I agree with your math but disagree with your conclusion :)

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.