Manufacturers of the single-tank solar/electric system place a single 240 VAC element about one-third of the way down from the top of the tank. With a 120-gallon tank, this assures at least 40 gallons of standby hot water—even if the sun doesn’t shine. The heat in the tall, vertically oriented tank naturally stratifies, with the hottest water at the top. The solar heat exchanger is located in the bottom half of the tank, using the sun’s energy to warm the coldest water first.
On a sunny day, the solar gains will exceed the electric element’s temperature setting, with solar energy heating the whole tankful of water to 140°F or more. A water heater timer can be used to keep the electric element off during the middle of the day, “prioritizing” solar energy over heating with electricity. (A tempering valve should be installed to ensure that scalding hot, solar-heated water doesn’t flow into the hot water service.)
In a single-tank solar-integrated system, solar energy is generally able to achieve temperatures well above the thermostat setting, and the heat lost down to that setting is all solar generated—and all free. The typical standby loss of a two-tank system can be 15 to 20% of the total energy required for the water heating system. In a single tank system, standby losses are about half this amount.