The District of Columbia has been ordered to pay $2.6 million in overdue electric bills for street lighting owed to the Potomac Electric Power Co.

If the Superior Court ruling stands Pepco customers in the District will feel a slight reduction in rates, a company spokesman said the exact amount has not been calculated and the eventual source of the funding the $2.6 million payment has not been determined.

The city had argued that Congress had ordered it to pay no more than 2 cents per kilowatt hour for street lighting, a level in effect since 1958. Superior Court Judge Joseph M. Hannon ruled, however, that when the Public Service Commission authorized Pepco to collect more than that amount in 1973, the city should have paid.

". . . Congress did not intend that its 2 cents per kilowatt hour spending limitation restrict plaintiff (Pepco to an arbitraryrate which may or may not be reasonable.' the judge wrote. "Had Congress intended to repeal or limit the jurisdiction of the Public Service Commission to set reasonable rates, Congress would have done so forthrightly and not by pursestring inference."

The 2 cent level was adequate until 1973 when the Arab oil embargo hit, a Pepco spokesman said. In that year, the commission authorized Pepco to assess a fuel cost against the city, but the city refused to pay. In the next three years the unpaid bill climbed to $2,564,179, and Pepco filed suit to collect in August 1976.

The commission ruled then that while the case was pending the extra amount would have to be paid some how, and allowed Pepco to roll the street lights' further fuel costs into D.C. customers' bills. The $2.6 million remained unpaid.

City Assistant Corporation Counsel Steven Baer said he based his case on the practice of Congress since 1911 of stating within the District of Columbia appropriations bill just how much could be paid for street lighting.

No decision has been made on whether to appeal the ruling, Baer said. The city has 30 days in which to file.

Pepco spokesman John Grasser expressed pleasure at the decision. "We're glad the court recognized that other D.C. customers shouldn't have to subsidize the street lighting costs," he said.