If I take 60% of $252.48, (since a solar collector system can only provide 60% of water heating), I would have $151.49, which is the amount I would save per year by using the solar collectors. 

 

Dividing that into the $5800, which is the cost for the system, I found that it should take us 38 years to pay this off !?  As this was way above the 10-15 years my contact Mr. Kelly predicted,  I asked him where I had gone wrong.  He informed me that I had forgotten that the price per gigajoule would increase over time.  Mr. Kelly suggested that I use a 13% price increase/year,  calculated from the ATCO Gas website of gas prices for the past 35 years.  He helped me with the spreadsheet below, which takes that increase into consideration.  For my family, I found it would take 15 years to payoff the solar collectors.

 

 

 

                                         

                                             Payback Time                          

 

Ellie Shifflett 

Grade 8, Cayley School

Last Edited May 1/07

Gas Price Increase   0.13   (35 year average from ATCO website)

Amount Saved in First Year   $151.49