said by dogma:
The OP invest $13,000 up front.
The OP has an average electricity expense of $140/Month.
The OP's Solar system zero's out his monthly electricity bill.
[...] That's a 13% IRR (Internal Rate of Return).
The assumptions are wrong.
OP invests 13,000 upfront.
The system produces 7000kWh/year. Based on a $0.11/kWh current rate that's $770/year. A bit shy of 6% return rate.
Take it even further... Start with a $0.15/kWh rate. It still takes 13 years to offset the initial investment.
If you compound the returns and take into account the real life loss in capacity, it's only a 2.5% annual rate. Add the inflation and it's still roughly between the return rate of treasury bonds and the return rate of an ultra-conservative, low risk portfolio.