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How many gallons of water do you need to power a lightbulb?

Virginia Tech researchers measure the water efficiency of fuel extraction and power generation, as fresh-water supply becomes a larger concern.

Here's a measurement you probably haven't thought of before: it takes between 3,000 gallons and 6,000 gallons of water to power a 60-watt incandescent bulb for 12 hours a day over the course of a year.

The water equation comes to energy. Virginia Tech

That statistic was published on Thursday by researchers at Virginia Polytechnic Institute and State University, who have studied how demand for a dwindling natural resource--fresh water--plays into energy.

The most water-efficient energy sources are natural gas and synthetic fuels produced by coal gasification. The least efficient are ethanol and biodiesel--two fuels booming in production because of supportive government policies, followed by rapid investment.

In terms of power generation, they found that geothermal and hydroelectric energy use the least amount of water, while nuclear plants use the most.

A United States-wide tally shows that power generation requires 655 billion gallons of water a year.

"There are several variables, such as geography and climate, technology type and efficiency, and accuracy of measurements that come into play. However, by standardizing the measurement unit (BTU, or British Thermal Unit), we have been able to obtain a unique snapshot of the water used to produce different kinds of energy," Virginia Tech professor Tamim Younos said.

Biofuels, in particular, are being increasingly scrutinized, as people start to measure the trade-offs of making liquid fuels from biomass.

Corn ethanol emits about 20 percent fewer greenhouse gases than gasoline, but it requires more water, and it has raised the price of grain and food.

Fresh-water supply is a serious concern among scientists studying climate change. Recent droughts in Europe and the southeast United States have been blamed for strains on production at nuclear and coal power generation facilities.