There are almost 100 passive houses registered in the Czech Republic, whose design was influenced by simulations coming from various design software from early stages with maximization of energy savings in mind. The accuracy of the simulations however, depends on the level of simplification and accuracy of inputs, such as climatic data, etc. Especially the energy simulations require databases with hourly or monthly climatic data. The questions are, whether these are corresponding with reality and to what extend the difference between the database data and reality will affect the simulations and subsequently the use of the houses.This paper deals with modelling of and energy-efficient house under several sets of climatic data. The analysis was done in monthly steps using Energie 2013 (methodology of ČSN EN ISO 13790:2009) and PHPP 2007 software tools and in hourly steps on the basis of reference data of a climatic year described in ČSN EN ISO 15927-4:2011 (TRNSYS) and from the database of Meteonorm (TRNSYS). The aim of this paper is to point out the differences in the determination of heat consumption using different calculation methods..