The D.C. Public Service Commission decided yesterday that more than 200 of the city's largest commercial electric customers will pay higher rates for using power in peak hours and get a discount at other times of the day or night.

The time-of-day rates ordered for Potomac Electric Power Co. customers will not go into effect for several months, at least. The amounts to be charged will be set as part of a case pending before the PSC in which Pepco is seeking higher rates for all customers, the PSC said.

The commission proposed the variable charges in June, justifying them by noting that additional electricity needed to provide for peak usage is the most expensive for Pepco-- or any utility firm-- to generate.

The proposal won the support from Pepco, which has suffered sagging profits in recent times. One city official said the higher payments from commercial users could increase the company's revenues and lead to realtively lower rates for residential customers.

A commission spokesman estimated that 240 customers would be affected by the new ruling.

Under the ruling, the highest rates would be in effect on summer weekdays between noon and 8 p.m., when the use of air conditioning is greatest. The lowest rates will be in effect from midnight to 8 a.m. and all day on weekends and holidays.

In yesterday's decision, the commission said the time-of-day rates should follow as closely as possible the company's cost of producing electricity throughout the various hours, days and months of the year.

Regulations covering usage by Pepco customers at various times of the day were in effect during the 1920s and 1930s, but were abandoned when energy became more plentiful.

The time-of-day rates adopted yesterday are made up of three major components-- a so-called "energy" charge that reflects Pepco's costs of running its generators, and two "demand" charges that cover the other costs of producing and distributing the power.