Both on European and national scale, standards are now being drawn to extend to summer air conditioning the evaluation of energy requirement in building-plant systems. To this aim different simplified methods have been taken into account, anyway all similar to the utilization factor method already used for the winter season.The results obtained by the application of these methods in the case of typical buildings are therefore compared here with those from a dynamic simulation of building-plant system by means of comprehensive computer programs like Energy-Plus. The analysis points out the possibilities but also the limits of these simplified methods.