The solar industry has made some inroads in the Washington area, but the sight of rooftop collectors is still an uncommon sight here.

About 20 businesses cater to the demand for solar equipment and service, but the growth of the industry is small here compared with the Sun Belt.

"The cost is prohibitive" for most home buyers, said Jean Bateman, of Bateman and Son, a Northeast plumbing store. "An average conventional water heater costs about $415. A basic solar water heating system runs approximately $3,000, and that doesn't include installation. The problem is convincing a buyer to invest in a system this expensive that will still need a conventional back-up system for most winters."

Passive solar design, using a structure that collects and stores heat, is cheaper than an active system, which transfers and distributes heat via pumps, tanks and pipes.

But an active system is easier to install in existing buildings. Passive equipment is bulky and is not as efficient as an active system in a cold winter zone such as Washington.

And the larger the home, the number of occupants and the consumption habits, the higher the cost.

For example, David Friend of Solar Science Industries estimates that it would cost $4,500 to $5,000 to install a solar hot water system in a four-bedroom, two-story home with five occupants. The cost is generally doubled if a buyer is interested in a space heating system.

It is cheaper to install such equipment in a house as it is being built, rather than retrofitting, but equipment suppliers here say that most systems are being sold here for existing houses.

"Builders and home buyers alike are reluctant to put any more frontend costs into a house and want to keep costs as cheap as possible. Out of the last 20 jobs we've had, only two were for new residential homes," said Dick Curtin of Solar Sciences Inc.

The store owners point out that there are tax breaks for owners who install such equipment. A 1980 tax law for renewable energy allows a 40 percent write-off on all the first $10,000 of a solar purchase. But solar businesses contacted here reported just a slight increase in sales last year.

Cost and geography are part of the problem, they say.

To retrofit a typical house occupied by a family of four, three 4-by-8-foot solar collectors are usually needed. For optimun efficiency, the collector plates must be placed on the roof or ground facing due south and 115 degrees west to collect morning and afternoon sunlight. This single prerequisite rules out much housing in the District and dense areas of the suburbs.

Last year, Washington had 156 cloudy days, which further necessitates the use of collectors that circulate an anti-freeze derivative, proylene glycol.

A 1,500-square-foot house in Washington needs a 750-gallon water tank for storage and 500 extra feet to house a space heating-system. The size of the storage tank could be reduced to a 50-gallon capacity if filled with propylene glycol instead of water. But the fluid is very expensive, highly corrosive to metal piping and needs to be changed every two to four years.

New piping and a pump must be installed to transport cool fluid from the storage tank to the rooftop collectors where it is heated. The fluid is then re-pumped down to the storage tank, adding heat for storage during the night and colder days.

A separate heat exchanger unit must be installed to capture the heat transmitted from the storage tank and circulate hot air throughout the house. At this stage, existing piping is utilized in circulating the air.

Forced-air systems are easier to retrofit because in older homes with radiators, an entirely new radiant baseboard system must be installed at an approximate cost of $1,500 to $1,600. This price excludes extensive retrofitting in many District homes and older homes in the suburbs.

During air circulation, if the difference in heat temperature between the water tank and fluid circulating from the collectors becomes too small, the pumping is shut off by a differential thermostat -- usually during the night. This prevents the tank from cooling down so much that heat is lost for the next day if it is cloudy.

If several cloudy and cold days run together, a backup system is needed to assist heating, and another differential thermostat enables automatic readjustment of the system.

There will be winters when no backup for the system described is needed.

Verna Burch is a part-time receptionist at Futuristic Solar Systems in Temple Hills who installed a solar heating system three years ago.

"I live in a huge house and last year, except for the summer, we spent only $25 to $30 a month for electricity," she said. "None of that was for last winter, because it was mild enough that we didn't need a backup."

Savings on fuel costs depend on the configuration of the house, how well insulated the house is, and the demands placed on the plumbing system by occupants.

Many factors determine how fast a solar system can pay for itself. Solar experts believe that, given prime conditions, eight years is the best a solar system can do.

Most of the retrofitting takes place in the suburbs, in part because zoning restrictions and building codes in the city are stricter, suppliers contend.

"In the suburbs, it only takes a couple weeks to get a permit to begin installation, but in D.C. the buyer must wait three months. We lose a lot of sales during the waiting period," said John Hyde of Sun at Work in Annapolis.

Because solar collectors need to be free of obstruction, shade from trees is an inhibitor. Most of the District's single family housing stock is old and well shaded.

Energy competition is also a factor.

The District has a higher percentage of homes using cheaper gas heat, whereas expensive oil and electricity are the major energy sources in the suburbs.

"It is so much easier to get solar energy into newer developments where there is little shading and lots of electricity. You won't see more solar in D.C. until gas prices get higher and people start looking more seriously at alternatives," said Tom Miller of Solar Sciences, Inc.

Other homeowners want to wait until solar heating becomes more popular so prices will go down, suppliers say. But solar technology is not like Polaroid cameras or calculators, these suppliers contend: the cost will only rise.