Pumped storage type power plants have been a feature of Japanese electricity systems since 1930 and contribute to the stable operation of a huge power network in Kanto distinct including the Tokyo metropolitan area. Tokyo Electric Power Co., Inc. (TEPCO) has 9 pumped storage power plants with approximately 10,000 MW total capacity, including one currently under construction. These stations provide peak load power sources, storage of electric power, spinning reserve, voltage support, ability to control reactive power and black start capability for power network recovery.

Pumped storage power generation uses two adjustment reservoirs located at different
altitudes and connected by conduits with reversible pump-turbines. They utilize surplus electricity generated during low-demand hours and weekends to pump water from the lower adjustment reservoir to the upper adjustment reservoir for later use during periods of peak electricity demand and/or in the event of an emergency.

The Midono pumped storage is one of the 9 operated by the Tokyo Electric Power Company and is located within the Nagano Prefecture. Commissioned in 1969/70, the power plant contains 4 turbines with an installed capacity totaling 245MW.