This review is concerned with the role of water as one of the many sources of energy. Other major sources include solid fuels, oil, natural gas, nuclear, biofuels and waste, together with smaller contributions from geothermal, solar, wind and heat. Hydropower, power from water, makes a relatively small contribution towards the overall provision of worldwide energy. By far the largest energy supplies come from solid fuels, oil and natural gas. Hydropower, however, plays a more significant role in the direct provision of electrical energy, 16% of the total world production in 2010.

Revised November 2016 (First published January 2013)

This revised ROCK is concerned with the role of water as one of the many sources of energy. Other sources include solid fuels, oil, natural gas, nuclear, biofuels and waste, together with smaller contributions from geothermal, solar, wind and heat.

The original ROCK was published in 2013 and was largely based on published data available at the time. This mainly referred to conditions in 2010. The purpose of this revision is to update the information for an energy market which is changing with time. Data is now available from governmental sources for the years 2013 and 2014 but, to some extent, conclusions are clouded by changes in the definition of renewable/non-renewable fuels.

The latter half of this ROCK describes the methods which are used to extract energy from water. These methods have remained largely unchanged in recent years.