YOU'RE NOT SUPPOSED TO HAVE FIGURED THAT OUT.
(By the way, ever think of all that cadminum and lead they're putting in those lead-acid batteries they're storing in the garage? Then having to recycle/dump in the landfill when the batteries die after 3 years: at $80.00-100.00 per battery, and around 40 lbs of lead and acid and plastic, it adds up to a lot of material for "free" power part of the time.)
The PV panels themselves have some 'nasties' as well and if the 'pay-back' is 8-13 years, (I wonder is they use a present-value calculation in figuring that) I would suspect that they would have degraded quite a bit over that time.
Unless it's a spot remote from the grid, I just don't see photo electric making much sense. Solar water heating would be a much better bang for the buck I would think.