The SI unit of energy is the joule (J). Many other units for energy are in use for the practical expression of energy quantities partly for historical reasons and partly because the small size of the joule demands the use of unfamiliar (for non-scientists) decimal prefixes. As a result, the international organisations have used units for energy of a size appropriate for expressing national fuel supplies and related to the commodities in use. Historically the ton of coal equivalent was used but, with the ascendance of oil, this has been largely replaced by the tonne of oil equivalent (toe) defined as 41.868 gigajoules. Many national balances use this unit but the tarajoule is increasingly used in accordance with the recommendations by the International Standards Organization (ISO).
There are several definitions of the calorie in use. The conversion equivalent between the calorie and the joule given here is the International Steam Table (IT) value which is defined to be 4.1868 joules. Similarly, the internationally agreed value for the British thermal unit (Btu) is now 1 055.06 joules. The Btu is the basis for the quad (1015 Btu) and the therm (105 Btu).
Conversion Equivalents between Units of Energy