WITH so much focus on carbon emitted from the nation’s power plants, another environmental challenge related to electricity generation is sometimes overlooked: the enormous amount of water needed to cool the power-producing equipment.

In the United States almost all electric power plants, 90 percent, are thermoelectric plants, which essentially create steam to generate electricity. To cool the plants, power suppliers take 40 percent of the fresh water withdrawn nationally, 136 billion gallons daily, the United States Geological Survey estimates. This matches the amount withdrawn by the agricultural sector and is nearly four times the amount for households.

Battles for water among these competing interests are becoming more common, and power plants are not always winning. A recent analysis by the Union of Concerned Scientists revealed many examples from 2006 to 2012 of plants that had temporarily cut back or shut down because local water supplies were too low or too warm to cool the plant efficiently.

Proposals to build new plants are also under increased scrutiny, especially in water-stretched regions. The proposed White Stallion coal plant in Texas drew opposition in part because of the plant’s water demands. The project was abandoned this year.