approximately equivalent of burning 100 cubic feet of natural gas.


British Thermal Unit is an older energy unit defined as the amount of energy it takes to heat one pound of liquid water one degree Fahrenheit.