Over the years I’ve read enough citations that I’ve accepted it on faith that solar panel performance degrades about 2% in the first year and then 1% every year thereafter. Today I had a chance to test that.
Yesterday afternoon I hauled 6 new solar panels up onto my roof and wired them up. Today they’re producing so I spot checked against the production of my existing 290W panels from 2016. The rule of thumb above plus math suggests that those 8 year old panels have degraded about 9% and likely produce about 290W * 91% = 264W. The new panels are rated at 405W so I’d expect each old panel to produce 65% as much as a new one.
Today is an overcast day, which may affect production differently on the new versus old panels, but when I average the production of 10 old panels to 210W and divide that by the average of the 6 new ones at 320W, I get 65.6%. I find it very satisfying when reality matches theory so closely.