Testing Panel Output (Used Panels)

[Sunking]

What is the standard means of testing a panel? Resistive load at rated output?

Using a environmentally controlled lab using a xenon strobe light with an intensity of 1000 watts per square meter with controlled at 25 degree Celsius, and 20% humidity. You cannot test them. All you an do is test Voc and Isc.

Panels are current sources and current varies with direct proportion of Solar irradiance input. You cannot use a fixed resistive load, it is a moving target. Only way you could use a fixed resistance is to control the Solar irradiance input of a calibrated source. That can only be done in a lab.

[posplayr]

Have you read this?

OK your link suggests short circuit load testing rather than rated load tests. What happens with a thin film panel? I assume they have less internal resistance and can not handle the short?

[posplayr]

Ideally you would need to connect the panels one at a time either to a properly chosen resistive load or an MPPT device to find out approximately what the Vmp and Imp are.

As an EE I understand internal impedance;, so my question is whether there panels follow a idealized Thevenin equivalent model (ideal voltage source in series with an internal impedance) around rated load.

What is the standard means of testing a panel? Resistive load at rated output?

To the OP, I don't think this is really thread jacking because the fundamental problem with your measurements is that they are all "unloaded". So the question becomes what is "a properly chosen resistive load"?

Is it as simple as R_load = Vmpp/Impp=41.0/5.49=7.47 ohms.

Rated Voltage Vmpp 41.0 V
Rated Current Impp 5.49 A

Standard sizes are 10 ohms so you can get four of these and bolt them in Parallel/Series and keep the same 10 ohms

http://www.ebay.com/itm/Mini-10-Ohm-100W-Watt-Power-Wirewound-Resistor-Metal-Aluminum-Shell-Case-/251958036841?pt=LH_DefaultDomain_0&hash=item3aa9de8969

If you fiddle you can find something closer to 7.5 ohms but probably not really necessary unless you are trying to stress the panel beyond rated load.

[Sunking]

Have you read this?

[oilerlord]

Just how are you measuring those numbers? By connecting to and MPPT GTI or CC?

The measurements were done with a multimeter - contacting the +/- leads from the panels to the leads on meter. Yes, it's a hot day (where i live), 86F, and the panels were very hot after sitting in the sun for 30 minutes. Initially, (at ambient temperature) they read nearly 47 volts but as they heated up from the sun; they leveled out to just under 43. I only connected them for a couple of seconds each time because I didn't want to risk damaging them.

It's also quite common for our temperatures to reach -20C (-4F) in winter, yet have clear sunny skies. For that reason, the ABB inverters were the only micro option that could handle the low temperatures / high voltage requirements for the permit.

OK, so what you measured for current was Isc. You cannot use that current in calculating panel power. You need to decrease that proportionally to get Imp instead. That would be about 5.37A.

And you measured Voc instead of Vmp, so at that temperature Vmp will be roughly your measured Voc times the nominal Vmp and divided by the nominal Voc.
That is 42.95 times 41.0, divided by 48.5. Roughly 36V.

36V times 5.4A gives you 196 watts, real world.

[inetdog]

It is easy to measure Voc and Isc with a multimeter and IF you have a good idea what the orientation and the insolation are that can be used for comparison to manufacturer specs.

But there are failure modes (weak cells, high resistance interconnects, blown bypass diodes, etc.) that will not show up at all in those simple tests.
Ideally you would need to connect the panels one at a time either to a properly chosen resistive load or an MPPT device to find out approximately what the Vmp and Imp are.

42.95 Volts X 5.84 Amps = 250.82 Watts

Just how are you measuring those numbers? By connecting to and MPPT GTI or CC?
The voltage looks like it could either be Vmp at STC or Voc for a very hot panel.
The current is higher than Imp and close to Isc, which leads me to thing that you are actually measuring Isc. That is not useful in testing the panels. You could get Isc from a panel 2/3s of which is bad and the bypass diodes are allowing you to see the Isc of the remaining 1/3 of the panel.

[oilerlord]

Hello everyone,

My panels are going up on the roof at the end of July; a total of 41 SunPower SPR-225's. I bought them used, and have put a multimeter to them to make sure they all work (which they do). I aligned them more or less at the roof pitch angle, and tested them at an ambient temperature of 30C (86 degrees), in full sun, and after 30 minutes so they reached max temperature. Generally, a sample of 5 yielded the following results:

42.95 Volts X 5.84 Amps = 250.82 Watts

I did the same test a couple of months ago on a cool day in full sun which yielded over 260 Watts. The panels are rated at 225. My installer says that the high number that I'm getting is somewhat meaningless, and not to expect anywhere near those numbers. The ABB micro inverters I'm installing are rated at 96% efficiency, and the cable runs are quite short. In full sun, at a temperature of 20C, is it reasonable to assume that the panels have the ability to produce over 240 watts of usable power? Here are the factory specs:

Electrical Data
Measured at Standard Test Conditions (STC): irradiance of 1000W/m², AM 1.5, and cell temperature 25° C
Peak Power (+5/3%) Pmax 225 W
Efficiency η 18.1 %
Rated Voltage Vmpp 41.0 V
Rated Current Impp 5.49 A
Open Current Voltage Voc 48.5 V
Short Circuit Current Isc 5.97 A
Maximum System Voltage UL 600 V
Temperature Coefficients Power (P) -0.38% / K
Voltage (Voc) -132.5mV / K
Current (Isc) 3.5mA / K

I was wondering if anyone else put a multimeter on their panels before their installation, and got similar results above factory specs and/or know the relation between testing on a meter vs actual performance. Thanks for sharing.