It's a rare sight on Shelter Island or almost anywhere else in the county: a residential roof bedecked with solar panels. But a turn-of-the-century house on Cartwright Road that has been undergoing a major renovation this year is packing enough solar punch that its electric meter often runs backwards.

"We're both very concerned about the environment," explained Melanie Stiassny, the curator of fishes at the American Museum of Natural History, who bought the three-bedroom shingle house with her partner Jacqueline Black last summer. "We decided that when we moved to Shelter Island, we didn't want to make a huge footprint."

"The option of solar power has been around for decades," added Ms. Black, a freelance photographer and artist. "It's always been in the back of my mind. When we bought the house, the roof was right for it -- sloping south toward the sun."

Like a lot of houses on the Island, the house sits on a well-wooded half-acre plot but there's enough open sky to make the solar panels function efficiently. Its owners both live in Manhattan now but plan to make their Shelter Island house their full-time residence, albeit with some frequent runs to the city.

"We're not air-conditioner-type people," Ms. Black added. "We're not 6,000 square feet and 10 bathrooms types. The savings this system can generate for us make a big impact." People with bigger needs, she said, would find the impact on energy costs proportionately less.

"But if enough people do it," Ms. Stiassny jumped in, "it will have a tremendous impact on the environment" by reducing the use of fossil fuels and their emissions into the atmosphere. And that's the whole point.

"You don't look at this just in terms of dollars," said Ms. Black. "You're not just buying something. We're trying to generate power so we're not burning fossil fuels."

"The real cost issue," said Ms. Stiassny, "is global warming and habitat degradation," which follow from burning fossil fuels. The very notion of getting energy from the sun, she added, is its own reward.

It so happened that their renovation and its array of 16 solar panels coincided with a LIPA rebate program designed to encourage Long Islanders to explore alternative energy sources. Since 1999, the utility has been offering a rebate that started at $3 per watt on photovoltaic cell systems -- solar panel installations -- that meet certain conditions and standards. The rebate went to $4 and then last December to $6. The rebate will drop back to $4 after July 31. Also, the state offers a 25-percent tax credit on the capital costs of installation.

Ms. Stiassny and Ms. Black's system, with a rated maximum capacity of 1,920 watts, cost them about $8,000 in out-of-pocket costs after the $6-per-watt rebate LIPA gave them and the state's tax credit.

The contractor who installed it for them, Go Solar of Riverhead, told them to expect a savings of about $33 a month in their electric bills, the women said. With energy-efficient appliances and no air conditioning, that should leave a pretty meager balance to be paid: maybe $10 or $20 a month, they estimated.

The system is connected to LIPA's grid: LIPA takes the power it generates and supplies electricity to the house in the conventional way. If LIPA has a power failure, Ms. Stiassny and Ms. Black's house loses power too. But when their electric load is light, their LIPA meter does run backwards, lowering their monthly consumption from the grid and therefore their bill.

When they first bought the house, the two had considered installing a windmill to generate electricity. They were deterred by the resistance LIPA found here to its proposal to site a wind turbine at the town recycling center. At least one neighbor wanted no part of it and the Town Board eventually nixed the idea.

Ms. Black and Ms. Stiassny chose Shelter Island to buy a house away from the city "because it's the UnHampton," said Ms. Black. "It's a gentle place. People seem to be interested in keeping things simple and protecting the wetlands and the environment. It's very appealing."

Since LIPA started its solar rebate program to encourage Long Islanders to reduce their dependence on fossil fuels, the utility has handed out about $2.88 million for 98 solar installations, most of them since the rebate was hiked to $6 a watt.

Before last December, in fact, rebates had been paid on only 16 systems. The 98 solar systems now in use on Long Island produce about 630 megawatts a year, enough to supply about 74 typical suburban homes.