Just a quick note to say that the 54 fourth-generation inverters (M215IGs and M250s) in my system have attained an MTBF over twice that of the 42 original third-generation inverters (M190s) which were finally fully retired in October 2017:
Fourth-generation inverter MTBF: 93 years (with 1 failure)
Third-generation inverter MTBF: 42 years (with 6 failures)
With more device-years on the overall population, the numbers are even more stark:
Fourth-generation inverter MTBF (INCLUDING original M215s): 332 years (with 9 failure)
Fourth-generation inverter MTBF (EXCLUDING original M215s): 471 years (with 1 failures)
Third-generation inverter MTBF: 36 years (with 246 failures)
So far it looks like the fourth-generation Enphase microinverters are achieving an MTBF which is about 10X that of the third-generation inverters. Time will tell if this improvement holds up or not.
Please update the status of the systems you are tracking, including row number from my spreadsheet.