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A Consumer’s Guide: Solar Hot Water or Solar Electricity?

So you want to go solar, but can't decide which type of solar energy system to install. Should it be a solar water heater or a photovoltaic (solar electric) system? It's not just a matter of technology. Household size is one very important factor. A standard solar water heater in Oregon is sized for a family of four, and will produce between 2500 and 3000 kilo-watt hours worth of energy each year. However, smaller families may not use all of this energy, leaving some of it wasted.

Is your household a fit for solar hot water?

Here is an illustration. I threw together a quick calculation to illustrate the benefits to home owners from a solar water heater. There are two assumptions. First, each person uses about 20 gallons of hot water per day. Second, the incoming water temperature is about 50 degrees and the conventional water heater is set to 120 degrees meaning that the water temperature has to be increased by 70 degrees.

To keep the calculations simple, I assumed that the solar water heater would satisfy 100% of the hot water needs for a certain number of days. You can think of these as "full day equivalents." For 1- and 2-person households, the solar water heater would accomplish this for more days. I chose 240 days or eight months. For 3- and 4-person households, I chose 210 days, or about 7 months. These assumptions are very generous for western Oregon.

You can see that a four-person household would use about as much energy in kWh as the typical solar water heater can produce. However, smaller houses would use less. So, even though the solar-heated water is available, it would not be used.

In western Oregon a 2-kiloWatt, grid-connected PV system would produce about 2200 kWh each year. If more energy is produced than is used on any particular day, the excess electricity automatically flows back to the utility. So, PV might be a better choice for household sizes of 3 people of less.

This situation is extremely sensitive to the gallons per day of usage. If you have teenagers in the house, hot water use may be quite a bit higher than 20 gallons per person per day. Lower water use through efficient dishwashers and low-flow showerheads—while extremely cost-effective on their own-- can make the economics of solar water heating even worse for small households. And for the smaller households, a gas tankless water heater or heat pump water heater would be a good alternative.

About the Author

Bruce Sullivan

Bruce Sullivan is the New Homes Outreach Program Manager and Green Building Consultant in Central Oregon for Earth Advantage Institute. He has been involved in energy efficient construction since 1983. He has worked for the Oregon State University Extension Service, Eugene Water & Electric Board and Iris Communications, Inc. In 1996, he established Oikos.com one of the world’s first web sites dedicated to green building. In 2006, Bruce’s personal high-performance residence, built by SunTerra Homes, was honored with the NAHB Research Center’s EnergyValue Housing Award and the NAHB Green Building Award. Bruce is based in Earth Advantage’s Bend office.