The aim of this paper is to develop a methodology that allows to know a optimal size of a photovoltaic (PV) generator in a building (dwellings, commercial buildings, etc.). The adopted design criterion is to optimize the profitability and the amortization of PV installations. There is an optimal maximum size for each building, the of the PV installation decreases strongly for sizes higher than the optimal one. This size minimizes the photovoltaic electricity fed into the grid and covers part of the building electricity consumption. Thus, for different groups of buildings, an optimal rate between consumed annual energy by the building and the size of the PV generator can be provided. This parameter will permit to define the appropriate size of the PV generator for a building, according to Its annual consumption and group A sensibility analysis to the optimal solution is important when the modules integration in the building doesn＇t allow to choose this option.