What is a BTU?

BTU is a basic measure of thermal energy and 1 BTU is the amount of energy needed to heat one pound of water in order to raise its temperature one degree Fahrenheit. It is equivalent of 252 heat calories and approximately 1/3 of wh (watt-hour). When referring to cooling power, the BTU works in reverse; the air-cooling power of an air conditioning system refers to the amount of thermal energy removed from the area.