Potomac Electric Power Co. won state approval yesterday for more generators at its Chalk Point plant in southern Prince George's County, sharply reducing the need for a controversial expansion at the Benning Road plant in Northeast Washington.

The Maryland Public Service Commission will allow Pepco to add four generators powered by natural gas at the plant on the Patuxent River. The company wants to increase its reserve generating capacity to cover periods of exceptional demand for electricity, which generally happen in the summer.

"Obviously, we're very pleased," said Luis Wilmot, associate District people's counsel, which has joined with residents in aggressively opposing the Benning Road expansion. "Our concern is: will that force {Pepco} to conserve more? Our problem is there's no tit for tat."

In contrast to the District neighborhood, there was scant opposition to the new Chalk Point generators, in a rural area 35 miles southeast of Washington. Prince George's County officials did not even comment on the proposal.

Nancy Moses, a Pepco spokeswoman, said the utility is "still pursuing a license" at Benning Road "just to have it in hand should something dramatic happen" in the future requiring greater energy production.

However, she said, with the new generators at Chalk Point and two more smaller ones to be built at Dickerson in Montgomery County, the utility should increase its reserve capacity from 10 percent to the desired 16 percent by 1993.

Moses said with the accelerated approval granted yesterday, the new generators should be operational for peak use at Chalk Point next June. She said that Pepco hasn't built a new generator since 1981.

In approving the Chalk Point expansion yesterday, the Public Service Commission said it "agrees that there is a need for the additional generating units to meet present and future demands for service and to improve system stability and reliability due to Pepco's low reserve strength."

Pepco's Chalk Point expansion was supported by the Maryland Department of Natural Resources and unopposed by the Maryland people's counsel, who represents the public before the Public Service Commission. In issuing his recommendation June 11, Hearing Examiner Joel M. Bright added 27 conditions proposed by Natural Resources intended to protect the environment.

Those conditions include the use of natural gas as the primary fuel, stringent air pollution limits and restrictions on the number of hours the units may operate.

"The new stuff's really very good, state of the art," said David Brownless, an environmental planner for Calvert County, which faces Chalk Point across the Patuxent River. "But any increase of nitrous oxides emitted into the air" will wind up in the river and harm the environment, he said.

Maryland People's Counsel John Glynn observed that "any fossil fuel unit in the long run is going to have significant environmental effects." He said his strategy has been to pressure Pepco to adopt "aggressive conservation measures, in the hopes future units can be delayed or avoided."

Bright noted additional concerns raised by the Calvert County commissioners and others "that the cumulative effect must also be considered in viewing the environmental effects" of the four new generators on top of the five already operating at Chalk Point. But Bright said the cumulative effect will still be "within permissible limits," under current federal laws.