Measuring micro-inverter output w/o monitoring

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Is there a standard way to test micro-inverter output on and old system with defunct monitoring?

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
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    In place... You can monitor the current output of the inverter/string of inverters:
    • 1 amp AC * 240 VAC = 240 Watts (single inverter)
    • 10 amps AC * 240 VAC = 2,400 Watts (string of 10 inverters)
    More or less, for a typical solar panel/array--Expect on a good (cool/clear/near solar noon) around 75-77% of rated output (typically "best" output):
    • 2,400 Watt array * 0.77 panel+inverter deratings = 1,848 Watt "best mid-day" output
    • 2,400 Watt array * 0.50 panel+inverter+problems (?) = 1,200 Watt sort of if better--probably mostly OK, if less, probably issues (on otherwise cool/clear/near solar noon day)
    The problem with measuring the current of a 10x inverter array--Your measurement inaccuracies are probably, at best, ~10% error (i.e., hard to say if 10x good inverters on a less than perfect solar day, vs 9x good inverters on an average solar day).

    If you can get to the wires on each panel... You can get a DC Current Clamp Meter and measure the current from the panel. Here are a couple choices to start your search (Note: this are really "AC+DC" current clamp meters):

    https://www.amazon.com/gp/product/B00O1Q2HOQ ("good enough" for our quick testing needs)
    https://www.amazon.com/gp/product/B019CY4FB4 (example of a mid-priced clamp meter)

    Say you have a 240 Pmp panel, and 30 Vmp Volt panel:
    • Imp = 240 Watts / 20 Volts Vmp = 8 amps Imp
    • If over ~4 amps--Probably "fine". If under ~4 amps--May be issues with panel or micro inverter...
    Again, on a cool/clear day near solar noon (no shading of panel, etc.).

    Note that using a Current Clamp meter... You need to clamp just one wire (+, - for DC side, or L1, L2 on 240 VAC side). You cannot just "clamp" the cable exiting a GT inverter... The sum of L1+L2 = 0 Amps (Clamp meters measure the "net current" inside the camp). If you have +/- or L1/L2 current flow--The current is flowing In/Out the two wires, and add up to Zero Amps. To measure current with clamp meter, you need to access an individual power wire. 

    If you had a "pile" of GT Micro Inverters...You could setup a test station with a "known good panel" and connect to the appropriate AC power source (120, 230, 240 VAC) and test each inverter individually (most micro GT inverters are "240 VAC" nominal in North America).

    Is this the answers you were looking for?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ToddF
    ToddF Registered Users Posts: 6 ✭✭
    edited April 2022 #3
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    Thanks BB for taking the time to lay it out in detail, very helpful.  That is about what I was figuring to do...measure current with my clamp meter. But as you noted, it is hard to get solid numbers due to the vagaries of insolation and other derates on an 8-year old system...like module degradation. According to Enphase, their early micros had a test port but I am working on AU Optronic AC modules using a Solarbridge micro that appears to have no ports and do have a single 2-leg AC cable that I cannot test current on. I have two strings of 12 each. I think besides testing AC string amperage and individual module DC amperage, I will also try disconnecting each module to kill it's micro, then re-connect each, one at a time, to read that micro's individual current at the J-box where I should be able to use my clamp meter. Thanks again!