SOLAR ENERGY is turning into everybody's favorite hypothetical solution to all energy crises, shortages and interruptions. It does not pollute, nor does it deplete. It streams down, absolutely free, subject neither to tariff nor embargo. Sunshine is marvelous stuff. It's so good that you can already hear the siren song. Why worry about all these unpleasant choices like raising gas prices and burning more coal, the sirens sing, when the world can reach up and collect energy out of the sky?

Unfortunately - as perhaps you suspected - it isn't that easy. Even the simplest solar collector to heat water for your home requires an array a panels and piping. The fuel is free, so to speak, but the plumbing is not. It's important to try to sort out the realistic possibilities of solar energy over the next decade or so - and the public policies that can encourage its development.

Congress has been taking a sharp interest in the subject, and some days ago the Joint Economic Committee published an extremely useful study by a group of economists at the University of New Mexico. The basic technology for solar home heating is well in hand. The speed with which Americans take advantage of it will depend, the study concludes, on comparative costs, Solar equipment is expensive. As long as federal policy keeps conventional fuels artifically cheaps, few homeowners are likely to go solar.

Take, for example, a solar array capable of providing half the space heating and hot water for a small house in the Washington area. That would cost about $6,000. People will lay out that kind of investment only if they are sure that it will save them money in the long run.

Building solar space heaters into all new one-family houses, the New Mexico economists calculate, could cut the country's requirement for fossil fuel by half of one per cent in 1990. That might sound like a small gain, but it's an ocean of oil. It's oil that will continue to be burned in people's furnaces, as long as it costs less than the mortgage payments on a solar system.

In Congress, President Carter's plan for an Energy Department is running into inordinate delays and sniping. Most of the opposition comes from people who fear that the reorganization would free the administration to raise prices. That would be unpopular, but it's necessary. Some of our readers have chided us as cruel for repeatedly emphasizing, over the years, the need for higher energy prices. Have we no social conscience? But in our view the greater cruelty is the national procrastination and evasion that subjects whole regions to experiences like last winter's natural gas squeeze, theatening people in their jobs and in their homes. Wise public policy requires gradual but steady increases in gas and oil prices to edge the country toward other and more reliable sources - of which the most attractive is, certainly, the sun.

That's the merit of this report to the Joint Economic Committee. It demonstrates the great sensitivity of the solar alternative to comparative costs.From the beginning, the sun has offered the cleanest and safest way to stay warm. But the country won't use it more efficiently as long as oil is cheaper.