The best time to get solar is when a house is built, or a roof is replaced, especially if you can combine the install with a financing event that's going to let you finance the system for 20-30 years with low interest rates. Not only is there a 26% federal tax credit for solar, but many states offer generous incentives/rebates as well. Capture them!
It's fiscally a no brainer. $20k over 30 years at 4% is $95/month. Mortgage rates will only continue to go down (and are already below 4%) due to macro issues, and residential solar install costs also will drop as the scale ramps. These mandates drive down costs, in the same way EV credits helped Tesla reach scale to offer the Model 3 at an affordable price.
Disclaimer: I review system quotes and advise on solar installs for friends, family, and internet strangers, but am not a solar professional (yet; if you're a utility scale developer in need of any role [system design, project management, finance], I'd be interested!).
[1] https://www.solarpowerrocks.com/wp-content/uploads/2016/12/s... (updated 10/23/2018)