Large consumers of electricity in the District would pay the most for power used between noon and 8 p.m. on weekdays under a proposed time-of-day rate schedule filed by Potomac Electric Power Co. with the D.C. public service commission.

Affected by the proposal, which was ordered by the commission late in 1975, would be more than 200 of Pepco's largest customers - including government agencies, the U.S. Postal Service, apartment buildings, universities, hotels and department stores.

All of these consumers of electricity use more than 1,000 kilowatts of power a month. The commission still must decide whether to order implementation of the new rates this summer or to provide an experimental period under which the large Pepco customers would get bills to be paid under current rates plus bills that would have to be paid if the time-of-day rates were in effect.

If the commission adopts the "peak load" pricing proposed by the Washington-based utility, there would be separate rates for consumption in three periods: the lowest rate for electricity between midnight and 8 a.m.; intermediate pricing for use between 8 a.m. and noon or 8 p.m. and midnight; and the highest charges for noon-8 p.m., when electricity historically is in greatest demand here.

The proposed rates are not designed to increase Pepco's overall annual revenues. Rather, the theory behind time-of-day pricing is to discourage use in the peak hours and thus reduce the need for construction of electric generating plants.