Solar Panel Tester Elejoy WS400A - Readings do NOT match simple load test

[awardblvr]

I picked up one of these cheapie PV Solar panel testers. It is showing the MPT of my 15-year old Sunpower SPR-200 (180W @ STC) panels at 179W... But, a 12 ohm (200W) resistive load voltage / current measurement/power calculation yields 133 watts.

Can anyone advise if the MPT reading on the WS400A should be trusted over my standard P=EI measured/calculated value?

[bcroe]

Your 12 ohm resistor is drawing 3.33A at 40V. That may be 133 watts, but most likely
not at the panels MPPT operating point, as used by the Elejoy. Bruce Roe

[awardblvr]

Ahhh You're right...

OK. So I bought a Kunkin KP184 Electronic load and wrote some python code to talk to it and try to figure out MPPT. (https://github.com/awardblvr/pv_mppt_test)

It starts at a high impedance and works down measuring voltage and current along the way and calculating total watts: (Details in readme of the above github.)

Here's one csv blob of the data...

Is this valid?

Volts,volts,amps,watts,state,mode_str,panelSN,resi stance,timestamp
43.911,43.911,0.0,0.0,0,CR,C03J00093546,100000,202 10913_113602.61
42.951,42.951,1.072,46.04,1,CR,C03J00093546,40.0,2 0210913_113604.01
42.934,42.934,1.085,46.58,1,CR,C03J00093546,39.5,2 0210913_113605.11
42.919,42.919,1.099,47.17,1,CR,C03J00093546,39.0,2 0210913_113606.22
42.9,42.9,1.113,47.75,1,CR,C03J00093546,38.5,20210 913_113607.32
42.881,42.881,1.127,48.33,1,CR,C03J00093546,38.0,2 0210913_113608.43
42.862,42.862,1.141,48.91,1,CR,C03J00093546,37.5,2 0210913_113609.53
42.841,42.841,1.156,49.52,1,CR,C03J00093546,37.0,2 0210913_113610.64
42.821,42.821,1.172,50.19,1,CR,C03J00093546,36.5,2 0210913_113611.73
42.798,42.798,1.186,50.76,1,CR,C03J00093546,36.0,2 0210913_113612.84
42.778,42.778,1.203,51.46,1,CR,C03J00093546,35.5,2 0210913_113613.95
42.756,42.756,1.22,52.16,1,CR,C03J00093546,35.0,20 210913_113615.05
42.733,42.733,1.237,52.86,1,CR,C03J00093546,34.5,2 0210913_113616.15
42.711,42.711,1.255,53.6,1,CR,C03J00093546,34.0,20 210913_113617.25
42.688,42.688,1.273,54.34,1,CR,C03J00093546,33.5,2 0210913_113618.35
42.668,42.668,1.291,55.08,1,CR,C03J00093546,33.0,2 0210913_113619.46
42.642,42.642,1.311,55.9,1,CR,C03J00093546,32.5,20 210913_113620.55
42.621,42.621,1.33,56.69,1,CR,C03J00093546,32.0,20 210913_113621.66
42.594,42.594,1.351,57.54,1,CR,C03J00093546,31.5,2 0210913_113622.76
42.57,42.57,1.371,58.36,1,CR,C03J00093546,31.0,202 10913_113623.87
42.547,42.547,1.393,59.27,1,CR,C03J00093546,30.5,2 0210913_113624.97
42.522,42.522,1.416,60.21,1,CR,C03J00093546,30.0,2 0210913_113626.07
42.494,42.494,1.439,61.15,1,CR,C03J00093546,29.5,2 0210913_113627.17
42.465,42.465,1.463,62.13,1,CR,C03J00093546,29.0,2 0210913_113628.28
42.436,42.436,1.487,63.1,1,CR,C03J00093546,28.5,20 210913_113629.38
42.403,42.403,1.513,64.16,1,CR,C03J00093546,28.0,2 0210913_113630.48
42.371,42.371,1.539,65.21,1,CR,C03J00093546,27.5,2 0210913_113631.58
42.338,42.338,1.566,66.3,1,CR,C03J00093546,27.0,20 210913_113632.68
42.302,42.302,1.595,67.47,1,CR,C03J00093546,26.5,2 0210913_113633.79
42.264,42.264,1.624,68.64,1,CR,C03J00093546,26.0,2 0210913_113634.88
42.231,42.231,1.654,69.85,1,CR,C03J00093546,25.5,2 0210913_113635.98
42.192,42.192,1.686,71.14,1,CR,C03J00093546,25.0,2 0210913_113637.08
42.15,42.15,1.719,72.46,1,CR,C03J00093546,24.5,202 10913_113638.18
42.109,42.109,1.754,73.86,1,CR,C03J00093546,24.0,2 0210913_113639.29
42.064,42.064,1.789,75.25,1,CR,C03J00093546,23.5,2 0210913_113640.39
42.013,42.013,1.825,76.67,1,CR,C03J00093546,23.0,2 0210913_113641.50
41.974,41.974,1.864,78.24,1,CR,C03J00093546,22.5,2 0210913_113642.60
41.924,41.924,1.904,79.82,1,CR,C03J00093546,22.0,2 0210913_113643.71
41.873,41.873,1.946,81.48,1,CR,C03J00093546,21.5,2 0210913_113644.80
41.815,41.815,1.99,83.21,1,CR,C03J00093546,21.0,20 210913_113645.91
41.758,41.758,2.035,84.98,1,CR,C03J00093546,20.5,2 0210913_113647.01
41.699,41.699,2.083,86.86,1,CR,C03J00093546,20.0,2 0210913_113648.12
41.635,41.635,2.133,88.81,1,CR,C03J00093546,19.5,2 0210913_113649.23
41.573,41.573,2.186,90.88,1,CR,C03J00093546,19.0,2 0210913_113650.33
41.505,41.505,2.241,93.01,1,CR,C03J00093546,18.5,2 0210913_113651.44
41.438,41.438,2.301,95.35,1,CR,C03J00093546,18.0,2 0210913_113652.54
41.352,41.352,2.361,97.63,1,CR,C03J00093546,17.5,2 0210913_113653.65
41.27,41.27,2.427,100.16,1,CR,C03J00093546,17.0,20 210913_113654.74
41.175,41.175,2.495,102.73,1,CR,C03J00093546,16.5, 20210913_113655.84
41.087,41.087,2.567,105.47,1,CR,C03J00093546,16.0, 20210913_113656.94
40.985,40.985,2.643,108.32,1,CR,C03J00093546,15.5, 20210913_113658.05
40.879,40.879,2.724,111.35,1,CR,C03J00093546,15.0, 20210913_113659.15
40.756,40.756,2.81,114.52,1,CR,C03J00093546,14.5,2 0210913_113700.26
40.626,40.626,2.901,117.86,1,CR,C03J00093546,14.0, 20210913_113701.36
40.495,40.495,2.998,121.4,1,CR,C03J00093546,13.5,2 0210913_113702.45
40.334,40.334,3.102,125.12,1,CR,C03J00093546,13.0, 20210913_113703.55
40.158,40.158,3.211,128.95,1,CR,C03J00093546,12.5, 20210913_113704.65
39.969,39.969,3.33,133.1,1,CR,C03J00093546,12.0,20 210913_113705.75
39.76,39.76,3.456,137.41,1,CR,C03J00093546,11.5,20 210913_113706.85
39.524,39.524,3.592,141.97,1,CR,C03J00093546,11.0, 20210913_113707.95
39.244,39.244,3.737,146.65,1,CR,C03J00093546,10.5, 20210913_113709.05
38.942,38.942,3.893,151.6,1,CR,C03J00093546,10.0,2 0210913_113710.14
38.579,38.579,4.059,156.59,1,CR,C03J00093546,9.5,2 0210913_113711.24
38.143,38.143,4.238,161.65,1,CR,C03J00093546,9.0,2 0210913_113712.34
37.616,37.616,4.425,166.45,1,CR,C03J00093546,8.5,2 0210913_113713.44
36.976,36.976,4.622,170.9,1,CR,C03J00093546,8.0,20 210913_113714.55
36.174,36.174,4.823,174.47,1,CR,C03J00093546,7.5,2 0210913_113715.65
35.122,35.122,5.019,176.28,1,CR,C03J00093546,7.0,2 0210913_113716.75
25.824,25.824,3.796,98.03,1,CR,C03J00093546,6.5,20 210913_113717.85
23.299,23.299,3.655,85.16,1,CR,C03J00093546,6.0,20 210913_113718.95
24.089,24.089,3.845,92.62,1,CR,C03J00093546,5.5,20 210913_113720.05
23.245,23.245,3.842,89.31,1,CR,C03J00093546,5.0,20 210913_113721.15
22.052,22.052,3.96,87.33,1,CR,C03J00093546,4.5,202 10913_113722.25

