The subject of this thesis is the energy performance of a building assessment performed according to the HRN EN ISO 52016-1 standard. The aforementioned assessment was compared to the detailed energy simulation by using the computer program TRNSYS. The standard, issued in 2017, is still in the transition process, and has not yet been put to use. The thesis aims at comparing the deviation of the overall heating and cooling energy demand, in cases where the calculation of the demand is carried out according to the standard, with the results presented by the detailed simulation. For the purpose of this study a computer algorithm was developed, which carries out the assessment defined by the standard. The algorithm is based on the hourly calculation method that is computed as a series of equations and logical operators. The equations are solved by separating their coefficients and, afterwards, sorting the coefficients in matrices in such a manner that the coefficients coupled with the estimated inside temperatures and energy needs for every hour form the constant matrix and the free coefficients form the known variables matrix. The comparison of the two assessment methods was performed by using the example of the reference nearly zero energy building for hospital purposes located in the continental area of Croatia. Both calculations were carried out on an hourly basis for a typical meteorological year in the City of Zagreb climate area. In order to complement the comparison, both methods were further analysed in cases where the model excluded internal gains and infiltration. Also, the alterations in properties of the nonopaque elements had a direct impact on solar gains, which consequently resulted in the most significant results deviations. When the assessment was conducted according to the standard, the overall energy heating demand was 47,6% lower and overall cooling energy demand was 29,4% higher compared to the demand calculated based on the simulation. In the case where nonopaque elements were replaced with opaque ones, the heating energy demand deviation was reduced to 8,9% and the cooling energy demand deviation was reduced to 1,1%.