We have had an issue with Mission Solar 410 modules with bad diodes.
Fortunately, I think all of them were on Enphase microinverters, so they are
easy to spot.
That's definitely a benefit of micros. I wonder how many strings are out there
with a bad panel that is undetected.
Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956
Florida Certified Electrical Contractor EC13013208
On Wed, Feb 11, 2026 at 2:05 PM Glenn via RE-wrenches
<[email protected]> wrote:
> We have seen a lot of Qcells with one diode failing and dropping module
> production by 1/3.
> I have a Solmetric curve tracer I am looking to part with...
>
>
> -Glenn
>
>
> On Feb 11, 2026 09:26, Dave Tedeyan via RE-wrenches <[email protected]
> wrenches.org [[email protected]]> wrote:
>
> > I must admit, this is a little embarrassing... this is what I get for trying
> > to work with a head cold. I didn't remember that there was a change order on
> > this one, and the original contract was for a larger system than what
> > eventually got installed. I was looking at the producion estimate from the
> > original larger system, which was around 17MWh/year. The actual production
> > estimate is closer to 14.3MWh, and the system has averaged 13.5MWh over the
> > past 5 years. So, 94% is not nearly as bad as I originally thought, although
> > it is still not good either.
> >
> >
> > The voltages that I am seeing on the Sunny Portal monitoring are still lower
> > than I would expect. I think it is still worth checking out with an IR scan
> > once the snow goes away and the sun comes back here in upstate NY. The
> > recommendations for doing a curve trace would be the best troubleshooting
> > step, although I have not yet invested in one of those. All this still leads
> > me to believe that these Qcell modules may be not quite be performing to
> > spec, but they are not as ridiculously far off as I feared.
> >
> >
> > Cheers,
> > Dave
> >
> > On Tue, Feb 10, 2026 at 8:50 PM Roy Rakobitsch via RE-wrenches
> > <[email protected] [[email protected]]>
> > wrote:
> >
> > > It would be fairly easy to spot bypass diode issues or cell current
> > > imbalance issues with a hand-held IR imager on a decently sunny day. Maybe
> > > time to go on the roof and take a peek?
> > >
> > >
> > >
> > >
> > > Roy Rakobitsch
> > >
> > >
> > > Windsine LLC
> > > 631-514-4166
> > >
> > > On Tue, Feb 10, 2026, 18:25 Jay via RE-wrenches <[email protected]
> > > wrenches.org [[email protected]]> wrote:
> > >
> > > > Looking at it from performance ie watts, how far off does it appear to
> > > > be?
> > > >
> > > >
> > > > Jay
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > > On Feb 10, 2026, at 2:07 PM, Dave Tedeyan via RE-wrenches
> > > > > <[email protected] [[email protected]]
> > > > > > wrote:
> > > >
> > > > >
> > > > > Hi All,
> > > > >
> > > > >
> > > > > I was just reviewing a site installed about 5 years ago and am finding
> > > > > it is not doing nearly as well as I would have expected. Most of our
> > > > > sites produce relatively close to our projections, but this one is
> > > > > pretty far off. There are 3 strings of 12 Q.PEAK DUO BLK-G6+ modules.
> > > > > Two strings go to a 7.7kw inverter and one string goes to a 3.8kw
> > > > > inverter - both of which are the -41 series SMA inverters. There are
> > > > > Tigo TS4-R-F rapid shutdown devices on all the modules.
> > > > >
> > > > >
> > > > > Vmp for 12 of these panels should be about 407V at STC. At 50C (which
> > > > > is really hot for upstate NY), the Vmp should be around 380V. On a
> > > > > perfect summer day, I am seeing Vmp of around 320V. On a perfect
> > > > > spring day (cooler, with some power clipping, I am seeing Vmp of
> > > > > around 355V. And on a perfect winter day (when there was no snow) Vmp
> > > > > is around 365V. All three strings are pretty consistent with each
> > > > > other, so I don't think that there are specific installation issues
> > > > > with any panels, unless we managed to forget to wire in two panels in
> > > > > all three strings which is extremely unlikely.
> > > > >
> > > > >
> > > > > So, I can only think of two things that might be at play here:
> > > > > 1. Maybe these modules are just crap, and are not at all performing to
> > > > > spec.
> > > > > 2. Maybe the Tigo devices are causing issues.
> > > > >
> > > > >
> > > > > Has anyone out there experienced production issues like this?
> > > > > Everything appears normal, it is on an open rooftop, and there are not
> > > > > things like partial shading, weeds, bird poop, or other soiling
> > > > > affecting the modules. I don't have another site with the same modules
> > > > > that also has good monitoring data, but maybe some of you do!
> > > > >
> > > > >
> > > > > Thanks for the consideration!
> > > > > Cheers,
> > > > > Dave
> > > > > --
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > [https://ci3.googleusercontent.com/mail-sig/AIorK4zoZ4JdaR79CFcLmy74s-VRVaVovMEuRmw5pzk3XBT09h8ZNlFbMgJ4yHW2Uxf91_2codJGB4DALADN?trnonsuspmrk=1]
> > > > >
> > > > > Dave Tedeyan, P.E.
> > > > > Owner | Sungineer Solar
> > > > > p: he | him | his
> > > > > a: 1653 Slaterville Rd. | Ithaca, NY 14850
> > > > > w: www.sungineersolar.com [http://www.sungineersolar.com/]
> > > > > c: (607) 270-0370
> > > > >
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