On a positive note, I have operated as an electric utility for the last two months, as I was a net seller of around 500 KWHRs of electricity to the grid. Whoo Hooo, I made about $45 over two months! Plugging that number into a calculator for return on investment is pretty depressing.
ROI really has two components, avoided cost plus net sold. For instance, If I generate 50 KWh per day and I only use 30 KWh of it, I have avoided paying for 30 KWH and I get to sell 20 KWh back to the power company. In my case, I pay $0.1070 per KWh and I get paid $0.0524/KWh for the excess. I calculate the ROI using 30*.1070 + 20*.0524 = $4.26. Multiplying by 365 to get it to a yearly rate and dividing by your system cost (actual cost - rebate - tax credits) should give you the ROI. Mine consistently runs in the 10-15% ROI range which is a lot better than any other investment I have made recently.