New European standards are now under preparation which also extend to verifying the energy requirement of a building-plant system for air conditioning. For this aim a simplified technique like the utilization factor method, just foreseen for the heating season, has been taken into account. Therefore the results obtainable in this way are here compared with those from the dynamic simulation of the building-plant system by comprehensive computer programs such as DOE and BLAST. The analysis points out not only the feasibility, but also the limitation of this method.