Unless all of the solar panels, the inverters, the wiring, the labor to install it, and the mounting hardware were all free, the electric bill is NOT $0 a month. If you were to amortize the expenses over any side by side time period, I bet the $400-600 a month was actually cheaper. Don't forget, when those panels go tits up, and they will, you will have to get rid of them.
$400-600 a month in electricity @ $0.10 a kWhr is using about 4,000-6,000 kWh each month. To produce 4-6 kWh's worth of electricity you'd have to install enough solar panels to provide between 35,000 to 50,000 watts according to a calculator I found online. At about $1 a watt just for panels, that's a sizeable chunk of change to invest. Installation costs for 250 panels is probably equal to if not more that just the cost of the panels. Take the entire amount of doing this, invest it in a stock index fund, and I'd wager that he'd have been money ahead to just keep paying the electric bill.
