A methodology is developed to determinate the optimal size of PV-hybrid subsystems and to optimize the stand-alone system management. Two particular parameters characterize the back-up engine generator: SDM and SAR, respectively the starting and the stopping thresholds calculated as a part of the nominal storage capacity. Using simulation calculations, the optimal configuration leading to the autonomy constraint, is determined on the basis of a minimization of the kWh cost. The operating back-up generator strategy study has shown that the smallest kWh cost is obtained for a system characterized by a SDM = 30% and SAR = 70% of the nominal battery capacity.