Nuclear power has critical cooling requirements that require huge amounts of water. Roughly 62 percent of U.S. nuclear plants have closed-loop cooling systems. Reactors with closed-loop systems withdraw between 700-1,100 gallons of water per megawatt hour (MWh) and lose most of that water to evaporation. Water withdrawals are even higher at open-loop cooled nuclear plants, which need between 25,000-60,000 gallons per MWh. Most of the water is returned, but at a higher temperature and lower quality.

In addition to fouling streams and drinking water through mining and coal-ash dump sites, coal-fired power relies heavily on closed-loop cooling systems which withdraw between 500 and 600 gallons of water per MWh and lose most of this via evaporation. Withdrawals for open-looped cooled coal-fired power plants are between 20,000-50,000 gallons per MWh. Most of the water is returned, but at a higher temperature and lower quality.

Under a so-called "Clean Energy Standard," biomass would become a much larger source of U.S. electricity generation; however, biomass also requires vast amounts of water. The report notes that a typical 50 megawatt (MW) biomass plant could withdraw roughly 242 million gallons of water per year and lose most of this. Adding 10 of these plants in a region would use 2.42 billion gallons of water per year. For dedicated energy crops, water use for irrigation can be considerable. One study estimates water use for most crops between 40,000 and 100,000 gallons per MWh, with some crops exceeding this range.

In 2010, EPA estimated that fracking shale wells can use anywhere from two to 10 million gallons of water per well. The water is often extracted from on-site surface or groundwater supplies. Such huge water withdrawals raise serious concerns about the impacts on ecosystems and drinking water supplies, especially in areas under drought conditions, areas with low seasonal flow, locations with already stressed water supplies, or locations with waters that have sensitive aquatic communities.

By contrast, wind and solar photovoltaic power requires little water in the electricity generation process. Concentrating solar power requires water for cooling purposes, but new technologies are placing greater emphasis on dry cooling. Solar power plants with dry cooling use only around 80 gallons per MWh - about a tenth of the low-end estimate for nuclear power and one-sixth of the low end estimate for coal-fired power generation.

The full picture for nuclear power water use may be even more dramatic. Estimates of lifecycle water use for three European reactors range from 2,600 to 6,900 gallons per MWh, not including cooling water use. (This compares with a lifecycle analysis of a parabolic trough solar thermal power plant at 1,240 with wet cooling and just 290 gallons per MWH for dry cooling.) In addition, nuclear wastewater production ranges from 6.3 to 7.4 gallons per MWh.

Coalbed methane recovery of natural gas depletes ground water: one estimate puts total groundwater removed between 1997 and 2006 at an astounding 172 billion gallons.

Estimates of the lifecycle water withdrawals from wind projects, including both onshore and offshore projects, range from just 55 to 85 gallons per MWh.