Bill Banack doesn't fancy himself an environmental "fanatic," but his showers, dishwasher and washing machine all use renewable energy these days. The source: three slim solar panels perched atop his 2,200-square foot Hadley, Mass., home. They don't provide electricity—they send him heat for hot water.

"I'm not belittling the green movement, but mostly we wanted to save money," says Mr. Banack, who now shuts off his gas boiler in the summer except during stretches of cloudy days. Mr. Banack estimates his heating-fuel bills...