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.

Unfortunately, most owners can't achieve anything close to this optimal efficiency. A typical household uses the most hot water in the morning—that is, shower time. But that's not when solar water heaters work best. In the morning, they've only begun heating water for the day. After sunset, the water in your basement tank gets cooler and cooler. So a good deal of that solar energy can go to waste.

Photovoltaic systems, on the other hand, don't waste surplus energy. Electricity demand peaks at different times depending on the time of year and local weather. But, for the most part, we put the most stress on the grid when the sun is up and producing kilowatt hours, especially in the summer months. And even if your roof panels produce more energy than you can use, the surplus feeds back into the grid, displacing energy that would have been produced by coal, gas, or nuclear plants. (Your electric supply is a two-way street. Just like the power company can push electrons into your home, you can push them back out.) In other words, a rooftop photovoltaic system can spread its benefit across the entire energy grid. Solar water heaters help only a single household.

To their credit, solar water heaters are much cheaper to install. A solar electric system will run you somewhere between $15,000 and $25,000, though that doesn't account for the tax credits and other incentives that local, state, and federal governments use to sweeten the deal. A water heater—which uses less advanced technology and fewer panels—will cost about one-third as much.

Still, solar electric systems are a better long-term investment. Photovoltaic panels carry 25-year warranties and most will probably last much longer, helped by the fact that they have no moving parts. Over that time, an average system will crank out about 150,000 kilowatt hours of electricity, at least twice as much energy as a typical household would use heating water. (Many manufacturers offer energy savings calculators to check your own potential savings.) To top off these basic savings, solar electric homeowners can make around $300 per year selling renewable energy credits to their utility companies.

This doesn't make solar water heaters bad investments. In fact, quite the opposite: Installing one is almost invariably a smart choice—just usually not as smart as installing photovoltaics. For homeowners who like long showers and own electric water heaters (which are more expensive than gas models) it often makes sense to install both systems, since solar thermal is more efficient at heating water.

At the risk of sounding like a salesman, the Lantern is baffled that so few homeowners have jumped on the solar bandwagon. While the systems aren't cost-efficient (PDF) without government incentives, those tax breaks and subsidies are massive enough to make your solar unit pay off. Twenty-two states offer some kind of tax incentive. New York for example, will pay 25 percent of the cost of your system and exempt it from the sales tax. The federal government is offering to pay 30 percent. That's free money to buy something that's going to save you even more in the coming years. Yet fewer than 1 percent of homes in California, the most solar-powered state, have solar panels.

The Lantern spends a lot of time answering your questions, so here's one for you: What, exactly, are you waiting for?

The Lantern thanks Scott Carr of Standard Solar and Brian Seal and Donald Kintner Jr. of the Electric Power Research Institute.

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Brian Palmer writes about science, medicine, and the environment for Slate and Earthwire. Email him at explainerbrian@gmail.com. Follow him on Twitter.

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