A cogeneration plant operates under very variable conditions within a reporting period (usually one year) and this frustrates the achievement of the high standards stipulated by the EU Directive from 2004. Taking into consideration these difficulties, one needs to start with the decomposition of the cogeneration plant in order to determine a virtual part of the plant that can meet the requirements of the Directive on the basis of conducted measurements and calculations. In this way, the plant can be partially qualified as eligible for economic and financial benefits. The focus of this paper is an analysis of the power loss coefficient of steam turbines for the generation of useful heat energy and electricity simultaneously. This applies to all cogeneration plants comprising extraction-condensing steam turbines and combined cycles (gas turbine and steam turbine) when heat is extracted prior to the bottoming cycle. The proposed procedure for calculation of power loss coefficient is easily applicable in industrial practice. The viability of the proposed method is demonstrated in the case of calculations for a 10 MWe (combined cycle gas turbine) plant. (C) 2013 Elsevier Ltd. All rights reserved.