This is why solar power will quickly dominate soon, IMO.
this is why Google is funding it -- getting an 11% return is great if you have giant piles of cash sitting around doing nothing. Those margins become substantially thinner if your cost of capital is 7%, 8%, 12%, etc.
edit to add: BTW, there's some great research that suggests when people "do the math" and figure out how much energy-efficiency/renewable energy technology will save, that they usually require ROI's of 50-100% -- i.e., "if your fluorescent bulbs/solar panels/low-flow shower head won't pay for itself within a year, i won't bother". It's sad, but it's a real obstacle.
I'd guess this is largely caused by our psychological tendency to overly discount future gains combined with loss aversion (people are used to things failing after a year, so if it hasn't paid for itself by then, it never will (example: I just had a 4-year fluorescent bulb I installed one year ago fail, about a month after I threw away the packaging with the warranty info on it)).
https://secure.wikimedia.org/wikipedia/en/wiki/Hyperbolic_di...
To clarify, as your total cost including everything $15k or $30k, I see both numbers in your post, and was that the actual prices, or were there subsidies as well?
More notes on the whole install
http://benjamin-meyer-home.blogspot.com/2011/08/solar-panels...
http://benjamin-meyer-home.blogspot.com/2011/08/solar-panel-...
Edit:
If I was going to retire soon and needed to start moving investments to a more predictive safe place to put some cash getting panels seems a win all around and removes one more unknown bill from my life. But again it is only ~10-20K of a retirement fund, but hey every bit helps.