@alberto_cottica The other problem is density. I installed 200W solar panels about 10 years ago, last time I looked you can get at least double that. Assuming the density keeps going up even at a few percent each year, then you might want to swap them out for higher density after a few decades. Of course that doesn't make the older ones useless, they can be moved to another part of the roof, a backyard, a carport, given to a neighbour/friend/community garden, or given away on a buy nothing group