"BTU" or the British thermal unit

Definitions

A BTU is defined as amount of heat required to raise the temperature of one pound of liquid water by one degree from 60 to 61 degrees Fahrenheit at a constant pressure of one atmosphere. As is the case with the calorie, several different definitions of the BTU exist, which are based on different water temperatures and therefore vary by up to 0.5%: A BTU can be approximated as the heat burned in a single wooden match or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet.

 BTU is also the equivalent of 252 heat calories, not to be confused with the kilo-calories of food, and of approximately a third of a watt-hour. When speaking of cooling power, the BTU also works in reverse. The air-cooling power of an air conditioning system refers to the amount of thermal energy removed from an area. Hence a 65,000 BTU heater and a 65,000 BTU air conditioner are of roughly the same capacity and size. The higher the BTU output, the more powerful the heating or cooling system.

In North America, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners.

When used as a unit of power, BTU 'per hour' (BTU/h) is understood, though this is often abbreviated to just "BTU".

 The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix, which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a term is used representing 100,000 or 105 BTU, and a quad as 1015 BTU.