Calculations are presented of the efficiencies of fuel-burning plants by using chemical energies and exergies of fuels. Studies of the available methods for estimating these characteristics for solid and liquid fuels were carried out, and one was chosen for practical application after some improvements To show the differences between calculations based on chemical energies/exergies and lower calorific values of fuels, the energy/exergy balance of a steam generator is considered and its energy/exergy efficiencies are estimated.