2000k system only puts out 500 watt why?

[sensij]

Sounds about right. Isc is the important number. Multiply Isc by Voc and that will give you theoretical max power. (Actual peak power will be somewhat less than that but not by much.)

No! Isc * Voc is meaningless. There is no theoretical way for the panel to operate on both ends of its IV curve simultaneously.

The Isc measurements are strongly related to plane of array irradiance. The data sheet is an STC value, at 1000 W/m2 of irradiance. That number can be corrected by a model that accounts for panel location, orientation (azimuth and tilt), and time of day (assuming a clear sky). Thermal effects matter too, but are secondary to irradiance.

Please let us know approximately where the panels are located, and at what orientation they are being held during the test.

[J.P.M.]

No! Isc * Voc is meaningless. There is no theoretical way for the panel to operate on both ends of its IV curve simultaneously.

The Isc measurements are strongly related to plane of array irradiance. The data sheet is an STC value, at 1000 W/m2 of irradiance. That number can be corrected by a model that accounts for panel location, orientation (azimuth and tilt), and time of day (assuming a clear sky). Thermal effects matter too, but are secondary to irradiance.

Please let us know approximately where the panels are located, and at what orientation they are being held during the test.

+1. But I'd change "corrected" to: "corrected to some degree".

[charles2,david000k]

panel #1 loop test

WOW When two or more of you guy's who know this stuff get talking I feel like I am trying to read lost code. lol

So the front of the panels are facing south where the sun rises on the left and sets on the left as for angle I am unsure it would be like a 5-12 pitch my roof is 4-12 pitch and they have just a little more angle then that.




My readings for panel 1 with loop for Isc reading with clear skies

10:40am 5.34A at 55F

11:50am 7.02A at 56F

12:32pm 7.64A at 54F

2:45pm 7.19A at 58F

Ap 7.19
Ah 44.917

I am happier then I was but am still disappointed that the panel did not hit its Isc of 8.62A I even provided the watts up meter with it's own power source so it would not pull the panel down any.

[jflorey2]

No! Isc * Voc is meaningless.

It's far from meaningless. Indeed, it is the best way to quickly test a panel. Let's take an example:

Case 1 - You have an 85 watt panel that someone is trying to debug. You test Isc and Voc and get 5 amps and 22 volts in full sun. That means the panel is working. If you put it under load, both will drop; a monocrystalline panel will drop about 20% in voltage and about 10% in current when operated at its peak power point. That gives you the panel's peak power.

Case 2 - You have an 85 watt panel that gives you 22 Voc and .05Isc in full sun. That means the panel is broken.

There is no theoretical way for the panel to operate on both ends of its IV curve simultaneously.

Correct. Those are the extremes. From the extremes you can infer what peak power the panel will provide.

[sensij]

Sounds about right. Isc is the important number. Multiply Isc by Voc and that will give you theoretical max power. (Actual peak power will be somewhat less than that but not by much.)

It's far from meaningless. Indeed, it is the best way to quickly test a panel.

My earlier response may have been too sharp. Of course Isc and Voc are useful ways to characterize a panel, that is what we've all encouraged the OP to do, and which he is doing successfully. Multiplying those values together does not give a theoretical max power. Multiplying them together is theoretically meaningless, although it does give an upper limit to the power that could be obtained if a magical material existed with a fill factor of 1. Applying a 0.8 fill factor is a reasonable shortcut for silicon, but still does not tell the whole story, and still needs to be adjusted for actual irradiance and environmental conditions.

The relationships between Isc, Imp, Voc, and Vmp are empirical relationships for a healthy panel, not theoretical relationships that hold true under all conditions. In actuality, a bad panel might yield good Voc and Isc readings, but still fail under load.

I hope that what you are saying is that, without a data sheet, you can tell by multiplying that Isc * Voc that a panel might be healthy. That sounds OK to me.

Edit:
Yes, theoretical approaches use Isc, Voc, and FF as a basis for understanding power conversion efficiency. In practice, fill factor still seems to be derived empirically.

[charles2,david000k]

So is this a good panel and should I move on to testing panel number 2 because on the last test the meter tracked a max peak watt of Wp 180.02 that's still 69.98 watts under its 250.00watt rating?

