The coefficient of performance (or COP) is the ratio of the energy output to work input.
For example in a heat pump the COP will be the heat transferred by the pump over the electricity input. It can also be expressed in terms of power as the rate of energy transfer into or out of the house.
The COP of a heat pump decreases as the outside temperature drops. As the temperature decreases there is less heat pumped and therefore the COP is lowered. --Fatima 12:50, 5 October 2007 (PDT) Template:Stub