British Thermal Unit
Heat energy is measured by the British thermal unit (Btu). Each thermal unit is regarded as equivalent to one unit of heat (heat energy).
Since 1929, British thermal units have been defined on the basis of 1 Btu being equal to 251.996 IT (International Steam Table) calories, or 778.26 foot-pounds of mechanical energy units (work). Taking into consideration that one IT calorie equals V860 of a watt-hour, 1 Btu is then equivalent to about V3 watt — hour.
Prior to its 1929 redefinition, a Btu was defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit. Because of the difficulty in determining the exact value of a Btu, it was later redefined in terms of the more fundamental physical unit.