ExtremeTech has published a Q&A about solar power. Here's a snip:
What's the payoff period?
It depends on several things. If we freeze our current usage in time, and continue with our current billing model and the amount of sunlight annually follows historical trends, the payback is roughly ten years.
But nothing remains the same. For example, when my two daughters headed-off to summer camp this year, we generated considerably less laundry and—dirty dishes—which meant the dishwasher and dryer ran less frequently. Plus, their computers were not running during that time.