If you've been thinking about installing a solar hot-water heater, try to do so within the next few months. Solar energy makes economic sense for many buyers only because of the big federal income-tax credit that sharply cuts the system's cost -- and that credit is due to expire at the end of 1985.

It is always possible that Congress will decide to extend the credit or to phase it out over a number of years. But a bid to continue the tax break for solar energy failed last spring.

Solar hot-water systems cost an average of $4,100 plus installation, according to the Solar Energy Industries Association (SEIA). The federal government lets you subtract 40 percent of the price from your income-tax bill, up to a total tax credit of $4,000. The average system, then, would save you $1,640 in taxes, reducing its effective cost to $2,460. (The same credit is available for installing wind and geothermal devices.)

About half the states give you an additional credit against state income taxes, or some other form of incentive, which lowers the price even further.

Even with tax credits, the efficiency of solar heating depends on where you live. If you heat water with low-cost natural gas, you probably cannot save enough with solar to make the switch worthwhile. But if you heat with high-cost oil or electricity, the savings may be large enough to give solar an edge.

If you could save $300 a year in electricity bills by going solar, you would pay for the equipment, after taxes, in around eight years. Depending on the cost of the system and where you live, it might be paid off in a shorter time, even as little as three years. Without the tax credit, the pay-off period might be so much longer that few consumers would find solar worth the money.

And two similar houses can produce very different energy cost figures, depending on how tightly the house was built, how much hot water the families use and what other steps they take to conserve.

Suppliers of solar products come equipped with computer programs that claim to measure your energy savings. Asked whether these devices are accurate, solar expert Donald Watson, a professor of architecture at Yale University, answers a "cautious yes," at least when they're used to measure the efficiency of one solar collector against another, and solar in general versus other forms of heating. But they cannot predict what the actual savings will be in anyone's home. Nor do these calculations include the cost of maintaining your solar equipment.

Solar hot-water heating (and swimming-pool heating) has been the most popular in California, Colorado, New York and other states with strong tax credits.

Solar heating for all rooms costs an average of $10,000 and usually isn't cost efficient, so it's rarely used. Solar air-conditioning is still in the experimental stage.

One risk any solar user runs is maintaining his legal right to the sun's rays. Only five states have laws preventing your neighbor from putting up a building that throws your rooftop solar collectors into the shade.

Although active solar-heating systems are not as popular as proponents had hoped, passive solar techniques are spreading rapidly. More and more new houses are built with big windows facing south, small windows on the north, and interior heat collectors such as tile over six inches of concrete or extra stone around a south-facing fireplace. Some super-insulated designs rely entirely on the sun, with no back-up heating units except a fireplace.

You get a 40 percent tax credit for engineered systems of passive heating, which distribute warmth from a solar-storage mass. On houses built before 1977 there's a 15 percent credit (to a maximum write-off of $300) for such energy conservation tactics as adding insulation, caulking, buying a more fuel-efficient burner and adding storm windows -- all of which is a lot cheaper than buying solar. These credits also expire in December 1985.

If energy prices stagnate, solar will look less attractive, Charles Ebinger, director of the Energy and Strategic Resources unit at the Center for Strategic and International Studies, Georgetown University, told my associate, Virginia Wilson. For this reason, he thinks that Congress should renew the solar tax credit.