to store hotter water in the tank—a mixing valve adds enough cold water in the pipes to keep the temperature at 120°F when it comes out of the hot water tap.

At night, having the timer turn off the element can save on standby losses too. Although it seems like this strategy would require more energy the next morning to raise the temperature again, heat loss occurs at a rate directly proportional to the difference in temperatures. A 120°F tank of water in a 50°F garage has a 70°F difference in temperature, or AT. When the tank's temperature drops to 110°F, the AT is reduced to 60°F, with a corresponding drop in the rate of heat loss. As the temperature drops, the percentage of heat loss drops as well. Even though the backup heater will be on longer first thing in the morning, it will not be on as long as it would have been if it had to maintain the temperature at 120°F through the night. Over the entire year, this amounts to many KWH of energy savings.

SHW System Costs

For most single-tank solar water heater systems, a 120-gallon tank will provide the best service. The single 4,500-watt element is strategically located a few inches above the middle of the tank and will effectively heat about 50 gallons of water electrically, with the remaining 70 gallons in the bottom portion of the tank heated by the sun. Although 80-gallon tanks are available, you'll run the risk of not having enough hot water on rainy days because the element will only heat the top 30 gallons.

For $4,000 to $5,000, you can purchase a single-tank drainback system kit that includes 40 square feet of solar thermal collectors, a 120-gallon tank heater, and all the other needed components. If you want to purchase the components separately, calculate your collector area based on the water in the tank that will be heated by the sun. In a 120-gallon tank, that would be the bottom 70 gallons. Divide this number by 1.5 to 2 gallons to get the recommended collector square footage. In general, most singletank systems work well with a 120-gallon tank and a collector area between about 40 and 60 square feet.

Federal tax incentives can help offset costs of a SHW system, and many states, cities, and local utility districts have incentives as well. The current federal tax credit pays 30% of a solar hot water system's cost, up to $2,000. However, qualified systems must be installed and operational on or before December 31, 2008, to receive the federal tax credit. And at least a dozen states offer incentives based on cost or performance, usually topping out at about $1,500. Utility incentives are also available in many regions of the country. To check incentives in your area, visit the Database of State Incentives for Renewables & Efficiency at You can also contact your local utility or solar contractor to find out what's available.

Conservation Pays

Keen energy awareness and dedicated conservation tactics can really make a single-tank system pay off, especially when coupled with a timer. There are always ways to conserve more, such as using low-flow showerheads and using hot water sparingly during cloudy weather. As your usage shrinks, the ratio of water heating by the sun increases. A retired couple from Portland, Oregon, once told me that I misrepresented their solar savings from the system I'd installed a year before. I was perplexed, since I had conservatively told them that they could expect to cut their water heating costs by 55% to 60%. But then they told me that their actual energy savings was 70%. I was amazed. After all, we're talking Portland, a place known for its soggy skies.

The key to the couple's exceptional savings was their enthusiasm. They were excited about their system and made the most of it, spreading their hot water use through the day to optimize solar efficiency and waiting for sunny days to wash clothes. Some of us don't want to change our lifestyles to accommodate the sun, but if you're so inclined, you'll reap extra dividends.

Single-Tank Success

Allen and Laura Bernstein installed a single-tank water heating system in August 2007, when their old water heater needed to be replaced. "We went with a single-tank system because we only had space for one tank in the laundry room," says Allen. The tank wedges between the wall and the laundry sink, with only a scant half inch to spare.

For their two-person household, the setup is ideal. They are energy-conscious and conservation-minded, and their routines allow them to set the timer to garner the most benefit from solar-heating hours. "The system is performing really well," says Allen. "We're getting plenty of hot water."

Allen is so impressed that he's planning to put similar systems on rental property the couple owns. "I think this makes sense for my business, especially with the tax credits available. I have a bunch of water heaters that are at the end of their useful lives and need to be replaced. I'm in the business of providing a comfortable place to live, and I believe my apartments will be more rentable as power rates go up."


John Patterson ([email protected]) is president of Mr. Sun Solar and inventor of the Sol-Reliant solar water heating system. He has installed more than 1,000 solar water heaters over the past 28 years.

Suzanne Olsen ([email protected]) is a writer and photographer specializing in renewable energy and the environment.

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