The derate factor for the PV module nameplate DC rating accounts for the accuracy of the manufacturer’s nameplate rating.
Field measurements of PV modules may show that the PV module powers are different than the nameplate rating or that they experienced light-induced degradation upon exposure (even crystalline silicon PV modules typically lose 2% of their initial power before power stabilizes after the first few hours of exposure to sunlight).
A derate factor of 0.95 represents that testing yielded power measurements at STC that were 5% less than the manufacturer’s nameplate rating.
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