Chemistry uses a series of energy units. The joule is the preferred, systematic unit.

The joule (J)

	the SI unit of energy.
	Energy or work is a force times a distance. Since force is a mass times an acceleration, the energy unit joule = kg m2 / s2 (where kg = kilograms, m = meters, and s = seconds).

The calorie (cal)

	the amount of energy required to raise 1 g of water 1 °C.
	1 cal = 4.184 J.
	The nutritional Calorie (note C not c) is really a kilocalorie (kcal) or 1 C = 1 kcal = 4.184 kJ

The British thermal unit (Btu)

	the amount of energy required to raise 1 lb of water by 1 °F
	1 Btu = 1055 J
	The Btu is still used extensively in engineering. Appliances like air conditioners are rated using these units.