These tanks are built in sizes ranging from 200 gallons to 10,000 gallons and larger. These tanks are made from stainless steel, fiberglass or high temperature plastic. Heat to be stored is transferred from liquid collectors into the tank with a coiled heat exchanger. Many manufacturers used to offer tanks with internal coils, but they are hard to find today. Solar water heaters generally have either one or two storage tanks.
Most systems have only one tank, which normally contains only an upper heating element.
The element serves as a back-up heater when solar energy is not available or when hot water demand exceeds the solar heated water supply. A dip tube at the top or upper side of the tank forces the heated water to return below the heating element, ensuring proper hot water stratification. This tank is plumbed to a second tank which contains a heating element or gas burner that can be used when no solar energy is available. Except for large residential or commercial type installations, the most popular tanks are glass lined or stone lined. These tanks have an outer shell of steel that must be protected from galvanic corrosion.