Shade level threshold where it makes sense to switch to microinverters

We have switched mainly to Enphase for residential installs, due to Rapid Shutdown Device requirements  (and poor SolarEdge reliability). However, we are looking at mixing in some SMA + APSmart RSDs. I was wondering if you have an approximate shade percentage threshold where you go to microinverters? I'm thinking maybe around 12% shade for systems less than 5 kW, maybe 18% for larger systems? Interested in others thoughts. I should add that Tigo has caused problems with Fronius' Arc Fault protection algorithms. I assume SMA has had problems, also, since they are dumping Tigo. Thanks!


  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    I am always a bit worried about partially shaded solar panels... Typically they have 2 or 3 parallel strings of cells, each with a "bypass" diode to allow current to "go around" dark cells (in dark, cells go "high resistance"--Diode typically keeps the "reverse voltage" across a dark cell to something like 12-18 volts max--Higher reverse voltage will cause cells to fail).

    And from what I have seen (no expert), the bypass diodes have little to no heat sinking, and if you put 2-4+ amps through the diode (mutliple panels in a series string), we have had a few bypass diodes fail (diodes usually fail open, but they can fail shorted too).

    Older panels, you could go into the J-Box on the rear, and test/replace bad diodes. Newer panels with sealed J-Boxes, not so much.

    For my 2 cents--I suggest that there should be really no shading in the middle of the day (something like 9am-3pm or so)--To prevent series strings of solar panels having bypass diode failures (localized overheating).

    Not sure how much of an issue this is (shading and failed diodes)--Perhaps some folks here with more experience than I can add there observations.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    The other thing to think about is how much shading "kills" a string or array...

    If you have a Low Voltage array (such as a 100 Vmp-array common with off grid MPPT battery chargers)--A partially shaded array with 3x 30 volt Vmp panels in series--The Vmp=90 volt array with one panel shaded ends up with that string somewhere around 30+30+15 volts or so (for shaded panel) with 75 volts (75/90=0.83 or ~17% mismatch) total shaded Vmp-string... Generally, that one string will not supply any useful current when paralleled with the other 90 volt strings (5% to 10% mismatch is typical rule of thumb).

    If you have 15x 30 volt panels in series for a Vmp-array ~450 Volts... One partial or total shaded panel will drop the one shaded string to ~435 rro 430 Volts Vmp-shaded panel... That is (420/450=0.93) or ~7% mismatch... And while that string will have a loss 10% or or so for that string, that is not a loss of that entire string's with of production.

    There are lots of variables here (different Vmp panels, different bypass diode wiring on panel, shading "one cell" vs shade (line of shade) across entire width or length of panel), so coming up with a "useful" rule of thumb is probably not possible/helpful.

    Being conservative, I suggest any partial shading on one panel will reduce that one string's current by 1/2 to 100% when paralleled with other strings.

    And my original concern--Possible overheating/failure of bypass diodes if under full sun for hours a day (such as a panel snugged up against a vent pipe or chimney). If the shading is outside of "prime harvest time", the loss of power and possible overheating of bypass diodes is much less of a concern (at least for me).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,970 ✭✭✭
    For NEC2017 and newer installations, it really make sense to just stick with Enphase. Since you have to meet the code for voltages between panels anyways, there is no cost savings using a string setup, or that's my take on the subject. Enphase from an installer point of view is also simpler to design, quote and support, it is lego blocks like which is a good thing. I'm in NEC2014 and have true old-school string inverter(s), but this isn't an option for most as the codes updates and adoptions role on.
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