This paper presents an evaluation of economical capacity of storage batteries equipped with residential PV systems. In around 2019, many of power companies' ten-year contracts with PV system owners will come to expire, pushing down the selling price of PV generated energy. This, if combined with a declining battery price, would make it more economical to self-consume PV generated energy than selling the electricity to the utilities. The authors explore the optimized storage battery capacity and charge-discharge pattern by using load and PV output data of 200 houses, and by linear programming. Results show 5.8 kWh battery is suitable for an average house with 4.5 kW PV system when the battery system price is about Y60,000/kWh. The authors analyze the daily storage start timing's impact on reverse power which affects power system operation, the optimum combination of PV and battery capacity, and each house's deciding factors for optimum storage capacity and so on.
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