Electric double-layer capacitors showing a remarkably high energy density (compared with conventional electrolytic capacitors) are now under development. Capacitors of this type have significant advantage; namely high durability against repeated charge and discharge and no need for maintenance. Therefore we have studied feasible basic circuit configurations and control methods required to apply electric double-layer capacitors as an energy storage element in output-power leveling systems for solar cells or windmill power generators, and in uninterruptible power supply systems. This paper discussed on operating methods for a capacitor bank to improve the efficiency. High efficiency has been obtained resulting from the simulations and the experiments.