Still, no matter how well a collector performs or how perfectly it’s placed, there’s no getting around the fact that it relies on an energy source that only works part-time. “You can never meet the demand exactly, because you can’t turn on the sun when you need it,” says Robert Waters of Viessmann, a manufacturer of solar thermal systems. In the some parts of the United States, solar panels might provide as much as 95 percent of a household’s hot water in summer, but as little as 20 percent in winter. And unlike solar-generated electricity, which can be stored in batteries or sold back to the power company, hot water is a fleeting asset; even a well-insulated storage tank turns cold after a few overcast days. That’s why virtually all solar hot-water systems are supplements to, rather than replacements for, conventional water heaters.

As is often the case, it costs money to save money. A typical setup for a family of four, with an 80-gallon storage tank, runs about $5,000, plus another $2,000 or so for installation. But compared with photovoltaic cells or wind generators, the payback periods are relatively quick—as little as five years, depending on local energy costs and state subsidies. (That factors in the current federal investment tax credit for 30 percent of the cost of a solar thermal system, up to $2,000.) Solar hot-water systems are also fairly easy to retrofit in existing homes. The solar-heated storage tank is simply linked to the existing hot-water tank, which switches on only when water from the collector falls below the water heater’s temperature setting. In new homes, a single tank can be heated by both solar collectors and gas or electricity. Either way, you can take your hot-water savings to the bank. Or you can hop in the shower and sing a longer song.

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