Sizing your
Solar Water Heater correctly

Sizing your solar water heater correctly guarantees adequate and abundant hot water supply. On the other hand a correct sizing will prevent unnecessary over investment and an necessary pouring of money. So what you look for is the "right" size, not too big and not too small. The right size is dependent mostlyon the family size.
The two components that you need to size correctly are the solar hot water tank and the solar thermal collector .

Sizing of Solar Hot Water Tank

If your household is 1-3 persons, the right tank size is 80 gallons
If your household is 4-6 persons, the right tank size is 120 gallons

Solar Thermal Collector Sizing

For the first two people in your home you need a 20 square feet (1.8 square meter) flat plate collectorIf your home is in the Sun Belt States you need 8 additional square feet (0.7 square meter) of panel size for every additional family member.
If however you live in the more northern state you’ll need 14 additional square feet (1.2 square meter) for each additional family member.

Protection from Freezing conditions

A reminder - if you do live in a a region where freezing condition may happen or happening regularly - you need to have anti-freeze fluid to flow in the loop between the collector and the solar hot water heater tank. You need to have a heat exchange in your tank. a pump is another element that should be added. This is by definition an indirect solar hot water system, even if freezing condition are very rare, it is a must if you want to protect your solar thermal collector from a freezing damage.

If for some reason you can't install the tank above the collector even though you might live in the Sun Belt states you'll need to have an active solar heater. It means a pump is required as well as the tank and the flat plate collector. You can however run water in the loop between the flat plate collector and the tank.

If you live in the really northern states, you will need to consider evacuted tubes collector and not a flat plate collector

Evacuated tubes solar collectors size is measured by the number of tubes, you'll find sizes between 20 to 40 tubes. Consult your supplier about the correct sizing your solar water heater if it is of the evacuated tube solar type