The acronym stands for British Thermal Unit, which is the unit used to measure thermal (heat) energy. Specifically, it is the amount of energy needed to raise 1 pound of water 1°F at sea level. The Btu is often used as a quantitative specification for the energy-producing or energy-transferring capability of heating and cooling systems such as air conditioners.

Please note that this calculator can only gauge a rough estimate. You should also consider factors like efficiency decrease of the heater or air conditioner with time, the shape/type of your home or room, insulation conditions, and other factors.