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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.
There's a typo in the title. If you go back to the original source (in french), they actually retain 79,5 % of their original efficiency, so even better than the article's title would have you believe.
Careful using the word efficiency there, as it has a different meaning when talking about solar panels - it indicates how much energy the panel can extract from the light hitting it. The best modern panels you can buy are below 25% efficient, and since these are from the 90s they were probably about half that when new.