Fig. 9 depicts the self-sufficiency ξ of the PV system. For a specific Pe, with the increasing C, ξ increases at first and then saturates to a certain value due to the poor PV generation in the winter. Even for the hypothetic case of the FESS without any auxiliary power demand (Pe = 0) and a very large capacity C,the self-sufficiency is limited to approx. 65%. With increasing Pe, ξ drops, as Pe has to be satisfied by discharge of the FESS when operating in consumption dominant mode, thereby the usable amount of energy is reduced. Actually, Pe increases with higher capacities depending on the technology applied. Therefore, an oversized capacity of the FESS is not reasonable.