"We switched to solar hot water."
Bob Alcock and Tim Wheeler, Lost River, West Virginia

How It Works: A flat-plate collector—basically an insulated weatherproof box containing a dark plate covered with glass or plastic—absorbs the sun's heat and transfers it to water or an antifreeze solution. The solution flows through a long loop of piping that heats water in an insulated tank. On a 90-degree day, the sun can heat this water to nearly 160 degrees F. On cloudy days, when the temperature of the water drops, a backup water heater can boost the temperature; it doesn't have to work as hard, though, because the water is preheated. The bottom line: You can cut your water-heating bills by more than 50 percent.

What These Homeowners Did: Two years ago Bob and Tim increased the size of their rustic home to 2,660 square feet. As part of a whole-house energy retrofit, they added five solar flat-plate collectors made by Alternate Energy Technologies, which is based in Jacksonville, Florida. Installed on the roof, the collectors heat two 80-gallon water tanks in the basement, so there's plenty of hot water on sunny days for household needs as well as for the new radiant heat system they added in more than two-thirds of the house. When their water drops below 110 degrees, a 75-gallon propane-heated backup tank kicks on.

What They Learned: The temperature of the hot water varies, depending on the weather and time of day. So they now time their showers—and encourage guests to do the same—to take full advantage of "free" hot water, which is most plentiful late in the day.

Keep in Mind: Panels should face south and catch the sun for most of the day. And solar-heated water is a more fleeting commodity than the electricity produced by photovoltaic cells, which can be stored in batteries.

Payback Period: 10 years
Their Cost: $14,770, installed, together with radiant floor heating (which they put in largely themselves), after about $2,000 in federal tax credits
Yearly Savings on Propane for Heat and Hot Water: $1,500
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