The energy certification of buildings implemented in the Member States of the EU with the transposition of Energy Performance Building Directive (EPBD) has stimulated the market for high-performance buildings, normally classified as A and A+. Such energy performance, however, is evaluated based on a theoretical calculation model that defines standard operating conditions. In Italy, as in most EU countries, performance assessments are not carried out under real operating conditions. The Lombardy region is a leading player in the field of social housing. In order to demonstrate transparency to the customer, the BEST Department of Politecnico di Milano was instructed to conduct a comparative evaluation between the actual and normalised energy performance of a high performance building located in Milan. The study showed that the real performance, calculated using data from three selected flats, was consistent with that calculated during the certification procedure. However real energy consumption is different even in identical flats, due to different behaviours of the users. (C) 2011 Elsevier B.V. All rights reserved.