Can someone help me with math?
I feel dumb that I can't figure this out because I'm normally pretty good with math. But I suppose it's not the actual math I'm having a problem with, it's figuring out the right equation to use. I'm making a spreadsheet to compare several systems and trying to figure out the 1, 5, 10, 15, 20, 25, 30 year savings and right now for the system I'm thinking about, my formula puts me at +$85,712 in 30 years. Maybe that's correct... but it doesn't feel like it. Especially because the same formula puts the number at +$35,235 at 20 years, which seems more correct to me, but I can't figure out what's going wrong.
So here are my numbers.
Production: The .995 is a 0.5% decrease in efficiency each year.
11213 kWh * .995^30 * 30 = 289,425 kWh
Consumption: This is based on my current average year kWh comsumption
10236 kWh * 30 = 307,080 kWh
Difference: Production - Consumption
289,425 - 307,080 = -17,655 kWh
Savings: The .1235 is my current effective monthly charge per kWh. The 1.035 accounts for a yearly 3.5% increase in energy cost.
(202868)(.1235)(1.035)^30 = $100,326
Subtract out paying for the difference
$100,326 - (17,655)(.1235)(1.035)^30 = $94,214
Subtract out the cost of the system
$94,214 - $14,622 = $79,592
I don't see what's wrong with doing the math this way, but everything I've read online says I should be at about half this at 30 years.
EDIT: I just realized that I get a different number than my spreadsheet when I manually calculate it using the exact same formula. I don't get it. I came up with $79,592 but my spreadsheet says $85,712.
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