Coefficient of Performance
The ratio of cooling or heating to energy consumption.
for heat pumps
COP = QH/W
for refrigerators and air conditioners:
COP = QL/W
QH = Heat In
QL = Rejected Heat
Typical values for COP are in the range 2-4. The energy output exceeds the input, because the system takes advantage of the heat released or absorbed by the refrigerant when it condenses or evaporates.
A heat pump that delivers two units of cooling for each unit of electricity also rejects three units of heat. This then gives it a COP of 2.0 for cooling and a COP of 3.0 for heating.
The upper limit of COP is based on reversed Carnot Cycle:
COPc = TH /(TH - TL)
COPc = TL /(TH - TL)