Coefficient of Performance (COP) measures the efficiency of a heat pump or refrigeration system by dividing useful heat output by electrical energy input. A COP of 3.0 means the system delivers 3 units of heat energy for every 1 unit of electrical energy consumed.
COP is most commonly used to describe heat pump heating efficiency at a specific outdoor temperature. Unlike HSPF2 (which averages performance over a season), COP is an instantaneous measurement. A heat pump might have a COP of 3.5 at 47°F outdoor temperature but only 1.8 at 17°F.
For comparison: electric resistance heating always has a COP of 1.0 (100% efficient conversion of electricity to heat). Air-source heat pumps typically achieve COP 2.0-4.0 depending on conditions. Geothermal heat pumps achieve COP 3.0-5.0. Higher COP always means lower operating costs.