In Japan, the first hydro-electric power was produced in 1890. After 1912, hydro-electric plants supplied more than 50% of Japan’s electricity. Only after World War II did the balance between power from thermal plants and power from hydro-electric plants tip in favor of thermal plants. This was when thermal plant efficiency was quickly increasing and the cost of oil or coal was low. Oil production and supply, of course, was virtually monopolized by American petroleum companies; cheap oil and American thermal technology were favored by the Allied Occupation authorities and succeeding Japanese governments.
The need to rebuild quickly after the war, however, also dominated decision-making and thermal electric plants were built so rapidly that by 1962, most of Japan’s electricity was produced from thermal plants. Fundamentally, all reasonable places where dams for hydro-electric power could be built were exhausted; the only additional power which could be obtained from water was in such remote areas that transmission to urban areas was impossible.
A major reason for the early dominance of hydro-electric power can be seen in valleys like the Kiso river valley. The river descends quickly and steadily during its short run and so it is easy to put up a medium sized dam and derive a great deal of energy. Although the rains are not absolutely predictable, they are regular enough to make water power a cheap and reasonable option.
Today, hydro-electric generating stations are still common, but the percentage of the nation’s power derived from the rivers has steadily declined. By 1973, thermal power produced about 74% of all electricity. This percentage has dropped as atomic power plants were rapidly built after the price of oil increased sharply. Hydro-electric plants are now commonly used to supply power at daily periods of peak consumption since they can be turned on or off quickly. In addition, ‘pump-up’ hydro-electric plants have become common. They use excess thermal power at off-peak hours to pump water from lower to higher reservoirs; the stored water can then wait for use again during peak demand. In 1978, 32% of hydro-electric power was generated in this way.