My friend invested $32,000 to install the solar panel on the roof of his house.
It produced 5510 kwH in 112 days from 15 June to 4 Oct.
He used 58% of the production (save 0.16 p kwH) and exported 42% (earn 0.14 p kwH) to the grid.
The weighted average is 0.158 p kwH.
His solar panel will produce 5510*365/112 = 18,000 kwH in a year and earn $2,855 a year.
The return on his investment is $2855/32000 = 8.9% per year.
However, the efficiency of the solar panel will deteriorate by 10% a year.
What is his real return if the cost of energy remain the same? If it increase by 3% a year?