This paper adopts a single-period newsvendor model with supply uncertainties to be used for optimally sizing an electrical energy storage system (EESS) for an apartment house with a photovoltaic system. Hence, typical inventory cost components and supply chain characteristics are translated into the EESS application. The results show that existing research approaches for inventory management can be adapted to determine the optimal size of an EESS. The proposed EESS-sizing procedure takes into account the total cost of the storage system including energetic losses as well as the costs for energy supply from the own energy systems and from the energy supplier.