What under the sun is wrong with Ronald Reagan?

He just won't have solar energy.

Using the sun for heating, cooling and power is so commendable that it is hard to figure out his hostility to a form of energy that is free, produces no toxic wastes, can't be taken over by terrorists or interdicted by sheiks and produces jobs instead of plutonium byproducts that can be used to make bombs.

Yet Reagan will have no part of it.

At the Knoxville World's Fair, he restated, although in terms that fell short of expectations, his preference for solar's deadly--and some say dying--rival, nuclear power.

The Clinch River Breeder Reactor in Tennessee, he said, symbolizes "our commitment to developing safe nuclear technology to secure America's energy future."

The tattered remnants of the once-powerful solar lobby, which four years ago staged rallies and celebrations throughout the country and turned out 25,000 people here for the first observance of Sun Day, held a wan news conference on Capitol Hill yesterday to protest that while the nuclear industry gets the subsidy, stepchild solar continues to make advances.

As Sen. Gary Hart (D-Colo.) ruefully remarked, four or five years ago as many as 40 members of Congress were lining up to introduce "an almost embarrassing rush of solar energy bills." He and Rep. Richard L. Ottinger (D-N.Y.) alone turned out for Sun Day's anniversary.

Hart said solar power is like arms control--overwhelmingly popular with people but not with government leaders.

Ronald Reagan made a 70 percent slash in solar energy funds in the 1982 budget. Congress restored $175 million, but Reagan has impounded the funds, and friends of the sun have gone to court to press for their use.

Reagan has sent solar out to play in the traffic of the marketplace, but he is far different with nuclear energy, which seems to be dying a natural death from economic causes.

Between 1978 and 1981, 35 planned nuclear plants were canceled. The industry suffered the equivalent of a stroke on March 28, 1979, when, because of a stuck valve, the Three Mile Island nuclear plant came within a half-hour of meltdown. The industry is bedeviled by skyrocketing costs. Of the "clean, quiet, safe" promise of nuclear power at its inception, only "quiet" remains uncontested.

Last year, the Nuclear Regulatory Commission recorded 4,000 "minor incidents" at the 69 plants still operating. For instance, Diablo Canyon near San Luis Obispo, Calif., which was supposed to start up as an answer to the crazies protesting at its gates, is still cold because, after the demonstrators had dispersed, someone discovered that construction engineers had misread a blueprint.

"All around the U.S.," according to the latest issue of Life magazine, "the looming shapes of idle and abandoned nuclear power plants stand as memorials to a brilliant hope that seems to be flickering out."

The Atomic Industrial Forum says that nuclear power is not dead but sleeping and that it will revive with the end of the recession.

But Wall Street, whose wisdom Reagan trusts implicitly in all other areas, has, since Three Mile Island, been quietly advising investors to put their money elsewhere.

Reagan will not quit. He asked Congress for a $1.1 billion subsidy for the industry. Congress subtracted $40 million, still leaving nuclear power with the largest kitty of any energy source.

It is futile to speculate where solar would be today if it had enjoyed the funds lavished on the nuclear industry. Over a 30-year period, the industry received from the government almost $40 billion in subsidies.

The Department of Energy, dominated by nuclear enthusiasts, dismisses solar's claims because it cannot compete with other energy sources, then makes sure solar can't by gutting government help for research.

But, as with arms control, the people are going one way and the government is going another, even in cold climates. In Vermont, the number of solar installations has doubled in the last eight years.

Obviously, the "easy, popular" course, as Richard Nixon used to call it, would be for Reagan to give the sun a chance. But it goes against his grain.

The people want him to stop the arms race; obviously, a meeting with the Soviets is imperative, although it's hard for him. At the White House, they are saying that although Reagan "sincerely" invited Soviet President Leonid I. Brezhnev to meet with him at the United Nations in June, Reagan hasn't sent an invitation because it is up to the United Nations to do the inviting.

He believes in nuclear arms, as he believes in nuclear power.

And he hopes that the popular insurgency against the arms race will die the way the public passion for solar has waned in the last four years. But the solar lesson seems to be that once people see a good thing, they're apt to pursue it, no matter how hard their leaders try to discourage them.