Hmmm, that seems broken. I installed 5.2KW of panels on the roof of my Bay Area home in 2002, at the time panels were a lot more expensive but the subsidies were better too, I ended up paying just about $20,000 for the complete system (after rebates and with all parts, labor etc). The system is grid tied (meaning it feeds back into the grid) and we switched our billing to a watt for watt basis (not time of use as my kids were being home schooled and we were in the house 24/7). My electricity bill is now annual (the net power usage is computed from the Anniversary date, and any power I've used in excess of my generation is charged at a flat fee). My typical annual power cost is < $200 and occasionally less than $100. There is zero maintenance other than rinsing the panels off twice a year, and trimming the trees to avoid shade.

By calculating my what my electricity bill would have been vs what it was, they have saved me more than their cost (at some point I should go back again and get exact numbers).

Its entirely unclear how much "additional" value they give the resale value of my house. It is clearly some, but I don't have a good way of knowing if it is $20,000 or more.

At the time we financed the purchase as part of a refinancing of the house, so we did know the interest cost for the money, we also went through the great Recession so in traditional financial analysis (the 'do nothing' option) we would have predicted our money would have had a better return than it actually did, although that didn't really change the math all that much.

So I can't see how you don't get it paid off in < 10 years these days. At this point I'm thinking of adding another string to offset the addition of an electric car.