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Possible to determine watts of panel when testing it?

wrybread

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I'm testing these 260 watt panels:

1654130376295.png

Doing the usual test of connecting to an amp meter set on 10 amps. Getting 8 amps and 35.5 volts, which is 284 watts... I'm guessing short circuiting the panel like ths isn't a good measure of the actual expected watt output of the panel?

If not, is there a way to figure out the expected output of the panels when testing with a multimeter without a load or any other equipment? Would be handy knowledge when testing panels if so.

On a sidenote, excited about these panels, just picked up 3 of them for $75 each. Hardly a scuff on them and all have nearly identical output.


1654130619514.png
 
As long as they test fairly close to the sticker ratings.
You can expect an output of at least 85% of the sticker wattage, In full sun.
 
Rated panel wattage is Vmp x Imp. You are measuring Voc and Isc. Voc x Isc is about 33% higher than rated panel wattage. So take your Voc x Isc and multiply by 0.75 to get expected max wattage. Of course that's all based on perfect STC which you certainly didn't have when you made your measurements.

You should expect about 213W (8A x 35.5V x 0.75) from the panel under the same conditions you had when you made your measurements.
 
If that picture shows up here without the solar panel in it, it will become the desktop on my computer. HINT HINT ?
 
What you need is a proper load for the panels to test their output. For a quicky test, what I did was just wire two 12V batteries in series, then connected the raw solar leads right to the battery terminals. Measured the amps flowing with a clamp meter. Keep in mind that the output voltage will drop to what the battery wants. I suppose you could create a better load by wiring two 12V car headlights in series. That would be good for testing a up to a 10A panel.

BTW, those RECs are a very good choice. Good score on your part. I myself picked up a half-dozen of the same model earlier this spring. The only mistake I made with that purchase was not buying more than I needed. As soon as he saw them, a neighbor wanted me to sell him a couple.
 
Isc is closest thing to true illumination current and its temperature coefficient is minor. If you are measuring 8.0 amps for a panel rated for 9.01 amps either panel is overstating its Isc spec or you have about 8.0A/9.1A = 88% full sun intensity., Likely have 88% of full sun level intensity. Lower sun is above horizon, the lower the sun intensity, which is a seasonal fact of life and your latitude on earth. Lower the angle of sun above the horizon the more atmospheric attenuation. Any haze or fog decreases intensity more.

solar-intensity-vs-incidence to horizon.png

Imp typically runs about 0.95 x Isc.
Vmp typically runs 0.81 to 0.85 x Voc. Voc is temperature dependent, dropping as panel warms up in sun. Vmp to Voc ratio depends on particular panel cells shunt resistance and series resistance. 0.81 factor is toward lower performance end, 0.85 factor is higher performance end of range.

So if you read 8.0 amps Isc and 35.5v Voc, you can expect 0.95 x 8.0A X 0.82 x 35.5v = 7.6 A Imp x 29.11v Vmp = 221.2 watts at max power point for the illumination condition and panel temperature you have during measurement.
 
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I'm testing these 260 watt panels:

View attachment 96844

Doing the usual test of connecting to an amp meter set on 10 amps. Getting 8 amps and 35.5 volts, which is 284 watts... I'm guessing short circuiting the panel like ths isn't a good measure of the actual expected watt output of the panel?

If not, is there a way to figure out the expected output of the panels when testing with a multimeter without a load or any other equipment? Would be handy knowledge when testing panels if so.

On a sidenote, excited about these panels, just picked up 3 of them for $75 each. Hardly a scuff on them and all have nearly identical output.


View attachment 96845
US $41.99 50%OFF | EY800W Upgrades Handheld Portable Photovoltaic Panel Power Supplys Multimeter Auto Manual MPPT Detection with LCD Display Screen

This tool, might be what you need, incorporates everything
 
Just test the short circuit current and Voc. Easy tests and you can use a meter that measures 10A
 
Just test the short circuit current and Voc. Easy tests and you can use a meter that measures 10A
Did that on a batch of poor performing panels, sc test was 4.1amps, then changed the meter dials and checked for Voltage. Voltage was 29v .
However when i line up the 3 panels in series to a 60amps mppt charge controller and halffull lithium battery pack, i got only 1.8amps and combined voltage of 56v.

Voc panels 37v
Make renogy
Wattage 250watts
Imp 7.9amps.

So am a lil confused
 
When I bought 40 used panels, I did a VoC and short circuit test *against* a brand new one of same watts - verified the used ones within 5% of of the new one and concluded they were perfectly reasonable. The test itself could easily have 5% fuzziness.
BTW - This simple test revealed 1 panel with a bad diode - replaced the diode pack and it was as good as the rest.
As in many things solar (panels, batteries), its good to do basic tests before install in my opinion - even if it takes a bit of effort or equipment :)
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You can mesure the live power using a dc/dc that read input voltage, and keep it stable by a feedback loop, you can use a TL494, feed input voltage on inverting input, reference by a adjustable divider on non inverting input, and use buck or boost topology for force.
Place an apropiate resitive load at dc/dc output, and adjust controlled input voltage to obtain maximal output current, that input voltage is Vmp, and power can be easy calculated from load current and voltage.

E.g. for a 60cell panel Vmp si about 25v, you can use as load 24 volts lamps, in parallel in total power near the rated power of the panel and use a buck converter to find Vmp, for common panels up to 500w you may use a irfp4110 or similar and a 30a schottky diode for converter, on a heat sink,
Load current may reach 20a, but test time is short, no special requirements for that converter, the inductor may be built on a sendust toroid, or a ferrite bone , if you not have one, you can bulid it in air, with more turns, or turn area, for 100khz 50 turns of 1 square inch are enough.
you may use an adequate conductor tickness for inductor, multiwire 12 AWG is ok. you can also use an inductor from a dead mppt controller, that covers expected current.
 
I found it quite simple to put an old car battery and spare mppt controller in a crate and used that. Need to make sure the battery has enough volts to power the mppt but nowhere near fully charged.
 
That battery must good enough to take entire current generated by that panel, but you can add a load to be sure that entire available power is absorbed.
For mobile testing a relative small LTO battery is better because it can be charged at over 10 c.
This method have some limitations if you try to test a large lot of panels, it takes time controller to settle on Vmp.
My method is good to test fast, if your load is an incandescent lamp you notice fast what panel is not good, if you use a boost converter you may use a 120v flood lamp at 250 or 500w in accordance with panels subject to test.
 
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That battery must good enough to take entire current generated by that panel, but you can add a load to be sure that entire available power is absorbed.
For mobile testing a relative small LTO battery is better because it can be charged at over 10 c.
This method have some limitations if you try to test a large lot of panels, it takes time controller to settle on Vmp.
My method is good to test fast, if your load is an incandescent lamp you notice fast what panel is not good, if you use a boost converter you may use a 120v flood lamp at 250 or 500w in accordance with panels subject to test.
yes i do have a little battery tester available to act as a load
 
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