FindSolar.com [1], operated by solar industry interests and the DOE, has a handy Solar Estimator [2] that calculates an estimated cost for a home solar PV system and provides a list of contractors in your area. It uses some curious formulas that I don't quite understand to calculate positive ROI, but it provides some interesting insights.

Some manufacturers also have calculators, including BP Solar USA [3], Sharp Electronics [4], and Kyocera Solar [5]. And the PVWatts.org calculator [6] estimates how much solar power you can generate for any given U.S. location based on a number of variables.

Where I live in Tennessee is listed at the high end of "good" on the "solar scale" of available solar energy. But a 3 or 4 kW system that costs $40K or so will only generate about $35 per month worth of electricity. That's a payback of about 95 years!

(Most of the calculators reduce the payback period based on tax deductions if you finance the system with a home equity loan. I don't understand how that works. It seems like the additional interest v. paying cash would make the system cost more over its life, not less. I'm not very good at math, though, so maybe someone can explain that?)

Folks living in Florida (where we lived for a while) can get a much better deal, because a) they get slightly more solar radiation, but more important b) they have a state rebate incentive that will pay for half the system. Plus they require utilities to provide "net metering" (meaning you can sell excess power back to the grid). The payback period is reduced by half or more, but it's still a stretch.

Conclusion: While good for the environment (assuming "green" manufacturing practices), residential solar PV systems are still too expensive and not yet cost effective. Expect this to change, however, as R&D produces better and cheaper solar panels and manufacturing ramps up.