Evaluation of thermodynamic efficiency of a power plant is usually performed using the method of so-called thermal diagnostics, based on energy balancing. Energetic analysis is however suitable only for a quantitative assessment and for comparing similar technologies. In order to properly assess the origins of energy losses in the given system, an exergetic analysis has to be applied. The paper describes the rules of exergetic diagnostics, which greatly extends the potential of classic thermal diagnostics. A calculation example of a combined cycle power plant is included. The example demonstrates the potential of exergetic diagnostics for locating exergy losses and explains the reasons for increased consumption of fuels by comparing two working conditions of the system: reference and operational.