COP. Coefficient of performance

A heat pump moves heat from a cold place (outside air, ground, water) to a warm place, using work (electricity). $COP_{heating} = \frac{Q_{condenser}}{W} \qquad Q_{condenser} = W + Q_{evaporator}$

Why COP > 1: the environmental heat $Q_{evaporator}$ is free; only $W$ (what you pay for) goes in the denominator. Energy is still conserved: out = $W$ + free heat.

Related: Energy balance, Sensible heat