Duke Energy could be said to have put out its own fire when it declared to the N.C. Utilities Commission that it planned to sell on the wholesale market the electricity produced by one of two 800-megawatt coal-fired plants that it wanted to build at its Cliffside site west of Charlotte.

In a thoroughly sensible decision, the commission last week ruled that Duke could build a single plant. The company now has to decide if going ahead with just the one plant makes financial sense. The upside would be a further buffer against the state's increasing demand for power. But the drawbacks would include more carbon emissions of the kind linked to global warming -- not at the volume that two plants would have produced, but substantial all the same.

The commission did its primary job, and performed a difficult balancing act, in acting to ensure an adequate supply of electricity while looking to guard the pocketbooks of Duke's 1.6 million North Carolina customers. Two new plants would have cost $3 billion, plus another several hundred million in finance charges.

Those costs, of course, would have been passed along to individual and business customers of the utility, which serves much of the central part of the state, including Durham and Orange counties. Duke and its stockholders, meanwhile, would have profited from the energy produced at the second plant and sold on the energy market.

A utility company's financial health is a matter of public concern. But given the expense and the environmental implications, a second plant whose output could be regarded as surplus from a North Carolina perspective, at least for the immediate future, was hard to justify.

Besides that, while coal-fired plants of the kind Duke proposes to build are cleaner than old-technology plants, they still pump tons of carbon dioxide into the air. The two plants would have generated the same amount of CO2, a so-called greenhouse gas, as a million automobiles. North Carolina needs ample supplies of electrical power as badly as any other growing state. But there's no reason to let utilities produce more than it needs, especially if that means adding excess pollutants to the atmosphere.

It's encouraging that commissioners also conditioned their approval of one plant on Duke's spending at least 1 percent of its annual North Carolina revenues on energy-efficiency programs. The amount is modest, but it sends Duke and other producers the message that using power efficiently should be part of their strategy. Not wasting power costs less than making it. And as demand increases, efficiency needs to do likewise.

Duke also has to close four smaller plants if it wants to build the additional one. And it still will need to persuade air quality regulators that the new plant won't harm human health. That review needs to be as thorough-going as that of the Utilities Commission.