Potomac Electric Power Co. said yesterday that it hopes to add thousands of customers to its energy management program in the next phase of a plan to defer construction of a costly new generating plant by saving energy.

Last summer, Pepco said it was conducting experimental energy management programs to reduce electricity demand and thus push back the need to build an $800 million plant from 1991 to 1995.

In a filing with the Maryland Public Service Commission, Pepco said it now wants to increase the number of commercial customers that pay "time-of-use" rates -- or rates varying according to time, day and season -- and make it mandatory for commercial customers and a handful of residential customers now participating in an experimental program. Under the program, which is now voluntary, rates are more expensive during times of greatest demand on the utility.

About 450 large commercial customers in Maryland and the District of Columbia and 250 large residential customers are billed under time-of-use rates. Pepco wants to apply the rates to 20,000 more commercial customers.

"Although time-of-use rates are mandatory, this gives people . . . more opportunities to save," said Nancy Moses, a spokeswoman for the utility. Savings result from using energy during the lower-demand parts of the day.

The utility, which needs permission from public service commissions to initiate or expand programs, plans to file the same request with the District of Columbia PSC later this month.

Pepco also has asked to add 500 to 1,000 users to another voluntary program in which 50 large commercial and government customers have agreed to cut electricity usage during periods of peak demand.

Likewise, Pepco hopes by 1992 to enroll more than 100,000 residential customers out of 250,000 with central air conditioning in a voluntary program in which air conditioners and water heaters are turned on and off by remote control during times of peak demand.

Pepco said the combined voluntary and mandatory programs will cut by about $12 million a year the cost of producing additional electricity to meet demand. The utility also is keeping down costs while meeting demand by refurbishing coal-fired generating units. The 10-year renovation work on Pepco's Potomac River Power Plant in Alexandria is estimated to cost about one-eighth what it would cost to build replacement units.