The objectives of energy systems diagnosis are: i) identifying components responsible for highest losses increments in comparison with design conditions, ii) quantifying energy-efficiency recovery when design conditions are restored in a particular system component. In this paper the energy diagnosis of an existing steam power plant is faced on the basis of three different formulae expressing global losses.A simulation model of a real 320 MW steam power plant has been implemented with a commercial modular energy systems simulation software (Aspen+). The functional decay of different components of the plant has been simulated using the model. The results show that the components actually responsible for the additional power losses can be identified and the effect of restoring design conditions can be quantified in most cases with sufficient approximation by using the General Formula for the Efficiency or the Lost work Impact Formula (developed in the ambit of thermoeconomics).