I wonder if SCE actually pays customers for excess generation.  4KW is way more than I need, even with 1 electric car, assuming 8 hours of generation a day.  I assume there's some basic monthly grid fee even if you generate more than you consume.  Ignoring this, it's still a 8 year payoff for me. though I guess electricity will only go up in price over time.

There's still the question of how do you replace your roofing shingles every 30 years and still keep everything working and warrantied, and how much extra that will cost.  Maybe just replace the whole system every 30 years along with the roof.

4KW is a small system. I am doubtful you have anywhere near 100% offset with an electric car if you had a system that size. California probably averages 5 hours of sun a day throughout the year. That's only 7300 kWh (not accounting for losses) which is not that much for a house without an electric car. We barely use air condition and if you remove our 2 electric cars from the equation we're probably averaging just around 7000 kWh annually.