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Solar Hot Water

System Sizing

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Proper System Sizing

One of the most important steps in buying a solar hot water system is to make sure it is sized properly for your hot water needs. If too large, you wasted money on a system that makes more hot water than you need. If too small, you spend extra money using your back-up heater that is powered by natural gas, electricity, or propane. The appropriate size and number of collectors, storage tank size, and overall system type that works best for your hot water needs will result in a smaller, more resource-efficient, and cost-effective system.

Collector Sizing / Roof Space

Flat plate collectors range in size from 20 to 48 square feet and can weigh from 80 to 150 pounds. Dimensions range from 3×6 feet to 4×10 feet with a 3-inch thickness. Evacuated tube collector sizes depend on the number of tubes used. A 20-tube collector (about 6×7 feet in dimension) would provide enough hot water for one to three people. They weigh more than flat plate collectors.

For a quick residential system estimate, the general rule of thumb is that your south-facing roof/surface needs a minimum of 20 square feet of collector area for each of the first two people in the home. For each additional person using hot water, add a minimum of 12 to 14 square feet. Using these guidelines, 52 square feet of south-facing roof or other surface area is needed for collectors that can provide 100 percent of the hot water for three people. Conserving water and installing water-efficient fixtures and appliances can reduce the size of collectors and overall system needed.

Solar Storage Tank Sizing

Some companies sell one-tank solar hot water systems (the tank stores solar-heated water and serves as a back-up), but most building owners keep the existing hot water heater as a back-up and purchase a new storage tank for solar-heated water.

The rule of thumb for storage tank size is based on an average of 20 gallons of hot water per person per day.

# of People Storage Tank Volume (in gallons)
1 to 3 30-60
3 to 4 80
4 to 6 120

For active systems (those typically used in Montana and Wyoming), a more accurate estimate uses the following guideline: 1.5 gallons of storage tank capacity is needed per square foot of collector area. This helps prevent system overheating when hot water demand is low. For our example, 52 square feet of collector area x 1.5 = 78 gallons of storage tank volume needed; thus, an 80-gallon tank would be used.

System Efficiency

While the type and size of a solar hot water system are important, so is efficiency. The more efficient the system, the quicker it pays for itself. Two efficiency numbers to be aware of and ask about:

If you refer to the Solar Rating and Certification Corporation’s (SRCC) website and compare complete systems (under the OG 300 Directory), you can compare SEF and SF ratings. System ratings are for all components of a system: collector, tank, pumps, motors, valves, piping, etc. SRCC system ratings are only for residential-scale systems. The OG 100 Directory provides only individual collector rating information.

While the rules of thumb and sizing worksheet provide a ballpark idea of collector and storage tank sizes, solar hot water system companies and installers can conduct a more precise assessment based on your water use, roof tilt, latitude, solar resource, and seasonal temperature variations.

Computer Programs: If you have access to a computer, there are programs that allow you to input your information to get an idea of system size, cost, etc. One to try is Solar Estimate.


National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (2003, December). A Consumer’s Guide: Heat Your Water with the Sun. DOE/GO-102003-1824.

National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (1996, March). Solar Water Heating. DOE/GO-10096-050.

U.S. Department of Energy. (2010, October). Sizing a Solar Water Heating System. Retrieved January 19, 2011, from: http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12880

U.S. Department of Energy. (2010, October). Solar Water Heater Energy Efficiency. Retrieved February 8, 2011, from: http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12900