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How is a British thermal unit defined?

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In a thermoelectric generating system a heat source—usually fueled by coal, oil, or gas—is used within a boiler to convert water to high-pressure steam. The steam expands and turns the blades of a turbine, which turns the armature of a generator, producing electric power. A condenser converts any remaining steam to water, and a pump returns the water to the boiler.
heating In a thermoelectric generating system a heat source—usually fueled by coal, oil, or gas—is used within a boiler to convert water to high-pressure steam. The steam expands and turns the blades of a turbine, which turns the armature of a generator, producing electric power. A condenser converts any remaining steam to water, and a pump returns the water to the boiler.

The British thermal unit (BTU) is a measure of the quantity of heat used in heating and cooling systems. Since 1956, it has been defined as approximately equal to 1,055 joules or 252 gram calories.1

Historically, the BTU was defined as the amount of heat required to raise the temperature of one pound of water by 1° F. This definition was eventually replaced because it depended on the initial temperature of the water.1

While the BTU is not part of the metric system, it remains a practical standard for rating consumer appliances like furnaces, stoves, and air conditioners, as well as for measuring fuel consumption. In the context of gas utilities, a larger unit called the therm—defined as 100,000 BTU—is frequently used.21