The Potomac Electric Power Co. was officially absolved yesterday of accusations that it discriminated against some neighborhoods and customers in restoring power after a severe thunderstorm knocked out service to thousands of customers in the summer of 1978.

By a unanimous vote, the three-member D.C. Public Service Commission concluded that a detailed inquiry by its staff failed to establish that Pepco favored some neighborhoods and types of customers over others in making repairs after a storm with winds of up to 80 miles an hour hit without warning on June 27, 1978.

Service to some customers was cut off for up to five days. During that period, Pepco got 51,485 telephone calls related to the storm. The PSC got so many complaints that it held an informal public hearing to receive public complaints, including charges that the utility favored affluent neighborhoods over those inhabited by poor people.

PSC Chairman Ruth Hankins-Nesbitt said yesterday that "there was no discrimination." The commission's general counsel, Lloyd Moore, said the staff investigation leading to that conclusion included a computerized survey of the pattern of the power outages and repairs.

D.C. People's Counsel Brian Lederer noted that the commission, in closing the issue with yesterday's vote, did not deal with the question of whether Pepco may have a legal responsibility to reimburse customers for food that spoiled in refrigerators and freezers or for business lost by stores that were forced to suspend operations.

In another action, the PSC voted to authorize the Washington Gas Light Co. to issue up to $300 million in debt securities -- probably debentures -- to finance the construction of new or replacement facilities. The PSC set a maximum interest rate of 16 1/4 percent and said the impact of the borrowing on the company's capital structure will be reviewed the next time the company asks for higher rates.