The DRAFT ISO/DIS 13790 (Thermal performance of buildings – Calculation of energy use for space heating and cooling, 2005) describes the so called Monthly Balance Method to calculate the heating and cooling demand for the European energy certificate and will be used very often to optimize buildings. Due to a lot of assumptions in the method the paper compares the Monthly Balance Method with whole building simulations. The energy demand for heating and cooling will be calculated by a systematic variation of building properties (like gain/loss ratio and time constant) and outdoor climate. Part of the variation is using of Night Ventilation Concept to decrease cooling energy demand. Especially for this strategy the coupling between different zones (staircase, offices etc,.) is very important and can not be accounted for in the standardized calculation method. Out of the large number of results recommendations for adaption and the limits for the Monthly Balance are presented.