For a long time, research and policy has focussed on reducing energy demand and increasing energy efficiency. However, with increasing renewable energy production, also the profile of this energy demand and the potential for demand side management needs to be assessed. Moreover, due to energy distribution restrictions, simultaneous power peaks on the grid need to be reduced or shifted. Regarding dwellings, for which heating still determines the bulk of the energy use in most of the EU, this means that reducing, or at least better characterizing and managing the heat load becomes more important. The current method for the calculation of the design heat load, according to the EN12831-1, is a static method and tends to overestimate the heat load. This research based on 50 in-situ measurement cases suggest indeed an oversizing between 25% and 100%! Further linear regression analysis and a parametric study based on dynamic building simulation was used to pinpoint possible physical causes for this "power gap". Following possible causes could be identified: The standard assumes a 100% simultaneous occurrence of all worst case boundary conditions (low design outdoor temperature - relatively high wind speeds and pressures leading to high infiltration losses - no solar gains - no internal heat gains) which does not occur in reality. Furthermore, the heat load for the heat generator (at building level) is defined as a simple sum of the heat load of each individual space. This might also lead to oversizing, as infiltration or ventilation are not at their maximum in all spaces at the same time. Finally, the difference between monitoring and simulation results suggests that users adapt their behaviour below 0 °C, by reducing their comfort expectations (e.g. less window opening) or other heat loss reducing actions (e.g. keep inner doors closed so that less heat is transferred to unheated zones). Formal conclusions could be used for eventual future standard improvements.