I live in CA. 4 years ago I installed a $70k ($50k after rebates) 8kW solar system on my roof and my garage's roof.
Prior to solar, my original electricity bill peaked at ~$1100/month, more commonly about $600. This is due mainly to my own choices, no doubt, but still that's what we are dealing with - in Summer, the AC is on quite a bit (it's been 106F this last week) and the pool pump needs to run 8 hours a day for good cleaning. There's also the 2 pond pumps which run 24/7 and the reef tank pumps which also run 24/7. Add in a baby (so lots of washing-machine and dryer activity) and it adds up...
After the solar installation, my bill peaks at ~$100, more commonly about $50. This gives me an average saving of ~$8500 per year, and if you divide $55k by $8.5k you get 5.88 years to pay for itself. By some definitions that's 5 years...
In terms of cost, certainly the main issue is that I consume a lot of electricity. I'm happier now that most of that comes gratis from the sun, but also partly it's California's (or at least PG&E's) electricity pricing which ranges in tiers (the below taken from www.pge.com)
$0.1323 ... baseline
$0.1504 ... 101% -> 130% of baseline
$0.3111 ... 131% -> 200% of baseline
$0.3511 ... 200% -> 300% of baseline
$0.3514 ... 301%+
My baseline is set at 7.5 kWh per day (or ~225 kWh/month) , and I consume about 40kWh per day on a "good" (no AC, pool closed) day (pumping water is energy-intensive...). For me at least, the maths works out. When you're looking at individual cases, using average numbers is not such a great idea...