The Post's support for Pepco's new power plant based on coal gasification {editorial, June 8} is disappointing because it overlooks two questions that must be answered prior to the construction of any power plant: Is the plant justified by projected demand? Is a new power plant the best answer to rising demand?

A Pepco spokesperson told The Post's Home section {June 11} that Pepco's conservation programs now in place (modest at best by industry standards) would save 400 megawatts of demand per year -- "approximately the capacity of a new generating plant, which Pepco will not have to build." In the meantime, Pepco has agreed to buy up to 450 megawatts of electricity annually from Ohio Edison for the next 18 years. That's the equivalent of two power plants, in addition to the proposed coal gasification plant.

Furthermore, plans for power production at the Benning Road incinerator forecast up to 21 to 24 megawatts of electricity by the next decade, while Blue Plains may also produce power by 1991. And Pepco told The Post {June 28} that Tellus, a New York-based cogenerator, might build a small plant in Rockville. According to Tellus, its plant would obviate the need for a new plant in Dickerson.

D.C. alone can expect to save an additional 51 megawatts by the year 2000 thanks to the passage of the National Appliance Energy Conservation Act of 1987, which Pepco has not yet factored into its calculations.

Finally, Pepco appears to ignore the major investigation by the D.C. Public Service Commission now under way into energy conservation and least-cost planning principles. Least-cost planning would require selecting options that reduce demand whenever such options are cheaper than increasing supply. The company has yet to demonstrate that it has thoroughly studied the programs now common to dozens of other utilities around the country.

Pepco's rate payers have a right to ask: What's going on here? Both Pepco and Washington Gas need to apply least-cost planning principles and put energy conservation (demand side) and power plants (supply side) on an equal footing. Neither has yet done so. In the commission's investigation, Pepco has included loss of sales stemming from conservation as a cost, rather than a benefit, in its calculations. This logic stands least-cost planning on its head.

The Office of the People's Counsel has advocated measures designed to overcome the most common twin barriers to consumer investment in energy conservation -- high initial cost and the need for quick paybacks. Such measures as utility company rebates on more efficient equipment for residential and commercial use and outright buy backs of older, inefficient second refrigerators have proved effective elsewhere. Some of the 59 utilities now offering such programs have produced peak savings of 0.35 percent to 1.38 percent -- savings for Pepco of anywhere from 19 to 77 megawatts by the year 2000.

The promotion by Pepco of coal gasification as a high-tech solution to rising demand is a red herring that only distracts the public from the much more basic question of whether Pepco needs a new power plant at all, and if so, when. There is an alternative, not only to traditional energy production, but also to coal gasification. Least-cost planning, including conservation, cogeneration and power purchases, can produce results unmatched by any other strategy. It's past time for D.C. rate payers to start reaping its benefits. FREDERICK D. DORSEY People's Counsel for the District of Columbia Washington