The Yuasa Battery Company and NGK Spark Plug Company have successfully completed installation, and started operation of a 1MW/8MWh Na/S battery energy storage plant in the Tatsumi energy storage facility of The Kansai Electric Power Company, under the sponsorship and direction of the New Energy and Industrial Technology Development Organization. The 1MW/8MWh Na/S battery plant has been constructed in two steps. The first half of the plant, corresponding to 0.5MW/4MWh, was constructed in September, 1990, and tested until May, 1991, when 85 charge/discharge cycles were counted. The second half plant corresponding to 0.5MW/4MWh was assembled in May, 1991, heated up to the operating temperature, and combined with the first half plant to complete a full 1MW/8MWh Na/S battery energy storage plant. After passing the official test, the operation of the full 1MW/8MWh battery plant officially started. The full size 1MW/8MWh Na/S battery plant functioned satisfactorily with charge/discharge efficiencies of about 86.3% on the DC/DC basis excluding auxiliary power consumption, and 126 charge/discharge cycles were completed by March 31, 1992. During this period, the first half 0.5MW/4MWh plant achieved of 211 cycles. The operation of the 1MW/8MWh full plant was officially continued till the end of March, 1992, followed by independent research by Yuasa Battery Company and The Kansai Electric Power Company.