Environmentally friendly hydroelectric power is well suited to Japan's unique topographical characteristics and climate, which provides abundant rainfall. Although hydroelectric power was once a main source of energy, the ratio of hydroelectric to thermal power has been reversed since 1959. Hydroelectric power plants can quickly be adjusted, and this makes hydroelectricity ideal for the role of compensating for potential shortfalls to meet peak demand. Pumped storage hydroelectric power plants, which pump in water during low-demand evening hours and use it to generate power during peak hours, have become the mainstream of hydroelectric power generation in recent years.
