IT'S A BRAVE householder who hasn't come to feel a twinge of apprehension upon opening the bill from the electric company. Who knows what kind of a surprise the computer may have in store for us this month? There is the dismaying example of the family in Poolesville, in upper Montgomery County, whose electricity bill for two winter months was $818. The house has electric heating, and perhaps there isn't much insulation. But that bill is only slightly larger than hundreds of others in the same area. The utility, the Potomac Edison Company, blames it on the recent coal strike. But other utilities - Pepco and Vepco among them - have suffered through the same strike without having to resort to anything like these big extra charges. What's going on here?

Potomac Edison, which serves western Maryland, uses coal to generate its power. Because coal is by far the cheapest of the conventional fuels, Potomac Edison's rates in the past have been low - which helps explain why some of its customers have not been very careful about insulation. When the strike arrived and Potomac Edison had trouble getting coal, it began buying a lot of electricity from neighboring companies. That turned out to be very expensive.

THe explanation lies in the way that power companies choose different kinds of generators to carry different loads. Suppose that you are running a power company. Your customers' demand rarely falls below a certain minimum - your base load. But it fluctuates upward, hitting peaks at sundown on winter days or at mid-afternoon during summer hot spells. To carry the base load, you invest in huge, exremely efficient plants - typically coal-fired generators or nuclear reactors. They are expensive to build, but cheap to operate. To handle the peaks, your engineers turn to the opposite solution: lighter equipment that usually runs on diesel oil. These peak-load generators are much cheaper to buy, which means that they cost the company less when they are turned off - and they are turned off most of the time. But when they are running, their operating costs are very high. Diesel oil costs several times as much as coal.

Now suppose that one of your neighbors - Potomac Edison Company - is running short of coal. It wants to buy power from you. Your efficient base-load plants are already fully committed to the needs of your own customers. You can sell surplus power - but it will have to be power generated by those oil-fueled peak-load generators. That's why the electricity that Potomac Edison has been buying this winter costs three of four times as much as the power that it produces in its own coal-fired plants.

Whether the fuel-adjustment charges in this case are precisely correct and justified in every respect is a question that we leave to the Maryland Public Service Commission. The PSC opened its hearings Monday. But he PSC's counsel, John J. Keane, is proposing deferral of part of the charges, to limit the effect oa any one month's bills on household budgets.

It's necessary to limit the rate at which the price of power can rise. We have been arguing for some tim that higher energy costs are needed to enforce conservation. But we can't say that we care for the Poolesville solution: the resort to electric-shock therapy. Electricity is a genuine necessity, and price increases have to be gradual if they are not to inflict dire and intolerable hardship on people. That's an important principle for the PSC to establish, for Poolesville and the Potomac Edison Company are not isolated examples. If anything is certain in the future of the power industry, it is the prospect of more squeezes, shortages, fuel disruptions and sudden jolts in prices.