OVER THE PAST YEAR, Americans' use of electric power has risen very little. That's hopeful for the country's hard-pressed electric utilities -- if it continues. But will it continue? Nobody knows, and the penalties for bad guesses, for both the utilities and the national economy, would be dire. Generating electricity has now become an extremely risky business -- and the customers share the risk.

The economics of power generation has been turned upside down over the past decade. Until the early 1970s, the industry's prospects seemed clear. Demand for power was rising rapidly and steadily. Utilities made money by generating more of it, as generators got bigger and more efficient. Then, without much warning, the rules reversed. With higher energy costs, customers' demand suddenly slackened. Even more disquieting, utilities found that growth was no longer profitable. Some of the reasons were technical; bigger generators no longer seemed to be cheaper to operate. Some were financial; inflation ensured that the newest generator was always the most expensive.

It takes at least 10 years to build a large generating station. Building too much wastes money, but no utility wants to run into a shortage with its threats of brownouts and industrial unemployment. The utilities are now desperately trying to decide what they will need a decade from now. As it happens, two companies at opposite sides of the continent illustrate some of the more interesting possibilities.

The Pacific Northwest is already gripped by a power shortage, and the Puget Sound Power and Light Co., serving western Washington, is in effect rationing electricity. New industrial customers can get only interruptible service. The company is taking no new residential heating customers at all. The region, with its huge federal dams, has a tradition of very cheap power and, because it's cheap, people have continued using more and more of it. But now there's no more hydropower. Turning to coal and nuclear plants, the region has run into the usual costs and delays. Pressing anxiously ahead with a very expensive construction program, Puget Sound is already bumping against the limits of its power supply. There will be no relief until, at best, the mid-1980s.

The New England Power Company is following a different strategy. It has deliberately decided to build fewer new plants than the conventional forecasts would require. Instead, it has embarked on an aggressive campaign of conservation. It is now testing new technology to hold down peak loads and to store power in off-peak hours. It's offering energy audits to all of its customers. It is committed to the new logic that conservation, and not rising sales, makes money for a power company. If this plan succeeds -- and the prospects are good -- over the next 15 years it will save the company $1.3 billion in capital costs. Most of it will be passed along to the customers in lower rates.

Public and private interest now both point in the same direction -- toward conservation. In the first century of the electric power industry, the rewards went to the companies that grew fastest. In the second century, it looks as though the rewards will go to those that work most actively, and successfully, with their customers to keep the growth rates down.