You should decide on the size of your hot water heating system with the advice of a solar professional. Here are the two primary considerations when sizing your system.
Solar Collector Size
The conventional estimates for surface area of solar collectors are as follows:
20 square feet of collector area for two family members
8 square feet for each additional household member in the U.S. Sun Belt region or
12-14 square feet for each additional household member in the northern U.S.
Storage Tank Size
In general, an active water heating system requires 1.5 gallons of storage per square foot of collector. Some specialists suggest installing an even larger storage tank – about 2 gallons per 1 square foot of collector – in very warm, sunny climates. The more tank space, the less likely the system is to overheat when demand for hot water is low.