A new report out from Boston-based Synapse Energy Economics raises a question that doesn’t seem to come up enough when talking about energy: what about water?

The report was commissioned for the Civil Society Institute, a nonpartisan think tank that often focuses on energy and climate change. It looks at coal, nuclear, biomass, solar, wind and natural gas and analyzes the hidden costs of each type of energy–like the ways each affects water, climate change, air pollution and subsidaries.

It concludes that in a time when water is becoming scarcer (just look to the droughts of this summer, and the perpetual problems in the Western United States), we’re using a lot of water for energy. Biomass is the biggest water hog, using up to 100,000 gallons of water for 1 megawatt hour of electricity. Coal uses up to 50,000 gallons for the same energy production, and nuclear 60,000 gallons.

From the release:

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.

The least water-intensive forms of energy are wind and solar, which use very small amounts of water.