Actual Solar Panel Watt Capability

[Experimentor]

So, I am new to solar power (and this forum) and wanted to get started with solar in a small way with experiments just for the fun of it and to learn as much as I could (and to have a minor backup if the power went out). I've recently become fascinated with generating my own power. I'm also living overseas temporarily and don't necessarily want to travel with tons of solar panels in my baggage! So I mail-ordered a couple of 50 watt mono-crystalline panels, a good charger/regulator, a sinus wave inverter and I bought a few deep-cycle batteries. I bought two 12v panels so I could make either a 12 or 24 volt system. I also bought a 130 amp DC in-line wattmeter for measurements, besides my regular multi-meters.

I wired it all up and noticed I was only getting about 35 watts per panel. I wrote the supplier and complained. Of course I got the mantra about clear sky, facing sun, humidity, temperature, wire size, etc. I assured them that I understood all that and it was all optimized. Then they mentioned an MPPT charger, which I was not familiar with, and upon reading up on that, I ordered one (which hasn't arrived yet) but it made sense about the voltage limitation of the battery and the charger. So I did an experiment with a ni-cad battery pack. It was a 12-cell pack which can easily go up to over 20 volts while charging and will accept a pretty hefty amperage. I wired it directly to the panel (for a short time). Guess what? Still 35 watts.

So I wrote them and told them this. They said measure the voltage and amperage without a battery or charger. I replied that you can't have amperage without a load (meaning a closed circuit) and assured them that I knew a bit about electricity. They assured me that they did too and sent me a video showing them measuring the no-load voltage of the panel, and then changing the meter to amperage and checking the short-circuit amperage. Of course, when I did this, I got about 20 volts and then 2.5 amps. Voila' - 50 watts! But I explained to them that this is an invalid measurement, since they are two separate measurements under two different conditions. I then made my own video using two meters, to show that when you short-circuit the panel, the actual voltage drops to just over one volt.

They then contradicted themselves by saying the panel wattage is figured with the maximum voltage (18 volts) and the maximum amperage (2.78) but that you will never get the maximums (what?!). Of course, both those are under load, since it also lists the open circuit voltage (22.18) and the short-circuit amps (3.34). So even though I clearly made my argument correctly, their ultimate answer was "you'll never get maximum." I could understand if it was 48 watts, or even 45, but 35? That's fraud in my book.

Here are the specs on the panel.
IMG_0288.JPG

My ultimate question is: is this industry standard? Are all panels grossly over-rated so that "you never get maximum"?

[Sunking]

So I wrote them and told them this. They said measure the voltage and amperage without a battery or charger. I replied that you can't have amperage without a load (meaning a closed circuit) and assured them that I knew a bit about electricity.

Then you clearly do not know near as much as you think you do. A solar panel is a current source, not a voltage source. What the manufacture was telling you to do was perform a Voc test (voltage open circuit), and Isc test (current short circuit). Measure th evoltage with the leads open only connected to the meter. Then switch th emeter to read current and your meter will short out th epanel and reas Short circuit current assuming you have noon bright sun with the panel facing direcly into the sun.

At very best a 50 watt panel with a PWM controller can only deliver is 35 watts. What you seen is exactly what you should have seen. A 50 watt panel has a Vmp of roughly 17 volts, and an Imp of roughly 2.9 amps. With any PWM controller Input Current = Output Current. So at full power on the output you will see battery voltage x 2.9 amps assuming the battery demads a charge. 12 volts x 2.9 amps = 35 watts.

Read this thread on performing a Voc and Isc test.

[Experimentor]

Wow, quick with an answer and quick to criticize. I wonder if you fully read my post? What you said is exactly what I did, what I knew I was doing, and what I said I was doing. Your quote from my post even reinforces this (I said you can't have amperage without a closed circuit which is what a short-circuit is). Is that how you always answer questions on this forum?

Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage.

But my question was, is this industry standard, and according to your incredibly rude answer, apparently it is.

[Sunking]

Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage.

Like I said you do not know as much as you think you do because you can certainly have current without voltage. That is exactly what you do with the Isc test. Riddle me this.

