The measure of a heat pump’s heating performance is called coefficient of performance (COP). It’s simply the ratio of useful heat output divided by required energy input, where both the output and input are expressed in the same physical units.

For example, if an air-to-water heat pump delivers 45,000 Btu/h of heat output to a stream of water and has a power input of 5,000 W, its COP, under those specific operating conditions, can be calculated as follows: