The Directive 2002/91/EC of the European Parliament and of the Council of 16 December 2002 (Energy Performance of Buildings Directive, EPD) requires member states to introduce an energy certification in order to reduce energy consumption in buildings. This new directive should help reveal saving potentials in real estate in order to achieve the aim of the Kyoto Protocol. It will surely set off an impulse for energy efficient restructuring of buildings. The transparency concerning energy consumption of buildings will inevitably apply to all property transactions and lease prices of buildings. In some EU member states the question of how the benchmark for heating energy should be calculated is being discussed: On the one hand it is possible to calculate the theoretic demand for energy by using physical and geographical characteristics of buildings. On the other hand it is possible to determine real energy consumption by collecting the annual demand for heating energy. The comparison of both approaches shows that the results differ fundamentally in many cases - differences of over 60% can be observed. In face of the fact that the two approaches lead to totally different results, this ìgapî is analysed in this contribution. Therefore, the annual heating energy consumption of 50 buildings was collected. Simultaneously the demand-orientated calculation was carried out. Based on deductive hypothesis testing, influencing factors like habits of tenants, micro location factors or physical building attributes were analysed to identify systematic differences between the demand-orientated and the consumption-orientated approaches.