According to the European Directive for the promotion of cogeneration, in order for cogeneration systems to be eligible for economic and financial benefits, they must comply with certain requirements regarding efficiency and primary energy savings. Therefore, the calculation of these two performance indicators, together with the electrical and/or mechanical energy produced in connection with the useful heat, is of crucial importance for the economic viability of these systems. In the present work, the main questions that have to be answered on this subject are posed, and a complete procedure to answer the questions and perform the necessary calculations is presented. This work not only is useful for the current needs, but also gives specific hints for future revision of the Directive on Cogeneration and the related texts. (C) 2012 Elsevier Ltd. All rights reserved.