Today’s guest post comes from Bob Ramlow, the instructor if our Solar Thermal Boot Camp. Bob Ramlow has more than 30 years of experience with solar-energy systems and is a co-founder and Board member of the Midwest Renewable Energy Association. He’s a solar thermal consultant for the Wisconsin Focus on Energy Program. We adapted this article from his book, Solar Water Heating, a Mother Earth News Book for Wiser Living. Bob teaches HeatSpring’s Solar Thermal Installer Boot Camp.

Click here to sign up for a free product training on real time solar thermal monitoring that is 50% cheaper, and 1/3 less time to install compared with traditional BTU meter based systems. 

Enter Bob Ramlow:

Rarely do we design solar water heaters to provide 100 percent of your hot water. There are just too many cloudy days over the course of a year. Nevertheless, a typical solar water heater will provide between 50- and 75 percent of your annual load. In hot climates, or during the sunniest times of the year, you can expect to get nearly 100 percent of your hot water from solar. And even during the cloudiest periods, you may get as much as 50 percent, depending on your climate.

In cool climates, you should allow 20 square feet of collector and 20 gallons of storage capacity for each person in the household. For large families, you can reduce this by 10 percent for each person over four members in the household. In warm climates, size the system with 15 square feet of collector and 25 gallons of storage for each person in the household, with the same reductions for larger families. These sizing methods will give the best return on investment. Smaller systems will work well, but your savings will be less.