They have never failed to replace a completely dead micro. The fuzzy part of the warranty language is what they consider to be "defective". Here's the wording from the warranty:
"During the Warranty Period, Enphase will, at its option, repair or replace the Defective Product free of charge, provided that Enphase through inspection establishes the existence of a defect that is covered by the Limited Warranty. Enphase will, at its option, use new and/or reconditioned parts in repairing or replacing the Defective Product. Enphase reserves the right to use parts or products of original or improved design in the repair or replacement of Defective Product."
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There's nothing in the warranty, which defines what "defective" means. Obviously a micro which produces no power is defective, but one which cuts power back and runs about 8% less than its neighbors? Apparently Enphase doesn't consider that "defective".
There's nothing in the language of the warranty which says Enphase is obligated to compensate customers for lost production and nothing says they are obligated to monitor customer systems and take action on their own to correct "defects".
The m190 and d380 models have been shown to be very unreliable. I'm not the only one having problems with m190s. I fully expect a majority of my m190s will be replaced in the next five years and none will survive the entire 15-year warranty period.
I've read that the m215 and later models are more robust, so I wouldn't write off Enphase completely. It's just that a lot of us m190 owners got bad batches.