A redox flow battery, one of the advanced batteries, has been in development since 1985 for the load-leveling of electrical power demand. A 10 kW class battery was successfully constructed and evaluated in 1986. At present, a 60 kW class battery system is under construction. To improve the system energy efficiency of this battery, several developing efforts have been done such as the scale up of the electrode area, membrane resistance reduction, a suitable design for piping and pumping system, etc. Preliminary test results have shown a high battery efficiency of 83% to 84%, and thus, a good system performance will be expected. This facility is to be installed at a sub-station of the Kansai Electric Power Company, Inc., and connected to a commercial power line for 500 cycle life test.