Testing a PV panel tester, you want same illumination for each test. If you don't have the lamp used for production testing, use the sun instead, intensity varies and without measuring you wouldn't know. Also, as panels heat up, their output changes. Read one panel over time and observe how much it changes - whatever variation is observed, don't expect two testers to differ less than that.
Need to switch panel between the testers several times and average multiple readings from each tester. Or put a different panel on each tester, read each one. Then exchange panels between testers and read again. If one panel consistently reads lower on all testers, it produces less. If one tester consistently reads low with all panels, its measurement is lower. Assuming several similar panels used, average of all readings by a given tester could be compared to the other testers.
A hand-held tester can't measure panel wattage under load, because it can't dissipate hundreds of watts. Same goes for automotive starting battery testers for cranking amps. It can draw "I" and measure "V" for a brief period; maybe that would read high because interconnect doesn't heat up; more likely low because a 100W 20% PV panel would still be getting 100W of heating, rather than 100W - 20% = 80W of heating.
I went through three Optima batteries in my Civic, each lasting about 20 months. The car would start eagerly time after time during the day, then one time nothing. Open circuit, voltage was around 12.5V. Headlights on, around 6V and a dim glow. Auto store battery tester showed 60 CCA (though obviously not "cold" cranking amps on a balmy day.) If the battery had actually been able to deliver 60A at 7.5V cranking voltage, 400W, it would have light the headlights just fine and held 12V. The switching load battery tester was not accurate. (I figure something inside the battery broke lose, leaving the tiniest amount of lead as an electrode.)
