Solar water heaters, also known as solar thermal systems, use the sun’s energy to heat water with a free energy source. Before the recent growth of solar PV, solar thermal was the most common way people used solar to offset energy needs.
Although they both use energy from the sun, solar thermal and solar PV work very differently. Solar PV panels convert sunlight to electricity. Solar thermal systems store the sun’s energy (heat) in a fluid that warms your water.
Solar thermal systems require a smaller upfront investment than do solar PV systems. Solar thermal costs roughly one-third to one-half less to install. This leads to a shorter payback period. Solar thermal systems are about 3-4 times more efficient than PV panels. This means they take up less roof space than PV systems.
Unlike solar PV systems, solar thermal systems do require routine maintenance. The fluid in the system needs to be changed every three to five years. Typical solar thermal systems provide about 75% of home water heating needs. They are sized this way so as not to over produce. Unlike PV systems, you can’t feed water a thermal system has heated back into the grid. That excess production would be wasted.
Solar thermal’s economic value depends upon two factors: how much hot water your home uses, and what energy source you use to heat your water. If you have a large household and use electric to heat your water, you may find that solar thermal is a smart economic choice.
Of course, solar thermal and solar PV doesn’t have to be an ‘either, or’ choice. It is possible to have both systems installed on a roof.