I apologize in advance for another short comment, but your reply leads to another interesting point: should entities (individual, companies) be using payback period as a criteria for investments? Everything I learned in b-school pointed us towards using NPV as the best metric, but many entities still use IRR or payback period. I'll try to follow-up tomorrow with some numeric examples so you and everyone else can see the implications of various methods. But the stuff I saw from calculations in France (the most generous market for building-integrated photovoltaic installations) was on the order of 10 years.

Your 6-year payback seems a bit optimistic but I last looked into this stuff 8 months ago and things may have changed.