For solar panels the total projected income is R14 500 for nominal annual returns of 17% p.a. and an IRR of around 10% p.a. over the 20-year term.
Year 1 nominal returns: 8%
Year 20 nominal returns: 32%
Monthly income reinvested at 7% p.a. = IRR of 16%
For solar, there is a guaranteed buy-back at the end of the 20-year period of R1 000 per panel and this is included in the calculation of your IRR.