A refrigerator is a 'reverse' heat engine that uses work input ($W$) to move heat from a cold reservoir ($Q_c$) to a hot reservoir ($Q_h$).

• Instead of efficiency, we use the Coefficient of Performance (COP).
• For a refrigerator, the desired effect is cooling ($Q_c$): $CO{P}_{ref}=\frac{\text{Desired cooling}}{\text{Work input}}=\frac{Q_c}{W}$. For a heat pump, where heating ($Q_h$) is the goal, $CO{P}_{hp}=Q_h/W$.