According to the second law of thermodynamics (see Carnot-cycle), the maximum theoretical efficiency of a cycle that uses heat from a high temperature reservoir at 80 °C and has a lower temperature of 20 °C is 1−(273+20)/(273+80)=17%.
Prema drugom zakonu termodinamike (vidi Carnotov ciklus), najveća teoretska učinkovitost ciklusa koji koristi toplinu iz ogrjevnog spremnika na 80 °C i ima nižu temperaturu od 20 °C je 1−(273+20)/(273+80)=17%.