A court overturned U.S. rules allowing power-plant operators to use a "cost-benefit" analysis when deciding what clean-water technology to use, a decision that may force the industry to spend more to protect the environment.

The federal appeals court in New York said power plants must employ the best technology available to protect public waterways.

The ruling may force power plants, which use billions of gallons of water a day as a coolant, to add costly new cooling systems that rely on less water and kill fewer fish, said Alex Matthiessen, president of Riverkeeper, a Tarrytown, N.Y.-based environmental group.

"This is going to force the old dinosaur plants across the country to use this technology," Matthiessen said. "No question - it's going to impose a cost on the companies."

Dan Riedinger, a spokesman for the Edison Electric Institute, a Washington-based trade association representing publicly traded utilities, said the decision was a setback for utilities that have been adding new power-generating units at existing plants.

"It creates more confusion about how we are expected to comply with regulations," Riedinger said. "Confusion isn't what we need."

The case focused on an Environmental Protection Agency rule that aims to protect fish from being harmed by cooling-water intake structures at large power plants.

Under the 2002 rule, the EPA allowed plants to use a cost-benefit analysis as they weighed which technology was the best available.

The U.S. Court of Appeals for the Second Circuit said Congress did not authorize the use of a cost-benefit analysis.

Matthiessen said there was wide agreement that the best technology was a "closed-cycle" system that recycled water, enabling it to draw as little as 5 percent of the water used by existing systems.

The legal challenge to the rule was brought by Connecticut, New York, New Jersey, Massachusetts, Rhode Island and Delaware, along with environmental groups including Riverkeeper.