What is the voltage developed with 2.9 amps flowing through 0 Ohms of resistance. A solar panel is a current source, not a voltage source like a battery. When you use a PWM controller you are not operating the panel at MPP, you are operating below MPP. A 50 watt Battery panel specs are:

Voc = 22 volts or open circuit no current.
Vmp = roughly 17 volts
Imp = roughly 2.9 amps
Isc = 3.1 amps @ ZERO volts short circuit.
Pmax = 50 watts or 17 volts x 2.9 amps.

Panel manufactures specify panel watt age in two measurement known as STC (Standard Test Configuration which is a lab measurement in a controlled enviroment) and is what they published for your panels. To be used in Grid Tied Application they are also required to specify PTC and CE ratings which are real world ratings. PTC and CE are roughly 90% of STC.

But that does not have much to do with what you are seeing because you do not understand current sources and PWM controllers. A PWM controller is going to pull down the panel voltage to the battery voltage. So you drop from 17 to 18 volts down to 12 volts. The input current = output current of 2.9 amps. 12 volts x 2.9 amps = 35 watts all day long.

A MPPT controller is a different animal as it is a POWER CONVERTER where Input Power = Output Power - Conversion Losses of a bout 2 or 3%. So now with that 50 watt panel you input 17 volts @ 2.9 amps (MPP aka maximum power point of the Current Resistance Curve) ) and on the output you have 12 volts @ 4.08 amps or 49 watts.

[Mike90250]

......Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage....

I think you need some edumucation about solar power. And the difference between sinus and sine.

http://en.wikipedia.org/wiki/Current_source

And you will have to wear a heavy leather Farrier's apron to protect from Sunking's sharp answers. This is not the solar molly-coddle form. You will (I guarantee it) get brutally honest, true answers. If you buy first and ask questions later, well, that's your style. I think there are better ones, but we'll try to deal with everyone.

[t5800512]

Yep, Sunking is brutally honest, and a little lacking on people skills. But he is usually right.

I don't want to get into the measurement aspect of this discussion. All I will say is that the true output would be better represented as a curve than a straight line.

So, there are many ways to measure the output of a PV panel. And of coarse manufactures do it under ideal laboratory conditions that do not indicate real world conditions. There are two rating to look at, STC and PTC. The STC rating is more that perfect lab test that the manufactures use. But PTC will get you closer to real life conditions (but still not spot on perfect). You should be looking for the PTC rating. Many manufactures do not offer the PTC ratings for obvious reasons. But for many of the panels without OEM ratings, the PTC has been measured and listed at sights like: http://www.solardesigntool.com/compa...s-modules.html

Another good link that does a better job of explaining what I just tried to explain is: http://www.civicsolar.com/forum/1036...-and-solar-ptc.

But until government steps in and forces PV panel manufacturers to conform to some standards, it is kind of the honor system, and ratings get fugged.

[Experimentor]

Wow,
Everyone here is a know-it-all. Sinus is how it's pronounced in German. Sorry I waxed to a different language there. Your link doesn't contradict Ohms law, does it? And there's a difference between being molly-coddled and just being rudely flamed. I came to the forum as an admitted solar newbie and what a welcome I get. Sorry I bought a couple of panels without seeking this forum's approval first.

Sunking: You can only have zero resistance in a theoretical circuit. Even experimental super-cooled circuits have some resistance.

I acknowledged an understanding that a controller is going to pull down the panel voltage to the battery voltage. Did you notice I mentioned connecting the panel directly to a higher voltage, low resistance battery pack without a charge controller? Though I don't claim to know as much as some people, the way I understand electrical theory, if a panel is truly capable of 50 watts, with this uncharged battery it should go to the maximum voltage capability (18 volts) and the maximum current (2.78 amps). As I said, that showed, again, that it was only 35 watts. So as I understand it, the actual capacity of the panel is 35 watts. If you can actually explain why I am wrong, instead of just flaming me, I would appreciate it.
Like I said, I would have expected a 10% difference in theoretical and actual capacity, but not a 30% difference.

[Naptown]

The other thing missing from the op' s post would be cell temperatures.
If say he were in a hot climate and the cell temp was say 50c in most panels that would equate to almost 25% less power. ( most panels lose .44% per degree above 25c)

Now to truly test you need to know cell temperature and calculate the losses from that in also

Did your test equipment include a pyronometer? If not how do you know how far off the lighting is or if you are indeed getting the 1000w per sq meter irradiance.

