How Big Should Your Solar Water Heating System Be?
This section helps you figure out how large a solar water heating system you need to acheive the level of performance you want in your climate. It will tell you how much collector area you will need to achieve a given level of performance, and will also tell you what the payback period is for a given size system as well as the yearly energy saved, cost of that energy, and the reduction in CO2 emissions.
These graphs will be mainly of interest to people building their own systems as they are DIY systems and include materials costs only, but some of the information (eg collectors size vs solar fraction) may be of interest to people having commercial systems installed.
Two sets of graphs are provided to tell you:
How large a system you will need to achieve an 80% Solar Fraction for 15 US climates. It also provides energy saving, cost saving, payback period, and CO2 emissions reductions.
80% Solar Fraction means that 80% of the enrgy to heat your water comes from the sun. This is often pretty close to what you want for the best return on your investment -- but, not always -- see item 2 just below to look at other Solar Fractions.
Click on the first graph above to take you to the page that has plots of collector size, cost, cost savings, emissions savings for a bunch of US cities. Read off the collector area, cost etc. for the city that most resembles your climate.
If you want to look at what happens to the system size, costs, and performance for systems with less than or more than an 80% solar fraction, then click on the 2nd graph to take you to the page that provides
Feel free to leave a comment or email me if you run into problems or have questions.
Assumptions Used
Hot water demand is 48 gal per day which is typical for a family of three
Collector efficiency curve parameters are based on the SRCC efficiency curve for a commercial flat plate collector with a black painted (non-selective) absorber.
Collector tilt is set to the latitude of each city modeled, and the collector faces south.
Cost is based on this cost table for the $2K system with the space heating left out. The cost is adjusted up or down depending on the collector and tank size used.
The existing water heating (before solar) is assumed to be electric at 12 cents per KWH.
See the description of the $2K System for the details on the collector, tank and controlls assumed for this system.
The tank size is done at 2.5 gallons per sqft of collector, but is not allowed to go below 100 gallons in order to have room for the pipe coil heat exchanger. It could be argued that the tank capacity per sqft of collector should depend on climate (larger for warmer climates) -- I looked at this a bit and it did not make much difference on DIY system cost or savings or performance.
The cost of the system is calculated as:
Total Cost = (Collector Area sqft)($8.5) + (Tank Cap gals)($1.8) + $426
So, a system with 60 sqft of collector and a tank size of (60 sqft)(2.5 gals/sqft) =
Where the $426 covers cost of plumbing, pump, potable water heat exchanger, controller, ...
All of these costs are for materials only with you supplying the labor.
