The important point they should ask about is my probability-adjusted return on investment.
Correct.
If you want to be even more correct :-) you should estimate your IRR (internal rate of return) and compare it with your opportunity costs for the money invested.
Yes, good point. It took me a while to figure out by myself the best way I should be calculating my rate of return on my variable-return investments, before discovering this in Excel.
But in this case, the panels produce (hopefully) a pretty constant annual amount of electricity, and the price I get is a fixed amount, so it seems that calculating IRR is easy.
As long as we are the topic, maybe the smart folks here can explain, mathematically, why the summation formula for IRR does not admit a closed-form solution? I asked on Quant StackExchange and didn't get much of an answer.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.