A refrigerator with coefficient of performance 4 extracts 400 J of heat from the cold reservoir per cycle. The work input required is:

The Carnot cycle alternates two isothermal steps (heat absorbed at T_H, released at T_C) with two adiabatic steps (no heat exchange); the enclosed loop area equals the net work output, and η = 1−T_C/T_H is the maximum possible efficiency between those two temperatures.