Fairfax City resident Jeff Cohen remembers the days when he had to turn off his washing machine to get a hot shower.

Now, thanks to four 10-foot rectangular solar panels bolted to the roof of his house and a 65-gallon water tank, Cohen takes all the hot showers he wants without worrying about whether there will be enough hot water left to clean his clothes.

The number of Fairfax County residents harnessing the sun's energy as an alternative heat source is on the rise. Domestic solar energy system sales in Fairfax County increased from 1,000 units sold in 1983 to 2,500 in 1984, and some local distributors have predicted that their business will double this year.

Donnie Woodrow, director of permits for the county's Environmental Management Department, said the recent rush for solar heating has been inspired by federal energy tax credits scheduled to expire Dec. 31, 1985. The tax credits are available to any consumer who invests in renewable energy equipment.

Congress sparked consumer interest in solar power when it passed the 1978 National Energy Act allowing taxpayers to deduct up to 40 percent of the cost of a solar system, for a maximum tax credit of $4,000. Virginia allows an additional 20 percent tax credit toward the first $5,000 of the same purchase (for a maximum credit of $1,000).

However, this state tax credit is scheduled to decrease at 5 percent increments per year until it reaches 10 percent in 1988. Solar systems purchased before Jan. 1, 1983 or after Dec. 31, 1987 are not eligible for Virginia's tax credit.

In 1980 the Fairfax County Board of Supervisors made solar investment even more irresistable by waiving the county's $21 permit fee to encourage energy conservation in the area. "These tax credits are the incentive for people to purchase a system," said Scott Sklar, director of government relations for the Solar Energy Industries Association, a D.C.-based trade association for the solar industry.

Woodrow said the hottest-selling solar product in the county is a domestic hot water system. This unit captures solar energy in elongated aluminum or stainless steel panels, which are usually filled with a nontoxic antifreeze fluid, and it uses the sun's energy to heat water.

Even though solar heat sales are booming now, said Sklar, distributors in Fairfax County should be aware of the potential for a decline in sales after the federal tax credit expires this year. Sklar said some stores will probably go out of business because "the young solar companies just don't have the capital resources to promote their product another way."

But Energy Consultants, a Vienna-based solar company, said business is too lucrative lately to think about future problems. Barry Thombs, general manager, said he expects to sell between 1,000 and 2,000 heating systems this year compared with its 1984 sales of 250 units.

"The people buying these systems are the smart ones," said Thombs. "There's only one decision to make: You can keep sending your money to the utilities and government, or you can put it back into your house.

"The only people not buying solar these days are the skeptics," Thombs said.

Monegon Limited, a Rockville-based distributor with 50 percent of its residential business in Fairfax County, said solar's popularity forced the company to double its 30-person staff.

According to the solar sales persons, most Fairfax County customers are families with three to five children and are college graduates earning between $30,000 to $60,000 per year. Thombs said these clients are primarily concerned about conservation and rising utility bills.

One Annandale resident saw an 88 percent decrease in his electric bill since he installed a domestic hot water system in his house.

And Dr. Mike Obremskey said the electric bills for his 4,000-square-foot, four-bedroom Oakton home are "very reasonable" since he bought his solar-heated home.

"Our electric bills are averaging about $170 per month -- and that's with an all-electric house," Obremskey said. "And at least 14 people can shower here in one day without running out of hot water."

He said his neighbors are becoming energy-conscious, too, especially with rising utility bills. "And of course the tax incentives help," he added.

Consumers should avoid installing a solar unit themselves, warns Sam. A. Cravotta Jr., a member of the Virginia Solar Energy Association.

"The risk is that people usually don't know what they're doing and could install the system incorrectly," Cravotta said. "You shouldn't get involved in the installations unless you're a licensed plumber or electrician."

Cravotta said a roof-mounted system could be hit by lightning and cut off a home's hot water supply, or a pipe containing the antifreeze solution could burst and cause further unit malfunction.