The coefficient of performance (COP) is the ratio of the work or useful energy output of a system to the amount of work or energy put in to the system as determined by using the same energy equivalents for energy in and out. COP is used as a measure of the steady state performance or energy efficiency of heating, cooling, and refrigeration appliances. It is equal to the energy efficiency ratio (EER) divided by 3.412. The higher the COP, the more efficient the device.