The method of determining the thermodynamic characteristics of gas-and-steam seat and power generating plants is presented. Considerable attention has been devoted to the influence of ambient temperature on the thermodynamic characteristics of the investigated systems basing on the formulated equations, including the steam part of the system. For the gas turbine, approximated equations have been presented, enabling one to evaluate changes in the flux of the consumed fuel and power rating depending on the ambient temperature. The choice of the minimum power rating of a heat exchanger was based on the criterion of the average annual efficiency of the transformation of the chemical energy of the fuel into the electric energy and heat.