How to test a solar panel performance

catclaw

I bought a used solar panel. It is labelled as a 120 watt panel. I measured the voltage across the + and - output as 18.0 (typical value) and the amperage varied from 4 to 5 amps. Do these measurements truly show the panel capability? Out of curosity, I measured the voltage across a car interior 12 volt light bulb hooked up to the panel. The bulb was lite and bright white. The voltage across the bulb was 13.4 volts. I measured the bulb's resistance several times and the resistance varied from 0.9 to 3,6 ohms. I'm not confidant about the meter's resistance measurements.

Did I accomplish anything meaningful with these measurements?? If so, what?

Cariboocoot

Re: How to test a solar panel performance

Welcome to the forum.

Good try and you have proved the panel produces some power because it will light the bulb. Getting an exact measurement, however, is nearly impossible in the real world; approximating the Standard Test Conditions (STC) that produce the specifications (Voc, Vmp, Imp, Isc) is not likely.

18 Volts open circuit (Voc) indicates a sub-standard panel: most 100 Watt panels will have a Vmp around 18 and a Voc of over 20. This is the easiest measurement to get as it does not require panel loading or full sun.

Measuring Vmp is practically impossible. Likewise measuring Imp. These are the two that give the panel its Wattage rating: Voltage at maximum power times current at maximum power. They require full illumination of the panel, controlled temperature, and controlled loading. Not easy to come up with.

The Isc on the other hand you can get an idea of by measuring the short-circuit current through a DMM capable of at least 10 Amps DC. It needs to be in full, direct sun for this. Between that and the Voc you can approximate the panel specifications; Voc will be higher than Vmp and Isc will be higher than Imp.

As it is you demonstrated the panel produced 13.4 Volts at 5 Amps or 67 Watts. The 18 Voc reading looks like this may be a 16 Vmp panel, or it may be defective. Or your meter may be; all measurements are only as accurate as what they are measured with.

PNjunction

Re: How to test a solar panel performance

catclaw wrote: »

Did I accomplish anything meaningful with these measurements?? If so, what?

As Cariboocoot noted, real-world performance depends upon a lot of variables.

One of these is the fact that these panels are tested under perfect conditions, usually flash-tested in the factory.

I would perform this test again, but try to get the best day possible, do the test near the middle of the solar-insolation period (around noon), panel aligned towards the sun with a proper angle. You didn't mention when this test was taken. Morning, noon, afternoon?

The power formula can get you in the ballpark - assuming your meter is on the ball too.

120W / 18V = 6.66A.

So if you tested near noon, and had steady sun not blocked by haze, clouds or smog, and the meter was accurate, you should get close to 6.6A *theoretically* in short-circuit measurement. If it won't get past 4-5A under these conditions, perhaps that used panel is deteriorating.

jonr

Re: How to test a solar panel performance

In looking at a graph for one particular panel, I'd say that 70% of Voc x Isc might be a rough idea of what an MPPT controller would put out in the same sun conditions.

catclaw

Re: How to test a solar panel performance

Thanks a lot for everyones input. Thanks. The panel was repaired before I got it. Even though it is "wounded" it will do what I need. I am using the panel in a barn lighting system. The lights are leds. I don't need a lot of power and the 110 volt wiring was not up to any safe standard. The former owner lost a cow to a short and I didn't want to worry about a shock or short.

Question: Help me understand "voltage drop". The light bulb leads show 13.4 volts across the bulb on the vom meter when there is 18.0 volts measured out from the solar panel. I have a brick wall stopping me from understanding what that means.

This hobby is a education and provides much entertainment to a retired guy. Thanx again

vtmaps

Re: How to test a solar panel performance

catclaw wrote: »

Question: Help me understand "voltage drop". The light bulb leads show 13.4 volts across the bulb on the vom meter when there is 18.0 volts measured out from the solar panel. I have a brick wall stopping me from understanding what that means.

When you draw current from the panel, its voltage drops. Sort of like when you start your car the headlights dim. Voc (oc = open circuit) of the panel is the highest the voltage can be. The light bulb is a load and the voltage of the panel drops, as it should.

--vtMaps

PNjunction

Re: How to test a solar panel performance

catclaw wrote: »

The panel was repaired before I got it.

Uh oh. If the break fractured the moisture barrier, then this panel is on the way out. That may explain the lower than expected output. Essentially the cells oxidize when exposed to air, and that includes the air left IN the panel after a quick fix.

You'll want to check on it from time to time and see if there is any discoloration, blue turning to a dull brown/grey, funky cell tab discoloration, etc.

Hope you didn't pay too much for it.

catclaw

Re: How to test a solar panel performance

Thanx again. The earlier repair has to do with the connecting ribbon cables, I think. With some guidance, I re-soldered (and re-stored) two disconnected junctions of ribbon cable at the top of the panel near the box. After soldering I appied silicon adhesive over all the exposed area, but I will re-check this. The panel is a very well known brand and is 15 years old. If I can keep the panel working as it is now, the cost will be ok. I believe in re-cycling anything useful, but I sure don't like to be taken.

Unless something really weird happens, I'll close on this. It is really nice to get some help when "lost in the Woods".