[LETitROLL]

It's far from meaningless. Indeed, it is the best way to quickly test a panel. Let's take an example:

Case 1 - You have an 85 watt panel that someone is trying to debug. You test Isc and Voc and get 5 amps and 22 volts in full sun. That means the panel is working. If you put it under load, both will drop; a monocrystalline panel will drop about 20% in voltage and about 10% in current when operated at its peak power point. That gives you the panel's peak power

I have a question to make sure I haven't misunderstood some of my previous teachings.

1) "You test Isc and Voc" This is of course the easiest and most common way to test a panel and see if it is close to spec or not, but aren't they usually (or always) measured one at a time? To my way of thinking it is not mathmatically possible for any panel to put out anywhere near Voc when it is being tested to Isc (short circuit). Since they are readings, that by nature cannot exist at the same time, I am trying to make sure i understand the value of multiplying the two together, and/or if it is a common practice in the solar industry to actually multiply those two together, or just know what they are individually.

[charles2,david000k]

I won't be home tomorrow but I think it should be another nice sunny cold day so tomorrow morning I am going to set up the watts up meter in line to track panel number 2 while it charges the batteries. I won't be here to take temps and time frame but I am hoping to get a jump to speed things up a little. plus I got to drive 2 hours to go pick up the new clamp meter.

[sensij]

I don't know where you are located, but from some things you've mentioned I'm guessing near Knoxville.

Using PVWatts, for the tilt (say, 25 deg) and direction (south) you've described, mid-day plane of array irradiance might be around 830 W/m2.

The Isc of 8.62 was taken under standard test conditions of 1000 W/m2. (830/1000) * 8.62 = 7.2 A, so the readings you are getting are in the right ballpark. Yes, moving on to the other panels and verifying the Voc and Isc for each would be a good next step. If they all read about the same, then you can be more confident performing a load test.

I'm not sure where you are coming up with the 180 W value. During an Isc test, the panel voltage should be approaching zero, so you won't get measurable power like that. For example, at 7.2 A, you would need 25 V to get to 180 W power (7.2 * 25 A = 180). I guess 180 W would be about right for power for a load test on this panel, if you hooked it up to your (discharged) 24 V battery. Is that what you did?

[charles2,david000k]

I don't know where you are located, but from some things you've mentioned I'm guessing near Knoxville.

Using PVWatts, for the tilt (say, 25 deg) and direction (south) you've described, mid-day plane of array irradiance might be around 830 W/m2.

The Isc of 8.62 was taken under standard test conditions of 1000 W/m2. (830/1000) * 8.62 = 7.2 A, so the readings you are getting are in the right ballpark. Yes, moving on to the other panels and verifying the Voc and Isc for each would be a good next step. If they all read about the same, then you can be more confident performing a load test.

I'm not sure where you are coming up with the 180 W value. During an Isc test, the panel voltage should be approaching zero, so you won't get measurable power like that. For example, at 7.2 A, you would need 25 V to get to 180 W power (7.2 * 25 A = 180). I guess 180 W would be about right for power for a load test on this panel, if you hooked it up to your (discharged) 24 V battery. Is that what you did?

The 180w was a load test which is what I did today the meter I have track and logs data throughout the whole day from the time you hook it up to the time power output stops. Like today I hooked into panel number two and got these results.

test started at 12:35pm to sundown at 4:55pm skies were clear and I did not get a temp for today but it seemed like it was pretty cool 55F I would have to say.

Ap 7.45
Ah 5.031
Wp 204.8
Wh 150.2
Vm 00.00

I think the Vm was changed due to I now have a backup power source to run the meter. From the readings I got today my peak watts were 204.8 my location is above Spring city Tn in a place called luminary or Bledsoe county but I live almost at the top of the mountain I can see watts bar nuclear power plant and the smokie mountains on a really nice day. you can find me here

https://www.google.com/maps/@35.7104717,-85.001619,98m/data=!3m1!1e3

But I am not really happy with the readings I got as of today so I will be running this same test again on the next good day

[Raul]

Let me see if I get this . Your panel outputs 200w+ and is 240w rated right? And you are not happy?
I think you're wasting your time with testing the panels . Those panels are performing as they should but your weak points are elsewhere in the system . It could be wire , contacts or the most important pice in the system for harvest CHARGE CONTROLER.
The panel will produce and if the system components are up to scratch to handle the power and efficiently harvest it when available.
Panel will only produce what the system can take even if the panel has a higher rating .Only a efficient system will be able the harvest it all.

