The difference between the peak and valley of daily electricity load of large cities is big, which causes adverse consequences such as efficiency decrease of power plant and grid. The large peak-valley load difference is mainly caused by the civil buildings. Therefore, Direct Current (DC) power distribution and storage in buildings is considered to be able to solve this problem fundamentally. This paper proposes the optimal design methodology for the battery capacity. The optimal design methodology is verified by taking a small office building as a case study. The annual hourly power loads are simulated using the energy consumption simulation software DeST. The optimal battery capacities are discussed for two different control strategies of the DC power and storage system, i.e. constant power use from grid and demand-response-based power use from grid, which proved the effectiveness of the proposed optimal design methodology.