Solar storage tanks are typically well insulated and recommended for the dedicated tank for storing hot water generated from outside thermal collectors. They work as a preheat tank to the second or typical domestic hot water tank or boiler. The second tank which is also the backup on cloudy days may be heated by gas, electric, oil or other solid fuel sources.
Sizing a solar tank depends on the number of people in the home taking showers, cooking, and running diswahsers.
Recomendations are 20 gallons for each of the first two people and 15 gallons for each additional person and size up the tank from there.
For example, a family of two would be a 40- 50 gallon tank. A family of four would require a 75 to 80 gallon tank, and 100-120 gallons of storage for 6.