Potomac Electric Power Co. yesterday was ordered by the District's public service commission to install new meters at its 212 largest customers - meters that will be used in the future to charge different rates for power consumed at different times of the day.

At the same time. the city regulatory agency postponed a decision on whether to require Pepco to begin charging new time-of-day rates by mid-July or to accept the company's proposal that an experiment be conducted for one year under which the large users would pay old rates but also get bills showing what they would have to pay under the new plan.

More than half of the 212 large power users in the District are buildings or facilities owned by the federal government. Also included are 68 office buildings; several hospitals, schools and universities; 3 department stores; and the Washington Post Co.

As the commission noted yesterday, the concept of peak-load or time-of-day pricing has been supported by environmentalists, conservationists, and some consumer and business groups because of two objectives:

Hopefully, it more fairly relates costs and revenues to various types of users, individually and as a class.

Higher costs for electric consumption during periods of overall peak demand should induce companies or government agencies to reallocate their consumption over the long run and thereby cut down on Pepco's peak demand needs, spread out consumption over more hours, create better efficiency and reduce the necessity of constructing more power plants.

For the time being, the 212 large customers will be paying about the same under new rates as under the old but the commission left no doubt that its ultimate goal is use of peakload pricing to increase rates to users who consume electricity during periods of peak demand.

Yesterday, the commission said Pepco should file proposed rates for the new system in two stages - based on average costs, May 20; and based on long-run incremental costs and marginal costs by July 8. A separate study on the cost of implementing the pricing - and who should pay for those cost - also is due July 8.