Measures of the Quantity of Thermal Energy

The unit of quantity of thermal energy used in the United States is the British thermal unit, which is the quantity of heat or thermal energy required to raise the temperature of one pound of pure water one degree F. (American National Standard abbreviation, Btu; conventional British symbol, B.Th.U.)

The French thermal unit, or kilogram calorie, is the quantity of heat or thermal energy required to raise the temperature of one kilogram of pure water one degree C.

One kilogram calorie = 3.968 British thermal
units = 1000 gram calories.

The number of foot-pounds of mechanical energy equivalent to one British thermal unit is called the mechanical equivalent of heat, and equals 778 foot-pounds.

In the modern metric or SI system of units, the unit for thermal energy is the joule (J); a commonly used multiple being the kilojoule (kJ), or 1000 joules.

One kilojoule = 0.9478 Btu.

Also in the SI System, the watt (W), equal to joule per second (J/s), is used for power, where one watt = 3.412 Btu per hour.