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Depending on how we are using heat, our local utility rates and weather conditions, our payback durations will all be different. That said, we can assume some reasonable averages and calculate the projections as follows:

Let's start with these assumptions:

- The sun provides 300 BTUs of heating per hour per square foot of collector

- The average collector is 50% efficient, returning 150 BTUs per square foot

- The average electricity cost in the US 1n 2009 is 12.05 cents per kilowatt hour (http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_a.html)

- 3.412 BTUs = 1 watt

- The weather conditions average 15 days with 4 hours of sun per month

With those assumptions we can calculate the value of a square foot of collector as follows:

- 720 hours of sun per year X 150 BTUs per hour = 108,000 BTUs per square foot per year

- 108,000 BTUs / 3.412 watt equivalents = 31,653 watt equivalents per square foot per year

- 31,653 / 1000 = 31.65 kilowatts per square foot per year

- 31.65 X 12.05 cents = $3.81 per square foot per year saved.

Based on these calculations and assumptions, simply take your total system cost per square foot and divide it by $3.81. If your cost is $4.00 per square foot, your pay back is just over a year. If your cost is $11.50 per square foot, your system will pay itself back in 3 years.

The short payback financial incentives of investing in solar are already compelling, but add in the total lifetime return on the investment and the case is rock solid. Let's add two more assumptions:

- Your collector will last 35 years

- Energy costs will rise at 6% annually

Based on the assumptions detailed above, the value of the energy generated by your collector is $3.81 per square foot per year based on 2009 energy costs. If energy costs rise 6% next year, the value will be $4.04 and the cumulative total for the two years will be $7.85. Run that same process out 35 years and the cumulative total is $424.57 per square foot!

Based on that value, if you build 100 square feet of collector, your cumulative energy cost reduction over 35 years will be $42,457! Build 200 square feet and you can expect to save $84,914!

Whether you choose to build a big array or a single small panel, every BTU that you generate with solar is one less BTU that you have to buy from your utility company. It's a lot more fun to keep those dollars in your pocket!

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