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Solar modules deployed in France in 1992 still provide 75.9% of original output power
(www.pv-magazine.com)
This is a most excellent place for technology news and articles.
Also I don't think we've really run out of suitable space to install new panels. If that ever happens it might be worthwhile to replace them, but as you said we can otherwise just run them alongside new installations until they break or maintenance costs surpass their profits
A significant portion of the cost of solar, is the installation costs. There’s not much you can do about how much work goes into installing panels, and this will get more expensive over time as wages rise. A more efficient panel means less to install, bringing down that installation cost
I don’t yet have solar panels so am not sure what they’d do, but at least half of my south facing roof is always shaded. Is the remaining half the roof sufficient space? I don’t know, but panel size is critical.