This paper analyses the accuracy of the calculation of the annual heating and cooling energy needs for modern Nordic buildings in Finnish climate. The results are based on a joint research – Nordic thermal mass – in which six energy simulation programs and one energy balance method based on standard EN 832 were used. The last mentioned is the predecessor of ISO DIS 13790 Thermal performance of buildings – Calculation of energy use for space heating and cooling. The inaccuracy in the calculated needs for heating energy was approximately 6 kWh/m2/a (9 %). In the cooling energy the inaccuracy was much higher, 5 kWh/m2/a (55 %) for the single-family house and 20 kWh/m2/a (45 %) for the apartment flat. The monthly energy calculation method gave heating energy needs which were fully comparable in accuracy with the results of the simulation programs for massive buildings (time constant 60 – 200 h). On the other hand the heating energy need for the extra-light building studied (time constant 17 h) was approximately 10 % too high compared with the results of the simulation programs. The reason for the too high heating energy of the very light building is, that the equation for the utilisation factor for internal heat gains in ISO FDIS 13790 gives too low values for very light buildings. This can be seen by comparing the utilisation factors of ISO FDIS 13790 with those calculated by the simulation programs of our study. On the other hand the utilisation factor of ISO FDIS 13790 seems to be correct, when the building’s time constant is at least 50 h. This means that the building should include at least one massive surface, e.g. a massive floor.