Fifteen years after the connection of the first PV plant to the distribution network of the city of Milan, the role of this technology is more and more crucial, considering the Italian and European energy transition goals. The availability of a zero-emission modular and dispersed electricity generation could allow the cities to meet the growing electricity demand in the coming years in a sustainable way. The PV plants that could be installed on the roofs of city buildings represent a precious resource, which has only been partially exploited. Quantifying this available surface can help to understand the electricity generation potential in relation to the future scenarios of electricity demand. This paper aims to analyze these aspects focusing on the city of Milan (Italy), determining the total power that could be installed on the roofs, its potential contribution to the electricity balance of the network distribution in 2020-2030 and the benefits for the grid in relation to the system losses.