Some of the nation's venerable power plants, already in service for decades, may find their normal careers of some 40 years stretched to 50 or even 60. That is what's happening in the electric utility industry in the United States as various economic trends force power companies to take a long, hard look at their futures.
Take the case of Pacific Gas & Electric (PG&E). The giant San Francisco-based utility has erased or postponed every major new project possible in the last few years, says spokesman Chuck Peterson. One of these was a coal-fired plant scheduled to be built at Collinsville, between San Francisco and Sacramento.
''It's completely off the boards,'' Mr. Peterson says.
Instead, PG&E is thinking about keeping a tiny, aging power plant in service beyond 1989, when it would normally reach retirement age. The Kern plant in Bakersfield will be 40 by then, and its two generators produce only 180 megawatts of electricity, compared with others in the system that are in the 2, 000-megawatt range.
PG&E is not alone in prolonging the life of power plants. Some utilities already have committed themselves to such a course.
Among their reasons: continuing conservation of electrical energy; the high cost of fuels; the enormous cost and seemingly interminable lead times needed to build even new conventional power plants; and moves to import electricity from Canada and Mexico.
As for nuclear power plants, only the daring are seriously proposing the construction of more these days.
Harold Bongarten, general manager for apparatus and service engineering at General Electric's Schenectady, N.Y., office, says utilities can extend the use of their older generating units for less than 30 cents on the dollar compared to what it would cost to build new generating capacity.
A power company can wring as many as 20 additional years out of a unit by modifying its turbine generators, Mr. Bongarten told a recent international conference of utility industry representatives here. He said this would:
* Avoid the effort and costs involved in complying with environmental-impact requirements in the building of new plants. It has been estimated that the lead time for construction and licensing of new facilities can be as much as 10 years.
* Preserve existing access to sources of water and fuel that might not be readily at hand if a new site had to be found. This would especially be true if developers run into not-in-my-back-yard-type opposition so common to the siting of any large facility now.
* Still allow new environmental controls to be retrofitted at a considerable savings over what a new system would cost.
''I believe that most utilities will nurture their existing generating capacity,'' John Kaslow, senior vice-president of the New England Electric System, told the same conference. ''Because they will not have large new units coming on line each year, or every few years, . . . they will be expecting and demanding higher availability from aging generating units.''
New England Electric is already in the fourth year of a 15-year plan that calls - among other things - for keeping older generating units in operation longer than originally anticipated.
The practice is not without its challenges, however, Mr. Kaslow cautioned.
''Lest I mislead you that keeping facilities in operation longer will lead to great increases in operating and maintenance budgets,'' he said, ''please remember that it simply does not work that way in a regulated business.''
He also emphasized the need to recruit ''bright, imaginative, and motivated'' people to the utility industry and to convince them that the utility business will still be ''exciting'' and ''provide rewards'' in the years ahead.
By all accounts, electric utilities were a growth industry from the end of World War II to the early 1970s. Energy consumption rose by 7 percent a year, new generating units increased in size and efficiency, and fuel costs and electric rates rose slowly.
But with the arrival of the '70s, and especially the Arab oil embargo of 1973 -74, the character of the industry changed dramatically. Fuel costs and electric rates began to rise, but consumption tailed off as Americans became conservation-minded. Inflation hiked the cost of building new power plants and stringing new transmission lines.
Today, industry sources note, many utilities are in financial distress, especially those with power plants under construction or those with nuclear units that have been shut down for safety reasons.
Meanwhile, energy conservation apparently has become a deeply rooted ethic. According to the federal Energy Information Administration, sales of electricity from the more than 10,000 generating units on the US mainland and Puerto Rico grew by only 7 percent from 1977 through 1982 - or from 1.94 million kilowatt hours to 2.08 million.