Do they eventually peter out, or become less powerful? Or can they be expected to last decades?
According to this webside http://www.otherpower.com/otherpower_solar_used.html
“Nobody really knows what the lifespan of a solar panel is, since many of the very first panels are still in operation.”
The problem is that it varies. A lot. And many of the specifics may be due to what kind of wear (read: environment) is around it and the construction. Kind of like windows. Some poeple have to replace theirs if they get a bad batch. Other people keep theirs for decades.
I used to do some maintenance on navigational aids that used solar panels for charging the 12v batteries. We used 10, 20 and 35 watt panels. IIRC, the only problem we ever encountered with the panels was an occasional bad diode. We could take those out of the field and just replace the diode for future use. Otherwise, unless the panel was actually cracked or damaged, we never changed them out. To be honest, we really had no idea how long some of those panels were in the field; not all of the record keeping was accurate. I certainly don’t recall any policy of changing panels after x number of years, which is the case with batteries.