[bcroe]

Looks like you found 176 W at 35V. Bruce Roe

[awardblvr]

Thanks Bruce.. I see that... BUT...in many of these highly technical papers on determining MPPT, they take into account a lot of other things like V_OC and I_SC and even W/M², and panel temperature. (I HAVE all those values also from a Fluke IRR1, and Thermal images (Flir one Pro.) )

Out of 28 Sun Power SPR200 panels, I think 8 are WAY below this. SunPower is saying their "Limited Warranty" (that is now on revision "F") at my panel's age (15.5 years) covers panels more than 20% below the STC rating of 180 Watts. They are, of course, doing everything they can to deny my claim of defective panels and I need to be able to prove categorically that my wattage measurements are valid.

So I am wondering how my wattage measurement might be validated? All the readings were done facing directly at the sun, and many were over 1000 W/M² on the Fluke, (All were over 925 W/M², all were @ temperatures well below 50^oC. (I have exact panel temperatures and detailed thermal images.

[bcroe]

Most panels already have ratings on them. If several identical panels under the same
sun vary too much to be used together, I would declare the weaker defective. That is
reality in the field, without the highly technical papers.

I have a couple 300W class panels that cannot always keep up with the rest. But they
may find service in my battery maintainer service, which takes care of all the batteries
on standby for up to years here, for starting my backup gen set, etc. And the tiny load
internet/com setup. Bruce Roe