Off-grid power systems have been installed in many countries to provide a stable electricity supply to remote island areas. However, off-grid systems with internal-combustion engines, such as diesel-turbine cogeneration systems, have significant operating costs, e.g., high transport, management, and fuel costs, and additional complications. In this paper, installation of an off-grid system using renewable energy, i.e., a photovoltaic (PV) and battery energy storage system (BESS), is considered and a cost-minimizing installation method is proposed. The optimal parameters for managing the BESS state of charge (SOC) using weather information, which can reduce the operational expense for an installed off-grid system, are verified. Hence, the PV output, BESS capacity, number of diesel-engine (DE) operations, and appropriate BESS SOC are determined using the proposed method, actual load, and PV output data.