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www.greenbuildermag.com 08.2012


~ How-to ~ 38 DO THE SOLAR MATH Weighing the fi nancial benefi ts of a solar hot water


package will only be accurate if the evaluation is extended to consider the full life expectancy of the system.


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OT WATER HEATING heating typically accounts for roughly 25% of the typical family’s home energy bill. Solar hot water heating can de- crease water heating bills by 50% to 80% ev-


ery year. Federal tax credits can off set 30% of the initial system cost. Many states and utilities off er tax credits or rebates. Payback for that initial solar investment is fi ve to seven years, but after that, the sun continues to provide hot water without additional cost—and without concern for rate increases—so this added value needs to be considered in any payback calculation. There are several solar hot water calculators available online. Some are more thorough than others.


One we recommend is here: http://apps1.eere.energy.gov/femp/solar_ hotwater_system/index.cfm


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How much hot water? Determine hot water demand for the home’s current or proposed number of residents. For example, according to the the Renewable Energy Resource Center, a family of four typically uses 65 to 75 gallons of hot water every day.


Evaluate the existing system. Determine the age of the existing conventional hot water system. Electric hot water tanks are likely to be the least effi cient, lasting only 10-15 years. Unlike gas, heat pump and solar tanks, electric tanks are not even included in the government’s Energy Star ratings. What will it cost to replace the current unit with a similar unit? More importantly, what is the cost per gallon for hot water produced by this unit?


Try some scenarios. Crunch the cost of various capacity solar hot water packages—including panels, pumps, water storage tank and installation. Keep in mind that you don’t have to supply 100% of demand to make the system worthwhile. Be sure to deduct tax credits from this cost. You can fi nd your state’s solar credits at www.dsireusa.org.


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