Solar systems for domestic hot water or electricity are eco-friendly investments

Green Lantern for 2/1/11
Green Lantern for 2/1/11 (Michael Sloan - For The Washington Post)
By Brian Palmer
Monday, January 31, 2011; 6:22 PM

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 vs. plastic, coal vs. natural gas or escalators vs. elevators, your dilemma is a win-win: In most cases, both of these products will shrink your environmental footprint.

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 - 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.

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 passes the sun's energy to your water through a heat exchanger. The heated water then flows into a water tank.

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.

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 to 70 percent of the sun's energy into heat, while high-end photovoltaics can 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 available to consumers and may never be.)

Such technicalities aside, 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 about 25 percent of the average home's energy use, a solar water heater can make a sizable dent in the utility bill. A 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 4,600 to 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 homeowners 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 generally we put the most stress on the grid when the sun is up and producing kilowatt-hours, especially in the summer months with air conditioning.

And even if your roof panels produce more energy than you can use, that surplus feeds back into the electric grid, displacing energy that would have been produced by coal, gas or nuclear plants. (Your electric supply is a two-way street. Just as 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.

On the plus side, solar water heaters are much cheaper to install. A photovoltaic system will run you about $15,000 to $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 potential savings.) To top off these basic savings, solar electric homeowners often can sell renewable energy credits back to their utility companies.

This doesn't make solar water heaters bad investments. In fact, installing one is almost always a smart choice - just usually not as smart as installing photovoltaics.

At the risk of sounding like a salesman, the Lantern is baffled that so few homeowners have jumped onto the solar bandwagon, given that the government tax breaks and subsidies are massive enough to make your solar unit pay off.

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 for assistance with this week's column. The Green Lantern is produced by the daily Web magazine Slate. Send your questions to

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