[Sunking]

Wow,
Everyone here is a know-it-all. Sinus is how it's pronounced in German. Sorry I waxed to a different language there. Your link doesn't contradict Ohms law, does it? And there's a difference between being molly-coddled and just being rudely flamed. I came to the forum as an admitted solar newbie and what a welcome I get. Sorry I bought a couple of panels without seeking this forum's approval first.

Sunking: You can only have zero resistance in a theoretical circuit. Even experimental super-cooled circuits have some resistance.

I acknowledged an understanding that a controller is going to pull down the panel voltage to the battery voltage. Did you notice I mentioned connecting the panel directly to a higher voltage, low resistance battery pack without a charge controller? Though I don't claim to know as much as some people, the way I understand electrical theory, if a panel is truly capable of 50 watts, with this uncharged battery it should go to the maximum voltage capability (18 volts) and the maximum current (2.78 amps). As I said, that showed, again, that it was only 35 watts. So as I understand it, the actual capacity of the panel is 35 watts. If you can actually explain why I am wrong, instead of just flaming me, I would appreciate it.
Like I said, I would have expected a 10% difference in theoretical and actual capacity, but not a 30% difference.

Like I said you do not understand current sources period. A solar panel is a Current Source. Until you understand current sources you will never figure out what is going on. I can help you, but you have to pull your head out of the sand and be willing to learn and listen. I am a professional licensed engineer with 33 years of experience and know what I am talking about. My bet is you are not even 33 years old. I might know something you do not and can teach you something. Right now you do not want to learn or listen. That is not our problem but yours alone.

[thastinger]

OP, how is the panel oriented azm and ele.

[Sunking]

The other thing missing from the op' s post would be cell temperatures.
If say he were in a hot climate and the cell temp was say 50c in most panels that would equate to almost 25% less power. ( most panels lose .44% per degree above 25c).

He is in Germany and has never seen anything above 30.

[Experimentor]

I'm 50. Wow, wrong on something; that probably hurts. I get it - it's a current source. But it still has to obey Ohms law, no? I am listening. Why do you think I keep asking questions? Perhaps your answers are not correctly tailored to my questions?

By the way, the temp is less than 25C.

[Experimentor]

Yep, Sunking is brutally honest, and a little lacking on people skills. But he is usually right.

I don't want to get into the measurement aspect of this discussion. All I will say is that the true output would be better represented as a curve than a straight line.

So, there are many ways to measure the output of a PV panel. And of coarse manufactures do it under ideal laboratory conditions that do not indicate real world conditions. There are two rating to look at, STC and PTC. The STC rating is more that perfect lab test that the manufactures use. But PTC will get you closer to real life conditions (but still not spot on perfect). You should be looking for the PTC rating. Many manufactures do not offer the PTC ratings for obvious reasons. But for many of the panels without OEM ratings, the PTC has been measured and listed at sights like: http://www.solardesigntool.com/compa...s-modules.html

Another good link that does a better job of explaining what I just tried to explain is: http://www.civicsolar.com/forum/1036...-and-solar-ptc.

But until government steps in and forces PV panel manufacturers to conform to some standards, it is kind of the honor system, and ratings get fugged.

Thank you for your civilized answer and the useful links.

[Experimentor]

The other thing missing from the op' s post would be cell temperatures.
If say he were in a hot climate and the cell temp was say 50c in most panels that would equate to almost 25% less power. ( most panels lose .44% per degree above 25c)

Now to truly test you need to know cell temperature and calculate the losses from that in also

Did your test equipment include a pyronometer? If not how do you know how far off the lighting is or if you are indeed getting the 1000w per sq meter irradiance.

No, it didn't include a pyronometer. I was limited to doing it on a clear sunny, but cool day, low humidity with the panel held directly facing the sun. I would imagine I can't get much better results than that other than in a lab. Like I said, I would have expected a 10% difference in theoretical and actual capacity, but not a 30% difference.

[t5800512]

I'm thinking you may not be testing under ideal conditions. There are many things to consider. Sun Radiance may not have been considered. I know with my Solar array, I see a difference in output due to dust and smog that is not even discernible to my eyes. I know this because after a good air cleaning rain my output goes up even at the same temperatures. If you are in Germany, your radiance will be lower than places south of you. Just saying.