Perhaps you thought only new homes could take advantage of the move toward solar energy. That's not true. Many older homes can have solar hot water systems added, and at affordable prices.

The cost ranges from $800 for parts alone to $1,300 to $1,700 installed. But it can save you 70 to 80 per cent of your water heating bill. That can mean $100 to $250 a year.

And if you're an experienced do-it-yourselfer, you might save as much as half the installed cost by doing the work yourself. Most manufacturers recommend buying an entire system from one company rather than bits and pieces from different companies.

The deciding factor in whether your house can be retrofitted with solar hot water is the roof exposure. For a solar collector to be efficient it must be on a south-facing roof. If you don't have the right exposure, the solar collector can be placed on the ground. But this is more complicated and expensive. And it's less efficient.

The most difficult part of installation in an existing home is running the plumbing line to the roof. In some cases this can be done along the outside of the building. Otherwise you must cut a section out of at least one wall. This may be a little messy, but it's not as complicated as it sounds. And the wall can be replastered (or the paneling replaced).

Installation of the remaining items is relatively easy. An electric pump keeps the system circulating. A heat exchanger is incorporated in the storage tank to transfer the heat in closed systems to the water. And a storage tank is added.

The storage tank is usually about 82 gallons, or not much bigger than your hot water heater. It can be placed wherever you have room: near your hot water heater or even buried in the backyard. It provides extra hot water without use of the conventional gas or electric system and so pays for itself in the long run.

Your conventional water heater will act as a backup. When the water temperature drops below the desired thermostat setting, it will switch on. This way you always have hot water, even on cloudy, rainy days and at night.

Itf you've decided to give solar energy a try, you must then decide which type of system to use. The three basic systems - air, water, and anti-freeze - differ little in price. Each has advantages and disadvantages. So your particular preferences or situation will dictate which is best for you.

In a water system, household water is circulated directly through the solar collector. It is the most efficient system. But that efficiency can be cut down if cold weather forces the system to be shut down or causes freeze-ups.

To minimize the danger of freezing, some manufacturers offer a drain-down feature. With this, water will stop being pumped to the collector when the temperature drops to a certain point. Water already in the collector drains down. And the conventional system switches on.

But the reliability factor of the drain down system is important. If it fails, the collector will freeze and break. And if it depends on electricity to work, a power failure could cause a freeze-up.

Leaks are sometimes mentioned as a drawback. But they are no more likely than in your regular household plumbing.

There is one additional plus. Your own water heater can serve as the storage tank if you don't have rrom for a second tank. Or if you just don't like the idea of putting one in. Less heated water is made available. And there is less efficient use of the solar radiation. But it is also simpler and cheaper to install.

Another system to consider is a closed system using antifreeze. This is favored by many experts over the water system because there is no possibility of freeze-ups. But there can be a health problem.

A leak could allow antifreeze to pollute your drinking water. Unless the leak is large enough to affect the taste of the water, it may go undetected. Your local plumbing code may have special regulations concerning this.

To prevent this problem, some manufacturers offer a heat exchanger on the outside of the storage tank. This way the antifreeze could never come in contact with the water.

Another closed system used only air. The air is blown through the solar collector where it is heated. Then it passes through a heat-exchanger (like the antifreeze) and heats the water.

This system is not widely used though it has few of the problems of the other systems. Leaks are inconsequantial. There is no danger from corrosion. And air can't freeze. But it does need more electricity for the blower.

One of these systems can probably be made to work for you. To get more information and a list of solar equipment distributors, write to the Solar Energy Industries Association, 1001 Connecticut Avenue NW, Washington, D.C. 20036.For $8 plus $2 postage and handling, they will send you their booklet entitled "Solar Industry Index."

Or call the National Solar Heating and Cooling Information Center toll-free at 800-523-2929. If they can't answer your question, they will refer you to someone who can.