The interconnection agreement with the Electric Coop is the first ever written in the Keys, said Hammerstrom.

Energy efficiency makes the PV system work, Hammerstrom said. He saves $10 on energy for every $1 he spent making the house energy efficient. The couple opted for the following:

• A white metal roof to reflect the sun's heat away from the house. They decided against a radiant barrier below this because it would only hold in heat rising from the house.

• Small windows and extra insulation on the south-facing wall.

• A 40-gallon solar water heater on the roof.

• Sealed, insulated duct work that is mostly contained within air-conditioned space.

• A gas range and cooktop.

• Miele low-energy and low-water dishwasher.

• Kenmore front-loading washer and gas dryer.

• A cupola with operable windows to draw warm air up and out of the house.

• Hampton Bay ceiling fans designed by Danny Parker of the Florida Solar Energy Center (sold at Home Depot).

• Low-flow shower heads.

• Bamboo floors and stairs. Bamboo is fast-growing and can be reharvested every five years.

• Formaldehyde-free insulation in the ceiling crawl space, exterior and interior walls.

• French doors with wood on the interior and vinyl-clad on the exterior with operable transom windows attached.

• Custom clerestory windows in interior walls for cross ventilation.

Following in a long Florida Keys tradition, Hammerstrom and Marshall also have a cistern. But this contemporary interpretation goes beyond the open, rain-fed concrete tanks of years ago. This one is rain-fed, but with slick differences.

Gutters direct rainwater from the roof to two large PVC pipes. The first rain off the roof, which holds the most dirt and debris, is allowed to drain away. Gradually, a Ping-Pong sized ball lowers into the drain and clean water then is backed into the cistern. After a day, the drain stopper gradually rises again.

CISTERN SPLIT

The cistern is divided in half, with the first compartment decanting water into the second. In addition, Hammerstrom has installed a series of filters on the cistern to clean the water before it is brought into the house: a 1-micron filter for fine sediment, a carbon filter for chemical contaminants and a UV filter chamber for water sterilization.

''The UV sterilizes in case there is any bacteria from bird poop, lizard poop or any other type of contamination that could get into the cistern,'' Marshall said. And like their solar/electricity set-up, Hammerstrom and Marshall have fashioned a split water system: They can use city water or cistern water, depending on their needs.

''We prefer to drink cistern water because it doesn't have chlorine and other chemicals required in the city water,'' Marshall said.

They sent up three valves to control where the water comes from and goes. Valve one delivers water to the kitchen island sink. Valve two delivers water to the sinks, laundry and showers throughout the house. Valve three delivers water to toilets and outdoor hose bibs.

The water in the kitchen sink is always from the cistern. If the cistern has more than 600 gallons of water in it, the laundry, sinks and showers draw from it, too. Early in the rainy season, the outside hoses and the toilet water comes from the cistern; as the season dries, the hoses and toilets are put on city water.

HOT AND COLD

To keep from wasting gallons of cold water while the hot water from the rooftop solar water heater circulates through the pipes, Hammerstrom has installed an ''on demand recirculation system'' that uses a small pump to move hot water more quickly. A button just beneath the light switch in each bathroom is pushed, ''you can change your clothes and in 90 seconds, there will be hot water for your shower,'' he said.

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