At present the law of the Czech Republic (Decreee on the building energy performance) and of the European Union (Directive (EU) 2018/844 of the European Parliament and of the Council of 30 May 2018 amending Directive 2010/31/EU on the energy performance of buildings and Directive 2012/27/EU on energy efficiency) require an elaboration of a Building Energy Certificate even before the realisation of the building. But the standard procedures of building certification calculations have some problems (one-month time step, solar heating, human factor etc.) and it may not correspond with the real use of the building. In the form of case study the paper deals with data of the operation of a family house, which is located in Rymarov (Czech Republic) by comparison with a results of the Building Energy Certificate, which is also calculated for the house. The study also includes a comparison with a dynamic simulation of the family house.