For example:
Suppose you live in Boston and have observed from the other section that you will need 60 sqft of collector to achieve a system with an about 80% solar fraction, but you only have room for a 40 sqft collector. What kind of hit will you take in solar fraction, cost savings, payback, and emissions if you drop down to 40 sqft of collector.
From the first graph you can see that:
The energy saved per year drops from about 2400 KWH down to about 2000 KWH, so a loss of 400 KWH or about 17%
The CO2 emissions saved drop from about 2.65 tonnes to 2.25 tonnes, so an increase in emissions of 0.4 tonnes
From the 2nd graphs you can see that:
The solar fraction drops from about 80% down to about 70%
The DIY system cost goes from about $1200 down to about $900
The 5 year cost saving drops from about -- to -- < need to look up exact values from spreadsheet>
This graph also shows the incremental saving you get with each increase in solar fraction -- as you can see, as you go above 80% solar fraction the extra savings for each increase in solar fraction drops off -- that is, a diminishing returns sets in.
This graph shows how the energy saved by the solar water heating system increases with collector size. The energy saved keeps going up as the collector size is increased, but the rate of increase in energy saved drops off for higher solar fractions -- a sort of diminishing returnds effect.
This is primarely due to the fact that a system with large enough collectors to keep producing a high fraction of your hot water needs with solar in the middle of winter will have excess capability in the mind summer that is wasted.
