I suspect your numbers may be off. A $15 000 investment that produced $150 in nominal cash flows a month for 20 years (low end) would produce an internal rate of return of 9.1%.

Yet solar panels do not produce nominal cash flows, they make energy - a real good. Energy inflation for urban consumers in the U.S. has averaged, month-to-month annualised, 6.7% over the past 10 years. This raises your IRR to 16.3%. The closest maturity-matched non-callable bond is TXU's 2034 6.55 yielding 15.2% to maturity and rated CC by Fitch.

Even allowing you to capture only 1/2 of the energy inflation, your return is 12.7%. A whole point south of Venezuela. Then again, a warranty from a solar company may be worth as much as a dollar lent to Caracas...