[fancyketchup]

I don't have a PV system, but I know people who do, so take this with a grain of salt: The default assumptions in that calculator are probably a little wrong.

First I think most people are paying closer to $5 per Watt, and I know someone who just refinanced his house in Long Beach at 3.5%, fixed. For comparison, the calculator assumes $10 per watt, and 8%. If you use halve the installed cost, there is obviously a big effect on the payback period (down to 6.5 years with tax incentives).