The Swedish environmental assessment certification system Miljöbyggnad rewards lower energy use in comparison to that of building regulations, related to total heated floor area. Previous studies suggest that this indicator shows the efficiency of the technical design of the building; not how well the building is utilized. Thus, a more suitable quantity/unit can be the one that takes into account the extent of how the building is used in a more efficient and sustainable way. The aim of this paper is to discuss if current energy units are sustainable enough to assess efficient use of buildings and investigate how indicators can affect the grades and criteria of a certification system. Normalization of simulated energy use of a school building, by using the total heated floor area, the total building’s enclosing area, number of occupants and the schedule of occupancy, give different results.