From an ecological point of view, the reduction of the "consumption" of energy is the first aim. The reduction of the CO2-emissions is only a subsidiary aim. On the other hand in industry, the reduction of the costs for energy is the main motive power to increase the efficiency of energy use. Therefore, efficiency may be interpreted in different ways and the understanding of efficiency is led by different criteria. Nevertheless, from a scientifical point of view, an efficient use of energy means the minimization of the input energy for a given energy application. This objective definition has been applied in the German directive for the evaluation of the efficient use of heat in buildings within the "Energie-Einspar-Verordnung 2002" [EnEV - Verordnung fiber energiesparenden Warmeschutz and energiesparende Anlagentechnik bei Gebauden - Bundesgesetzblatt Teil I Nr. 59 vom 21. Nov. 2001. S. 3085 ff.], the German directive for the saving of energy in buildings. In this paper, a criterion for the rational use of energy in industry. The starting point therefore is the exergy and the corresponding input energy. A project for the efficient use of energy in industry that is run in Germany [Schaumann G, Pohl Ch. Transferstelle Bingen Projekt since 2000; www.energie-industrie.de.] is presented and the formulae for the cogeneration of heat-and-power is shown as an example. (c) 2007 Elsevier Ltd. All rights reserved.