Duke Energy Corp. may build only one 800-megawatt coal-fired generator at its aging Cliffside plant in Western North Carolina, not the two that Duke requested almost two years ago, the N.C. Utilities Commission ruled yesterday.

The commission said that Duke, based in Charlotte. failed to prove that it will need both units to meet increasing consumer demand. The brief “notice of decision” also indicated that Duke had been considering putting as much as half of the power generated by a two-unit facility on the wholesale market.

Duke must also invest 1 percent of its annual retail revenue from electricity sales in energy efficiency programs, the order said.

One commissioner, Robert V. Owen Jr., dissented, saying that the application should have been entirely denied. The commission said it will release complete details of its decision soon.

A spokeswoman for Duke Energy, Paige Sheehan, said that the company must see the complete ruling before assessing its impact.

“There’s not a whole lot we can say in terms of the next steps,” Sheehan said. “It’s a significant step in our plan for new generation. Clearly (the commission) recognized there is growing demand.”

Opponents expressed satisfaction with at least some aspects of the ruling.

“It is a kind of victory. The glass is certainly half-full,” said Michael Shore of the North Carolina office of Environmental Defense.

Others said that even one coal-burning generator will be bad for the state’s air and add to the greenhouse gases blamed for global warming, and some wondered whether Duke would find it cost-effective to build only one unit.

A few noted that Duke Energy’s chief executive, Jim Rogers, has joined chief executives of nine other major U.S. corporations to push for a nationwide limit on carbon-dioxide emissions in order to combat global climate change. They hope that the required investment in energy efficiency will lead Duke to abandon the Cliffside project.

“It’s clear to me that if (Rogers) is serious about energy efficiency, building these coal plants would have sucked all the air out of the room,” said Stephen Smith, the director of the Southern Alliance for Clean Energy.

Permission to build the single unit and related power-transmission facilities at the Cliffside Steam Station is conditional upon Duke Energy retiring the four existing 100-megawatt units as soon as the new generator is operational, the commission said.

Duke Energy, with 2.2 million customers in the Carolinas, first sought a permit for the Cliffside generators in May 2005, saying that they would be the most reliable, acceptable way of meeting increasing demand.

Opponents argued — and the company acknowledged — that the project would lead to more carbon-dioxide emissions, and complained that Duke Energy didn’t fairly consider adding capacity through conservation or such renewable fuels as solar or wind.

Critics were also angered this fall when the company restated the full project’s cost as $3 billion, 50 percent more than originally estimated.

Several opponents said they will now shift their attention to the N.C. Division of Air Quality, which must decide whether to grant an air-quality permit. That permit must also be approved by the federal Environmental Protection Agency.

“This is really only the first step in the process for Duke,” said Gudrun Thompson, an attorney with the Southern Environmental Law Center. “We are going to be very involved in fighting the air-quality permit for this plant.”