Update: Sometimes I re-read something I have written and realize I left a lot of important information out. Homepower Magazine had an excellent article about sizing solar hot water systems in an easy to understand article called sizing solar hot water systems, but that has been taken off of their website. Also, they have an excel spread sheet which you can down load. By way of example, here is my excel spread sheet that I used to calculate the size of my solar hot water system.

I may be a little strange, but for some reason, I like tinkering around with formulas and little math problems. Perhaps that is why I like doing this kind of thing. As with any system design, the first thing is to determine what you need and how much. That is easy, we need hot water and as we are a growing family of four, we will need quite a bit of it. The average family of four uses about 60-120 gallons of hot water per day (15-30 gallons per day per person). I will have 80 gallons of water storage in the solar tank, plus another 40 gallons in the backup tank for a total of 120 gallons. The backup tank will have an electric element connected as a back up source of heat for prolonged cloudy periods.

I will be using a tempering valve which mixes cold water with the output temperature of the hot water tank. This will prevent scalding injuries if the solar heated water gets over 130 degrees F. I will set the output temperature at 115 to 120 degrees, which is what our hot water temperature is now.

Sizing the solar collector depends on how much water storage the system has and where the house is located. The basic rule of thumb for my area is 1 gallon of storage per 1 square foot of collector. Therefore my collector square footage will be 80 square feet.

My idea here is the solar tank will function as a water pre-heater. In the summer time, I should be able to pre-heat the water to about 160-180 degrees F. In the winter, I think I should be able to get the solar tank to around 80-90 degrees F and the electric element in the backup tank will take it the rest of the way to 115-120 degrees F. Our ground water temperature is 52 degrees year round, so I think this is not an unreasonable goal. Thus for 6-8 months out of the year, the solar system should handle 100 percent of the water heating and for the other 4-6 months, it should handle about 50 to 60 percent. This seems to be about the standard around here.

I drew this 2D basic system diagram (click for hi-res picture):

There are several good sources of information on solar hot water heating:

Arizona Solar Center
Home power Magazine
Alternative Energy Technologies
Green Builder.com

And, there is one very good reference book called Solar hot water systems, lessons learned 1977 to Today written by Tom Lane from Energy Conservation Services of North Florida. This book is a wealth of knowledge based on Tom’s experiences installing solar hot water heating systems. You can order the book and information about tax rebates at their web site.

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