Heat Pump COP efficiency explained
The COP of a heat pump stands for the "Coefficient Of Production" - which is basically how efficient it is.
It is the ratio of: Heat out : Electricity in. So if a heat pump has a COP of 1 then for every 1 kWh of electricity it uses it would produce 1 kWh of heat. A COP of 1 would be rubbish though! Most heat pumps have a COP of over 2.5. So for every 1 kW of electricity they use they produce over 2.5 kWh of heat.
Ground and water source heat pumps can have even higher COP's - of 4 or more. That is because it is easier to extract heat from the ground or water than it is to extract it from air. Just think about the difference between being outside in 10ºC air, or in 10ºC water - you get cold a lot Water source can be even higher.
And that's all about how easy it is to "pull" the heat out from whatever the heat pump is pulling the heat from. Each of those things transfers heat a little better than the next. That's why you get colder more quickly in water than if you were buried in earth, and you get colder buried in earth than just wandering around in the air.
The COP is also effected the temperature of the medium that the heat pump is pulling the heat from. The graph below shows how the COP of an air source heat pump varies with outside air temperature. This graph is based on the heat pump supplying water at 55ºC - the COP will also vary depending on what temperature water the heat pump has to produce.
Heat pumps are also very efficient at producing low temperature heat. If the heat pump has to heat the water to 60ºC then its COP will be lower than if it only has to heat water to 45ºC. It still runs well at 60ºC, it is just important to consider these things when designing heating systems powered by heat pumps.