Organizing under way in 15 cities for what promoters yesterday called "the biggest planned event in American history": An outpouring of grassroots demand for solar energy development beginning May 3, a day called Sun Day.

The organizers, Solar Action Inc., got together a balanced ticket of a labor chief, a consumer advocate, an inner-city poverty worker and a Republican politican to join in a news conference announcing Sun Day plans.

"We came out of the environmental movement," said Denis Hayes, Solar Action board chairman, "but the 'we that organized Earth Day (in 1970) has changed . . . This time 'we' involves a very strong coalition, right from ground zero, of labor, farmers, businessmen, education . . ."

The environmental consciousness raised by Earth Day on April 22, 1970, has spread to cover most of American society, Hayes said. "This time, though, we're not defining a new problem like pollution but celebrating a new solution."

He admitted during questioning that investment required to reach a goal of a 40 per cent solar-powered American society by the year 2000 would be "staggering, comparable to the gearing-up we did for World War II." But he said it would still cost less than gearing up conventional energy sources to meet the projected demand that year.

Such an effort would provide millions of jobs for every segment of society," said Bill W. Winpisinger, national president of the International Association of Machinists and Aerospace Workers. He called for a federal initiative to put solar panels and windmills on all federal buildings.

Events being planned for May 3 include sunrise concerts, energy fairs, seminars, teach-ins, exhibits, lectures, films and tours of solar energy facilities. A greenhouse will be dedicated in Chicago, while Seattle's World's Fair grounds will be the site of a solar festival. Solar magic will be on display in San Francisco and at a fair on Washington's Mall, while an information clearinghouse for consumers will be set up in New York as a sunrise concert plays at the United Nations.

"It's time to start exporting in the energy field," said Rep. James M. Jeffords (R-Vt.). "We are dropping rapidly behind the Germans and the Japanese and the French in this area."

Charles Tisdale, antipoverty program director in Bridgeport, Conn., said a nationwide move to solar power would offer the kind of local control, do-it-yourself, labor-intensive resource most needed by the nation's poor. Lola Redford, head of Consumer Action Now (CAN), said a public increasingly aware of energy problems is demanding more information on ways to solve those problems safely and economically.

Solar Action directors Peter Harnik and Richard Munson emphasized the local, decentralized nature of a solar-powered future in which each home or small group homes has its own power source.

But even going all out after Sun Day, the U.S. would not be able sufficiently to organize the necessary solar laws, institutional changes, industrial retooling and job retraining to provide more than "a few percentage points" of the total U.S. energy demand with solar power by 1983 or so, when the international energy pinch is widely expected to become serious, Hayes said.