Understanding heat pump performance
Switching to a heat pump?
Nigel Griffiths
explains why lab-tested CoP figures don’t tell the full story on efficiency and real-world savings
Heat pumps are often sold based on impressive-sounding CoP (Coefficient of Performance) figures. But what’s behind these numbers, how reliable are they and do they reflect what a heat pump system will actually achieve in real-world conditions? To make sense of the data – and understand whether your install will save money compared to a boiler – you need to ditch the sales pitch. When investigating true heat pump performance, a good place to start is the difference between the three main measures: CoP, SCoP and SPF.
What is CoP?
A heat pump’s coefficient of performance (CoP) is a measure of efficiency conducted in a laboratory environment. Put simply, a CoP represents the heat energy you get out of the appliance, divided by the amount of electricity required to run the pump:
CoP = Heat Output / Electricity Input
So, if you could produce three units (usually kilowatt-hours, kWh) of energy out of a heat pump for every unit of electricity input, then the CoP would be three (3/1 = 3). People selling heat pumps often quote the CoP figure – but the fact it’s achieved in laboratory conditions means it isn’t generally replicable.
Remember, the CoP is only a snapshot over a short period of time in standardised conditions. This is typically meant to represent a 7°C external air temperature and 35°C water output (which essentially relates to an optimal system in a well-insulated home with underfloor heating, so CoP in a retrofit is likely to be lower). Depending on the heat pump type, there may also be values for 55°C and 65°C output (which is the same range a boiler would deliver).