Volumetric distribution of the built form really affects the capability of an urban environment to integrate renewable energy, with specific regard to active and passive use of solar radiation collected by the building envelope. The solar potential of a generic surface does not directly correspond to the energy received, but is related to the effective possibilities for exploitation. This study deals with defining an index of horizontal solar collection which expresses the amount of direct solar radiation received by roofs compared to the corresponding overall gross floor area of an urban block. This parameter represents a significant tool in assessing the efficiency of different types of buildings with regard to technical use of solar gains. The proportion between height and the plan of a building assumes a crucial role. The comparative analysis presented here seeks to examine the relationship between slenderness and potential of an urban block to match its overall domestic hot water through solar gains. The results allow formulation of early considerations about the most appropriate distribution of horizontal and vertical exposure surfaces and about the combination of active and passive use of solar energy.