I live in Sunny Arizona and has owned solar panels for years (full cash purchase) for 2 of my primary homes. The current I live in has solar panels (13.3kw) + 2 Tesla batteries installed back in 2020, and has seen remarkable results and ROI. We drive two Tesla EVs, so if we count in the $ delta we saved from gas $ to charging at home, our projected ROI on the total system (~$40k post rebate) is about 8 years. Our electricity bill went from $4100 / year to $833/year (with added 2 EVs)
With that being said, to answer the question of ROI, it is SO dependent on your usage and local pricing scheme, and you really have to do your own homework to dial in the right system set up to achieve the optimal results. For example, in AZ, SRP offers "net metering" as well as super low pricing if you are on the "customer generation plan", therefore if the system is dialed in to eliminate all on-peak usage using battery, the saving is huge comparing to choosing a different SRP plan or choosing a different system set up (such as solar panel only).
I've considered putting solar panels on my investment properties, however, I have not pulled the trigger yet. Largely is because renters are not going to watch their electricity usage patterns, which makes it is hard to dial in the right system config to anticipate the load. This means, you will either over purchase the system (too many panels, batteries, etc) or under estimate, both making the outcome not optimal.
In conclusion, I think a general assumption of ~15 years ROI is reasonable, and I believe if you really dial in the usage and match the system config to optimize, you could drive down to a 8 year ROI. Given the life span of the system, it would still be worth it.
Also, never listen to the sales people on their saving projection, I've yet see someone who does a good job of data analytics to truly project it correctly. They take your electricity bill for the year and do a straight line calculation which is almost always wrong because your electrical company charges vary by the hour, and solar production vary by the hour / season etc. You need to download your electricity usage by the hour by the day, overlay the solar production projection by the hour by the day, and truly see what can be offset.
Iris