British thermal unit

Despite its name, the British thermal unit, or BTU, may be more widely used in North America than in Britain. Air conditioners, furnaces, and stoves are generally rated by BTUs. (Though "BTUs" is often short for "BTUs per hour;" in air-conditioner ratings, for instance, "BTUs" really means "BTUs of cooling capacity per hour".) Fuels such as natural gas and propane are also compared using BTUs. The BTU first appeared in 1876 and isn't part of the metric system--the metric unit of energy is the much smaller joule--so it isn't much used by scientists, but its practicality keeps it popular for consumer goods and fuels. A better-known heat unit is the calorie; a BTU is equal to about 252 calories. (Since the familiar food calorie is actually a kilocalorie, a BTU equals only about a quarter of a food calorie.)