HOW MUCH MORE electricity is this country going to need in the years ahead? In the aftermath of the Three Mile Island accident, the question takes on a new urgency and it is the basic question with which policy has to begin.

Current planning assumes that the demand for electrictiy is going to rise at a rate of more than 5 percent a year over the next decade-requiring the nation's generating capacity to expand by two-thirds within those 10 years. According to the projections of the federal government and the utilities, about 40 percent of the next decade's additional electricity will come from nuclear plants. Nearly all of the remainder will come from coal-fired generators.

It is possible, within narrow limits, to shift the balance away from nuclear power and toward coal. Perhaps that will happen automatically, if the accident in Pennsylvania results in extensive construction freezes for redesign and improvement of future reactors. But coal represents a significantly greater danger to the health of the general public than nuclear power. Generating electricity on a large scale inevitably creates risks to health, and the scale of those risks depends on the amounts of electricity that consumers demand. But unexpected and hopeful things have been happening to the trend of public demand.

Up until the pivotal year 1973, this country's consumption of electricity had been soaring upward at a rate of 7 percent a year. But when the revolution in oil prices arrived, electricity suddenly got much more expensive and people-from large industries to individual homeowners-began cutting back. Through the 1960s and early 1970s, electricity production had been going up nearly twice as fast as the growth rate for the national economy as a whole. But currently it's expanding at just about the same rate as the economy. This sudden change does not seem to be hurting the economy or causing unemployment. As far as anyone can tell, it merely seems to represent a widespread inclination to be a bit more careful. There's nothing ideological about it, and nobody seems to have sacrificed much comfort.

Perhaps the present trend in power consumption can be pushed even lower. The congressional Office of Technology Assessment, in a study published this week, concluded that energy use in both existing and new housing can be cut 30 to 60 percent through the skillful use of insulation, and conventional heating and cooling equipment.

This country has been building new power plants at a forced pace to produce electric heating for houses with no insulation. Heating the sky is expensive. For people who worry about the dangers inherent in both coal and nuclear generators, and who don't much care for the choice between them, there is one good way at least to slow down the rate at which the country has to build them. It requires thinking carefully about all the things that Americans have attached to the other end of the wire.