Yes it's 10 years for me as well. Most of that is because we have a 30% efficiency hit because we didn't want to cut down a tree in our backyard that shades it for 2 hours a day during about 6 months of the year. Our system is also larger than our current usage, because we plan to use more (electric car, more kids, etc).

Another benefit to think about is you've effectively locked in your electricity rate. With some companies that might not be a big thing, but here in CA PG&E has been relentless with their rising kwh costs.