On grass-wilting days this summer, the central air conditioner is likely to switch off for up to 7 1/2 minutes every half hour in Ruth and Jack Yarbrough's two-story, cedar-siding home in Cobb County, just north of Atlanta.
The same thing will occur periodically in some 13,000 other Cobb County homes , but few complaints are expected. Temperatures rise only two to three degrees during the shut-offs, and monthly electric bills are averaging $1.43 lower than they would have been without the program.
Elsewhere, some 200,000 Detroit Edison customers have volunteered to let the company periodically switch off power to their water heaters. Participants in the program get a 20 percent reduction in their electric bills.
Despite some initial gloomy predictions, the number of participants in voluntary programs to switch off some of their power during peak demand periods is steadily increasing as more Americans do something more than just complain about rising electric bills. Some 400,000 persons nationally now are participating in such programs, and in Wisconsin another 150,000 are in the process of signing up for a similar one, according to a national study by Energy Utilization Systems, a private research firm.
Like a highway system built for the commuter rush hour, electric power systems are designed for peak demand periods -- such as the afternoon and early evening hours in the summer, when air conditioners are used to the maximum. At other times, especially late at night, power systems -- again, like the highways -- are vastly underused.
By switching more use to hours of less demand, the rate at which costly new power facilities have to be built can be slowed somewhat. This, in turn, can help keep down customers' electric bills.
US Department of Energy (DOE) electric power specialist Philip Overholt estimates that if the rapidly growing interest in such programs continues, an equivalent of 200,000 barrels of oil a day could be saved by 1985. Up to $15 billion in new power-plant construction costs also would be saved, he says.
Electric cooperatives see quick payoffs in holding down "rush hour," or peak, electric demands. But utilities producing their own power are being pressed by soaring costs of fuel, construction, and environmental and regulatory delays. Building more power plants is less attractive to the industry than it once was.
Cobb County Electric Membership Cooperative, based in Marietta, Ga., began its program without offering volunteers specific financial incentives. But, says Mrs. Yarbrough, who helped sign up volunteers: "Most people were for anything that will cut down the bill and won't cost them money."
In four years, the cooperative has saved $3.2 million as a result of the program, says cooperative official Robert Elsberry. The cooperative buys most of its electricity from Georgia Power Company at a price based on the cooperative's highest peak demand during the year.
This summer, Georgia Power hopes to begin its own smaller version of voluntary switch-offs after having the idea on the "back burner" for several years, a company spokeswoman says.
And an estimated 50 electric power companies are experimenting with ways to hold down peak demands, says Dorman M. Miller, vice-president of customer services for American Electric Power Service Corporation, which sells power to seven states from its base in New York.
His own company is experimenting with a program seen by many utilities as potentially an even better long-range way to control peak demands than switching off power to volunteer customers. Some 75 families in five Northern states are using ceramic brick furnaces and specially insulated water heaters that are heated to high enough temperatures at night (when power demands are lowest) to provide service all day. A fan circulates the hot air from the furnace.
The furnace-water heater package costs about $1,200 above the normal cost says Mr. Miller. But users are billed at a lower rate, typically shaving $240 a year off the family electric bill, he says.
In a related power-saving development, at least two companies -- Etech of Atlanta and Energy Utilization Systems of Pittsburgh -- have developed a heat pump that attaches to the family water heater. By drawing in heat even from cold Northern basements, the devices can reduce the electricity used by the water heater by up to 50 percent, says Orin Zimmerman, of Electric Power Research Institute, in Palo Alto, Calif.