COP
COP (Coefficient of Performance) measures the efficiency of a heat pump as the ratio of heat moved to electricity consumed. A COP of 3 means the unit delivers 3 units of heat for every 1 unit of electricity input — the other 2 are pulled out of outdoor air or ground. Modern cold-climate air-source heat pumps run COP 3+ at mild temperatures and COP 1.5-2 at design conditions; ground-source heat pumps maintain COP 3-4 year-round. COP is the metric to compare heat pumps against; SEER and HSPF are seasonal averages.