[charles2,david000k]

Let me see if I get this . Your panel outputs 200w+ and is 240w rated right? And you are not happy?
I think you're wasting your time with testing the panels . Those panels are performing as they should but your weak points are elsewhere in the system . It could be wire , contacts or the most important pice in the system for harvest CHARGE CONTROLER.
The panel will produce and if the system components are up to scratch to handle the power and efficiently harvest it when available.
Panel will only produce what the system can take even if the panel has a higher rating .Only a efficient system will be able the harvest it all.

No it's a 250 watt panel putting out maybe 208W and we cut out most if not all the other factors by only testing just the panel only when we do current or amp's and on perfect condition day I won't even set up to test if there is a chance of light clouds it has to be a perfectly clear day before I waste my time setting up. That is probably why some readings were off today. An I got to be pulling over 250 watt because my inverter puts out 6000 watts easy and sucked the bank completely dry and burnt out my second to last 250 amp fuse in the process but the fuse burn up because they are rated at 32V and the panels should peak at 38V and 8 amps I have solenoid cc and MPPT built in to the inverter I even open the inverter up to blow out dust and check it has huge coils and a mess load of the flat diode I have seen in other MPPT cc and for $2000 it better really be MPPT. I will have to do a update when I get more time this thread is getting long.

[jflorey2]

1) "You test Isc and Voc" This is of course the easiest and most common way to test a panel and see if it is close to spec or not, but aren't they usually (or always) measured one at a time?

Yes. The easiest way is get a meter that is rated for more current than the panel is capable of, then test Voc with the meter set to volts and Isc with the meter set to current. (Two separate tests, generally with two different connections on the meter.)

To my way of thinking it is not mathmatically possible for any panel to put out anywhere near Voc when it is being tested to Isc (short circuit).

Right. The definition of "short circuit" is zero voltage across the device, just as "open circuit" refers to a zero current condition.

Since they are readings, that by nature cannot exist at the same time, I am trying to make sure i understand the value of multiplying the two together, and/or if it is a common practice in the solar industry to actually multiply those two together, or just know what they are individually.

If you multiply the two together you get an artificially high power - because the voltage will be lower under a heavier load, and the current will be lower under a lighter load. Fortunately the relationship between Voc and Vmpp, and Isc and Impp, is fairly well known (and shown explicitly on the panel's data sheet.) If you don't have access to the data sheet, and it's a monocrystalline panel, you'll get about 70-75% of the product of Voc and Isc at the maximum power point.

[jflorey2]

No it's a 250 watt panel putting out maybe 208W

Sounds about right. You almost never get rated power out of a panel. If you are getting 208 watts calculated out of a 250 watt panel it is doing its job - especially with a simple solenoid based charge controller.

and burnt out my second to last 250 amp fuse in the process but the fuse burn up because they are rated at 32V and the panels should peak at 38V and 8 amps

Voltage does not burn out fuses. Current does. (However, once you blow the fuse, then the voltage rating becomes VERY important - and you'll see fireworks if you use a low voltage fuse on a high voltage system.)

I have solenoid cc and MPPT built in to the inverter

Inverters don't do MPPT charge control. That only happens in the charger (the thing between the panels and the battery.) A standard inverter does not have an MPPT, nor does a solenoid based charge controller.

I even open the inverter up to blow out dust and check it has huge coils and a mess load of the flat diode I have seen in other MPPT cc and for $2000 it better really be MPPT.

Surely you are not thinking that your inverter has any given functionality simply because it has "flat" components in it?

[LETitROLL]

No it's a 250 watt panel putting out maybe 208W

For middle of November in North America that does not seem bad, no matter how big and bright the sun is, in most of North America right now it is at a "Poor Angle" and the atmosphere is eating part of your energy. Also if I remember correctly your panels are fixed mount, so even during mid-day sun when things look good to the naked eye, if the elevation of your panel(s) is not perfectly square to the sun (which is almost impossible with fixed mount) it makes it impossible to match the lab conditions that your panels nameplate wattage was created with.