A BTU is a measurement of thermal energy where one BTU is equal to the amount of energy needed to raise the temperature of 1 pound of water by 1° Fahrenheit. Even small space heaters can produce thousands of BTUs. A central air conditioner, which uses hundreds of thousands of BTUs. BTU, short for British thermal unit, is a basic measure of thermal (heat) energy. One BTU is the amount of energy needed to raise the temperature of 1 pound (0.45 kg) of water by 1° Fahrenheit (0.55° Celsius).