For John and Susan Rust of Alexandria, solar energy has become a passion, a challenge, a lifestyle and a high-priced risk from which they hope to turn a profit - someday.

At the moment, however, the Rusts are still trying to get back in the form of lower utility bills the $22,000 it cost them to install a solar system in their home. They expect it will take 30 years. They never did find a buyer for a second solar house they own despite its location in booming Old Town Alexandria.

They have had to rent it.

"We made some concessions in that house to make it more appealing," Susan Rust said, explaining that the house contained four large fireplaces which are not considered energy efficient because they draw heat from the rooms. It also featured a large kitchen, and which is not solar efficient because rooms ought to be small so they are easier to insulate and it also had multi-paned windows through which heat can escape. Mrs. Rust said the $160,000 home should "have sold for its prices" but that the market was not ready for such innovations.

John Rust said he overheard one real estate agent showing a prospective buyer the house and saying, "You know, this is a solar energy house. It's really fantastic but, of course, you have to be an engineer to run it."

"I think if I could take the entire system and hide it . . . in the closet, it would solve part of the problem," Rust added. Although it takes only two or three steps to sitch the system from heating to cool, Rust said that the exposed gauges, thermometers and water storage tanks "tends to frighten people."

In a way, the unsold Rust house at 619 1/2 S. Pitt is evidence that solar energy has not been widely accepted by the public as a realistic and affordable alternative to more traditional energy sources. Tomorrow, the nation will celebrate "Sun Day" in an attempt to make people more aware of the potential of solar energy.

John Rust said he finally gave up selling the house after six months, an almost unheard of situation in Old Town, where real estate prices have rocketed in recent years. Today, the Rusts became upset whey they walk by the house and see that their tenant is not home but all the lights are on.

Over the weekend, the Rusts sat in the three-story, plant-filled atrium of their home and told a visitor that despite the frustrations and money solar energy costs, they are convinced it can become competitive with other energy sources and that the United States must make extensive use of it if truly serious energy shortages are to be avoided in the future.

The John Rust home at 615 Royal St. features few windows and a sharply sloped roof where the panels that collect the sun's ray are located. The atrium where the Rusts sat during their interview is called a "passive" solar energy system because the plywood walls and flagstone floor collect heat from the sun.

When the room reaches a certain temperature, dampers open automatically and suck in air from other rooms in the house. The heated air is, in turn, rotated to the rest of the house.

Even the fireplaces in the house are part of the system because they contain pipes through which water passes, is heated and then circulated. Rust said 1,000 gallons of water can be heated in four hours to 150-degree temperatures, but he admits that no solar system has been developed which can be relied on entirely during the cold northeast winters.

Nevertheless, he estimated that he saves 60 to 75 percent on heating costs on an annual basis.

Rust said that one serious problem with solar energy systems is that while each part is guaranteed by the manufacturer, the system as a whole is not. "The only person who can guarantee the whole system is the one plaining that frequently they are individuals like himself.

But Solar Energy pioneers such as the Rust are beginning to get some help from the government. Last year the Alexandria City Council passed a law which will exempt the value of the solar energy system from the total assessment of the property.