A BTU, short for British Thermal Unit, is a basic measure of thermal (heat) energy. One BTU is the amount of energy needed to heat one pound of water one degree Fahrenheit, measured at its heaviest point. In other words, if you placed 16 ounces of water at 59°F into a stovetop pan and turned on the gas burner, it would take one BTU to raise the temperature of the water to 60°F. As more BTUs continue to flow from the gas flame, the water will eventually reach the boiling point of 212°F.

A 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.

Strangely enough, the British Thermal Unit is rarely used in Great Britain anymore, where it is considered a non-metric measurement. Even in countries which use the BTU as a standard measurement, there is some disagreement over the formula used to derive it. The thermal energy needed to raise water one degree Fahrenheit can depend on the original temperature and the method used for heating. Therefore, it is possible to get several different definitions of a BTU from different sources. This rarely has a palpable effect on consumer product information, however.

Most heating and cooling systems produce thousands of BTUs, almost rendering the measurement of one BTU pointless. One is more likely to encounter smaller BTU figures during scientific experiments, where the slightest change in thermal energy may need to be calculated in terms of calories. When dealing with central air conditioning units and commercial pizza ovens, however, the BTU numbers can easily reach the hundreds of thousands. A unit of measure called the MMBTU is the equivalent of a million BTUs. Few man-made objects can generate this level of thermal energy, however.

When shopping for heating or cooling systems, keep in mind that even the smallest window-mounted air conditioner or space heater can produce thousands of BTUs. The BTU numbers should primarily be used as a comparison between systems. Larger and more expensive systems should provide significantly higher BTUs than smaller ones. When deciding between similarly priced units, compare the BTUs for a better gauge of performance.