THE FASTEST and most direct way to expand American energy supplies is to begin, forcefully, to shift oil-burning power plants to coal. That strategy is second only to conservation in the speed with which it could make a real difference in a tightening fuel shortage. Making synthetic fuels out of coal is very much worth doing, for the long term, but synthetics can't contribute much before the late 1980s. In contrast, burning more coal to generate power could free perhaps 1 million barrels of oil a day - the equivalent of the present national oil shortage - within five years. The coal strategy is an obvious choice, but it has been proceeding very slowly. Why?

One reason is uncertainty over environmental rules. Congress intended to rewrite the Clean Air Act in 1977, but got bogged down in a long quarrel that lasted until late 1978. Then the new law left the key decision on power plant standards to the Environmental Protection Agency, and the final ruling appeared only this month. Perhaps these delays were inevitable. Coal smoke is toxic, and requires careful regulation. But during that long debate, the utilities had no way of knowing what it would cost to use coal. That was a powerful incentive to avoid any great commitment to it.

But economic uncertainty has also slowed progress. To the great surprise of the utilities, the country's consumption of electric power is no longer rising as fast as it used to. That's conservation of the most useful and beneficial kind. But, with demand far below their expectations and a recession probably coming, the utilities have grown more cautious than ever about embracing large construction programs.

At this point, the government has to end the uncertainty. It has to tell the utilities that it will be in their own interest, as well as the nation's, for them to move rapidly and steadily toward greater use of coal. Tax cuts and other subsidies on a substantial scale would be justified to get a fast response. Coal generates about 44 percent of the country's electricity, compared with 17 percent for oil and 14 percent for natural gas. (The rest is nuclear and hydroelectric power.)

Public policy now needs to push hard to replace that oil and gas with coal, by converting those oil- and gas-fired plants that are capable of it, and by buying the rest into early retirement. It will take a lot of money, both public and private. But when you consider the costs of severe and repeated oil shortages and disruptions, the price of coal conversion begins to seem entirely reasonable.