Solar water heaters: Are they better than solar electric panels?

Illuminating answers to environmental questions.
Feb. 1 2011 7:01 AM

The Best Investment Under the Sun?

Should I get a solar water heater? Or solar electric panels?

Solar Power Panels.

The days are getting longer, and I'm thinking I ought to make use of those extra rays to help the environment. The big decision seems to be between a solar electric system and a solar water heater. Which one should I choose?

This question is pure joy for the Green Lantern. Unlike paper versus plastic, coal versus natural gas, or escalators versus elevators, your dilemma is a win-win: In most cases, both of these products will shrink your environmental footprint.

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Just how much you'll shrink it depends on how much sun strikes your roof, how much hot water you use, and how many panels you can install. Of course, manufacturing solar electric systems and solar water heaters requires a good deal of energy and raw materials. But you might be surprised how quickly a solar system "pays back" its own carbon costs. Studies have shown that photovoltaic systems—that's the industry term for solar electric generators—recover the energy used for their production in just three to four years. Solar thermal systems can take just two years (PDF).

A residential solar water heater consists of one or two rooftop panels. In some systems, tubes carry your water through the panels, where the sun heats it up directly; in others, those tubes carry an antifreeze-like liquid that later passes the sun's energy to your water through a heat exchanger. The hot water then flows into a water tank that looks just like your standard hot water heater. (The two tanks usually sit next to each other in the basement.) Systems in the sunniest climes can heat the water to almost 200 degrees—but even in cooler, grayer areas, most solar heaters should still achieve temperatures similar to those a traditional water heater can reach.

Photovoltaic systems are bigger and more complicated. Homeowners typically blanket their roofs with 24 to 40 panels. When the sun's photons hit a panel, they knock electrons from one material in the panel to another, creating a flow of energy.

Technically, solar water heaters use sunlight more efficiently than photovoltaic systems, partly because of the complex series of interactions that happen in the photovoltaic panel. In addition, the silicon used in photovoltaic systems can't use as many wavelengths of light as the water heater, so some light goes to waste. Solar thermal systems convert 60 percent to 70 percent of the sun's energy into heat, while high-end photovoltaics top out at around 24 percent efficiency. (In the laboratory, researchers have developed photovoltaics that exceed 40 percent efficiency, but those stripped-down systems aren't currently available to consumers and may never be.)

Technicalities aside, however, photovoltaic systems are the better choice for the vast majority of consumers—and for the environment. That's because some of the energy used to heat the water goes to waste as it cools off in your basement, and because you can share electricity (but not hot water) with your neighbors.

Americans use a lot of hot water. Because heating it accounts for 25 percent to 30 percent of the average home's energy use, a solar water heater can make a sizable dent in the utility bill. An average household of two might use 64 gallons of hot water per day—that's two 10-minute showers, a load of laundry, a dishwasher cycle, and four minutes of running the hot water tap. Over a full year, a traditional electric water heater would use between 4,600 kilowatt hours and   5,000 kilowatt hours to meet that demand, at a total cost of $580. Generating that much hot water with a gas heater would cost about $266. A solar water heater operating at optimal efficiency, on the other hand, might eliminate the bill entirely.

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