If Potomac Electric Power Company is allowed to sell electricity at bargain rates to the owners of the Three Mile Island nuclear plant, it will cost the average District of Columbia household $2 to $4 this year, Pepco officials said yesterday.

Pepco asked the District of Columbia Public Service Commission to approve the sale, and to do it quickly, without public hearings.

But two environmental groups told the PSC they object to the plan and the District of Columbia People's Counsel urged that hearings be held so the public can ask questions.

Pepco and other utilities in the Pennsylvania-New Jersey-Maryland Power Pool want to charge lower rates to General Public Utilities Co. for power GPU must buy because its Three Mile Island nuclear plant has been shut down.

The utilities have asked state regulators in the District, Maryland, Virginia, Pennsylvania and New Jersey to approve the unusual discount to GPU which has been pushed to the brink of bankruptcy by the nuclear accident at Three Mile Island.

If the states don't go along with the request, the Federal Energy Regulatory Commission could step in and order the utilities to sell to GPU at even lower rates.

If that happens, Pepco executives said, Pepco's customers could lose any benefit they get from sale of power to the pool.

The power pool, known as PJM, is a group of utility companies that routinely sell each other surplus electricity or buy power from each other when they are short.

Usually the price that's charged is calculated by splitting the difference between what it would cost the buyer to generate the electricity itself and what it costs the seller to make it.

To save money for GPU and its subsidiary that owns Three Mile Island, the other companies are seeking authority to charge a lower rate. They want to charge GPU 10 percent more than it costs them to make the power.

Doing so will mean that Pepco collects at least $2 million and perhaps as much as $4 million less than it would otherwise get for power sold to GPU. Since money Pepco earns from wholesale sales of electricity is credited against the bills charged Pepco's regular customers, the regular customers will make up for the loss.

Selling power to GPU at a discount won't actually cost Pepco customers money, but it will eliminate $2 million to $4 million they would otherwise have saved, Pepco officials told the PSC.

D.C. Peoples Counsel Brian Lederer said he has not decided whether to oppose the discount for GPU. When the Public Service Commission opened discussions on the topic yesterday, Lederer raised a series of questions about its effect on Pepco customers.

Lederer said he fears that Pepco's customers may have to bear an unfair share of the burden of bailing out the owners of Three Mile Island.

Pepco normally generates only about 10 percent of the electricity in the power pool, but it will supply about 33 percent of the purchases of GPU, because Pepco has more surplus power to sell than other companies do.

Lederer said he wants the PSC to look into federal studies of the Three Mile Island accident before it allows local residents to pick up part of the bill.

Representatives of Potomac Alliance and the Clean Water Action Project said they oppose any aid to the Three Mile Island owners.