A battery energy storage system (BESS) is a promising technology to augment the benefits provided by photovoltaic (PV) power generation. This study proposes a method to evaluate the optimal capacity of a BESS for commercial electricity consumers with PV systems. This method considers the aging effect of a BESS and the PV output uncertainty through linear programming and a real-time control method. The benefit of BESS installation is assessed through cost-benefit analysis by using the net present value (NPV) in the determination of the optimal BESS capacity. Subsequently, the method is applied to the numerical simulation of a commercial load. In the numerical simulation, an optimal capacity is demonstrated at which the benefits of the BESS are maximum, especially the NPV. In addition, we highlight the impact of PV output uncertainty on the benefits of the BESS and verify the effectiveness of the proposed real-time control method in mitigating this impact. The importance of considering the degradation of a BESS in the evaluation of the optimal capacity is also discussed.