They use 20 years and in the past I have heard that the average time for home ownership is typically 7 years. I don't know the true figure today (wasn't able to find it when I just searched) but for most people a 20 year payback is way to long and I would imagine this certainly contributed to lack of adoption of solar in addition to other factors obviously.

Edit: Note this: https://www.google.com/get/sunroof/faq/#financial-considerat...