The SI unit of heat is the joule (J) . Heat is a form of energy that is transferred from one body to another due to a temperature difference. The amount of energy required to raise the temperature of a given mass by one degree is represented in joules. For example, 4.184 joules of heat energy is necessary to increase the temperature of a unit weight (say 1 g) of water from 0 degrees to 1-degree Celsius. Other units of heat include the British thermal unit (BTU) and the calorie, but the joule is the standard unit of heat in the SI system.