COP (coefficient of performance)
Also known as: coefficient of performance, EER
Short definitionThe ratio of the heat a system removes to the electricity it uses. At a COP of 2.5, each 1 kW of electricity takes 2.5 kW of heat out of the room.
COP reduces efficiency to one number: cooling capacity divided by the power drawn by the compressor. The higher the number, the less electricity the same job needs. In air conditioning a similar ratio is given as EER, and its seasonal average as SEER. Catalogues list capacity and power side by side, so you can work out the ratio yourself.
The value is not fixed; it drops as the evaporating and condensing temperatures move apart. A compressor giving 3 in a 0 °C room falls to around 1.2 in a −25 °C blast freezing room. For this reason, COPs in competing quotes are read at the same operating condition. A dirty or undersized condenser, a room set colder than needed and a poor defrost setting directly hurt the COP, and so the bill.