The objective of this paper is to propose an adapted method of calculation to evaluate the solar potential on building facades on a large scale. Autonomous electricity supply is nowadays an important issue, particularly during the winter. Thus the facades of buildings, which are better irradiated in winter than the roofs, can provide a response to this issue. Research has been focusing for several years on fine modeling of radiative phenomena in an urban environment. The first challenge is to obtain a physically realistic model considering the phenomena of local reflections. The second challenge is to succeed in implementing this type of simulations on large scales. These two issues are antagonistic: the first implies making the model more complex while the second implies simplifying it. This paper proposes a calculation strategy to reconcile the two issues based on a simplified radiosity method. The results obtained via this strategy were compared to those obtained by measurement and simulation on a theoretical canyon. Subsequently, this calculation strategy has been deployed to real districts in Geneva, Switzerland.