British Thermal Unit (BTU):

In some applications, a totally different unit of heat is used: the British thermal unit (Btu). You have heard this unit mentioned in advertisements for air conditioners and furnaces. When someone talks about Btus only in regard to the heating or cooling capacity of a furnace or air conditioner, this is an inappropriate use of the term. They actually mean to quote the rate of energy transfer in Btus per hour, not the total quantity of energy transfer in Btus.

The Btu is stated as the amount of heat which will raise or lower the temperature of accurately one pound (1 lb) of pure liquid water by one degree Fahrenheit (1°F). Does something seem flawed about this definition? If you are anxious about it, you have a fine reason. What is a pound? It depends where you are. How much water weighs 1 lb? On Earth's surface, it's around 0.454 kg or 454 g. On Mars, though, it takes around1.23 kg of liquid water to weigh 1 lb. In a weightless atmosphere, like on board a space vessel orbiting the Earth or coasting via deep space, the definition of Btu is meaningless since there is no such thing as a pound at all.

In spite of these flaws, the Btu is still used once in a while; therefore you must be familiar with it. Specific heat is occasionally identified in Btus per pound per degree Fahrenheit (Btu/lb/°F). In common, this is not similar number, for any given substance, as the specific heat in cal/g/°C.