That’s a good question. Setting aside climate change and fossil fuel issues, the general answer is something like when the present value of a new installation, minus installation cost, is greater than the present value of the existing installation. Estimation can’t be precise but after 20 or 25 years, the homeowner will have some idea of the remaining efficiency they’re dealing with. They might know the reliability of their inverters, too.
The replacement installation should be cheaper since initial permitting and electrical hookup is in place, and the roof mount might be reusable.
Tax incentives to modernize old solar installations will factor significantly.
Put another way: how high did your electric bill have to be to justify installing panels in the first place?
I was thinking of industrial installations, but similar issue with residential. I assume that not many roofs are good after 20-30 years, so by the time the solar panel warranty starts to expire, it is time to get a new roof. Might as well upgrade panels at the same time.
I forgot that for residential, in many areas the replacement panels would probably come about like a typical roof replacement: after a storm, subsidized by insurance. That may be the case with industrial/commercial as well, but I’m not familiar with insurance trends there.
The replacement installation should be cheaper since initial permitting and electrical hookup is in place, and the roof mount might be reusable.
Tax incentives to modernize old solar installations will factor significantly.
Put another way: how high did your electric bill have to be to justify installing panels in the first place?