Below is some information from AskRod.com about sizing a solar hot water system for your home. AskRod.com is created by Rod Hyatt, HTP's National Solar Manager. --
In late spring, I published a chart that helps size a solar hot water system for your home, but after hearing from some people I realized I needed to make it more simple.
So, to back up and provide a bit more information:
This chart shows what system is right for you, using actual sizing logic instead of guessing. (Believe me, I see that more than you’d think.) Instead of a rule of thumb, it considers a southern or northern location in the U.S. to determine what the incoming water temperature might be. The amount of energy you’ll need is based on raising the water temperature from where it starts as it comes into your home. With a little math, we can easily determine how much solar will be needed to fill up the tank, based on the incoming temperature.
The chart lists four choices of collectors with differing measurements. For example, if the load requires two 10-foot collectors and your roof height is only 7-feet tall, you still have several choices to make them fit nicely on your south-facing roof. (Unless you live in Brazil, then please face them north.)
To read the original story, click here: http://askrod.com/temp-of-incoming-water-for-sizing/