The paper deals with an evaluation of building energy performance during its operation. The Energy Performance functionality is presented. The functionality is a part of the intelligent building management system within the project „Extension of the intelligent building control system product“, which is part of the Applied Research and Experimental Development Program "TRIO" announced by the Ministry of Industry and Trade of the Czech Republic. This project is being developed at the Department of Indoor Environmental and Building Services Engineering of the Faculty of Civil Engineering, Czech Technical University in Prague. After the description of the principles of the functionality and its outputs, the case study of a family house is performed.