What is Efficiency and coefficients of performance?

The efficiency of an engine is the work delivered per unit heat taken from the hot source, $\eta = -W/Q_h$; the coefficient of performance (COP) of a refrigerator is the cold produced per unit work, $e_f = Q_c/W$; that of a heat pump is the heat delivered per unit work, $e_p = -Q_h/W$. Each is the ratio “what one wants” over “what one pays”; an efficiency is at most 1, a COP can exceed 1.