DIY and how to choose the right size of a solar water heater?

When talking about sizing the solar water heaters, I will use the recommendation from the US Department of Energy.

There are two elements that have to be considered when sizing a home solar water heater; solar panels (collector area) and water storage tanks.

This recommendation is for the southern parts of US, such as Texas State; 20 square feet of a collector area for each person of a two-person family, plus 8 square feet for an additional member. That means that for your family of three, the approximate area of solar panels is 48 sq. ft. and that will be sufficient to provide enough hot water.

In a case of a water storage tank, the recommended capacity is in a range from 50 to 60 gallons, and can be larger if using hot water often. If the recovery rate is higher, the tank can be smaller, but if it is a budget-type unit, you might need a larger capacity than 50 gallons.

Consider also recommendation from the contractors which applies on the active water heating systems in Texas, the size of a water heater tank can be dimension in a ratio of 2 gallons of water storage per 1 sq. ft of a collector area.

Hope this helps.