Vapor compression refrigeration machines (VCRM) are used in numerous and various cooling systems. While manufacturing them, there is a tendency to reduce specific energy consumption for cold production. One of the ways of reducing costs in the operation of the VCRM is organization of its work with an optimum cooling temperature. To develop an effective VCRM, we need to have a two-parameter model of the compressor, which describes its volume and power characteristics. Development of such a model based on experimental study of a hermetic reciprocating compressor has been reported. These data were used in determining the optimum cooling temperature corresponding to the maximum values of exergetic efficiency of VCRM cycles with different losses from internal and external irreversibility. It has been established that considering these losses, optimum R134a refrigerant boiling temperature is the same, equal to 243 K. There is detected conservatism of optimal cooling temperature in case of significant losses from irreversibility in the VCRM cycle.