Page "Heat pump" Paragraph 30

When used for heating a building on a mild day, for example 10 ° C, a typical air-source heat pump ( ASHP ) has a COP of 3 to 4, whereas an electrical resistance heater has a COP of 1. 0.

That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.

Note that an air source heat pump is more efficient in hotter climates than cooler ones, so when the weather is much warmer the unit will perform with a higher COP ( as it has less work to do ).

Conversely in extreme cold weather the COP approaches 1.

Thus when there is a wide temperature differential between the hot and cold reservoirs, the COP is lower ( worse ).