Heat energy is measured by the British thermal unit (Btu). Each British 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 one Btu being equivalent to 251 IT (International Steam Table) calories, or 778.26 ft.-lb. of mechanical energy units (work). Taking into consideration that one IT calorie equals 1/860 of a watt-hour, one Btu is then equivalent to about 1/3 watt-hour.

Prior to its redefinition in 1929, one Btu was defined as the amount of heat necessary to raise 1 lb. of water 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 electrical unit.

1 BTU = 1055.05585262 Joules = 0.251995761111 kcal

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