Fair enough, I read a university study ~ 1.5 years ago that panels lost about 2% in the first year, and leveled off to about 0.5% annual degradation by year ten. It could have been an old study, so could not be up with current technology.
Either way, as said it wouldn't surprise me to see average household usage at +50% within 5-10 years. I doubt a 13KW is massively overbuilt if selling it in that time frame.
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