There is little magical or mysterious about solar energy. The same principle that heats hot water in a solar collector heats the inside of an automobile on a sunny day.

When you return to your car after a day on the beach and find the seat so hot you can't bear to sit on it in your bathing suit, it is because the car, with the windows shut, preventing the wind from cooling it, has trapped the sun's rays inside.Black upholstery on the seats absorbs even more of these rays, energy repleased by the sun in the form of light, visible and invisible.

Solar collectors take advantage of this so-called "greenhouse" effect. Most are built in an aluminum box. Inside is a flat metal "collector plate," usually black, that absorbs the sun's energy. Attached to the plate are tubes, usually copper (though some of the newest enclose the tubes in mirrored glass). Water circulates through the tubes, absorbing the heat transferred by the plate.

To trap the sun's rays and increase the temperature inside, the collector is covered with "glazing," one or two layers of glass or another transparent material. Insulation is added on the backside of the collector to keep heat from escaping. The entire unit is sealed tight.

One problem with solar energy is that the sun does not shine every day. Nor does the sun's radiation reach the earth with the same intensity in winter as it does in summer. The sun's rays are strongest when they strike the earth at an angle perpendicular to the planet's surface. As the earth tilts on its axis in the winter, these rays reach the surface at an angle steadily decreasing from perpendicular. They must penetrate more and more of the earth's atmosphere and, in the process, become weaker and weaker.

Solar water heaters will work in the winter and, to a degree, even on some cloudy days. But no manufacturer can guarantee to deliver 100 percent of your hot water every day of the year. A solar system requires a conventional water heater as reinforcement.

On some days, the water heated by the collectors is sufficiently hot for the tap. Most systems are equipped with a storage tank, practically indistinguishable in size and shape from a normal hot water heater. When the collectors are unable to bring the water up to tap temperatures, the hot water heater turns on. The water heater, however, draws water from the storage tank, water pre-heated by the collectors on a sunny day. It need not heat nearly as much as it would if it were using 55-degree water from the outside main.

Sensors tell a pump when temperatures in the collectors are high enough that it is beneficial to send more water up to be heated. The pump turns itself on and off automatically.

In this way, a properly installed solar system with 40-60 square feet of collector area can be expected to deliver about 50 percent of the hot water needed by a family of four over the year. Typically, this system will cost about $2,500. More collector area can provide up to about 75 percent of the family's hot water. Much depends on the local climate and the amount of water the family uses.

There are two different types of systems: closed loop and open loop. The open loop system circulates potable water through the collector tubes. The closed loop system circulates an anti-freeze liquid and heats the tap water with a heat exchanger. Closed loop systems are recommended for areas such as Washington, where freezing might cause water trapped in the collectors to expand and burst the pipes.

But potential buyers should remember that a standard is still being developed for solar heater safety. (Under-writers Laboratories [UL] is expected eventually to test collectors and award a certification label.) In some systems the heat exchanger is located inside the storage tank. In others, outside the tank. Although manufacturers design tanks with several safety measures to guard against the posibility of heat exchange fluid leaking into the tap water, and some use a non-toxic liquid, owners such as Washington architect Francis Donald Lethbridge have opted for a heat exchanger outside the tank.

The Washington Suburban Sanitary Commission, despite manufacturer precautions, requires a back-flow valve on systems in Montgomery and Prince George's Counties. The valve adds about $200 to the total cost.

For more information on solar, write the National Solar Heating and Cooling Information Center, P.O. Box 1607, Rockville, MD, 20850, or call toll-free (800) 523-2929. The center provides brochures on different kinds of solar systems, directors to manufacturers, distributors and installers, lists of persons in the Washington area who will show you their solar system, and a bib-liography.