2000k system only puts out 500 watt why?

[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 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.

[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]

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?

[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.

[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.

[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.

[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".

[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.

[jflorey2]

We got some really good weather today clear blue and no clouds so I decided to start testing again early. But as of 10:42am I was only getting an amp reading of 4.2 amps of current from panels number 1 temp 52F the same panel I tested last. Isc is 8 . . .

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.)

[charles2,david000k]

loop test started 11/13/15

We got some really good weather today clear blue and no clouds so I decided to start testing again early. But as of 10:42am I was only getting an amp reading of 4.2 amps of current from panels number 1 temp 52F the same panel I tested last. Isc is 8 but it is still early and we will see what she peaks at around 12 to 2 pm high noon.

[charles2,david000k]

The old spinning disk meters will NOT work with DC. They use the AC to induce eddy currents in the aluminum disk which makes it rotate.

Ok good to know AC power only.

[sensij]

when i tested my panels for current i just used a $10 multimeter, put the switch to the 10amp position then put the red pin to the +ve on the panel and the black pin to the negative on the panel. the meter read 9amps when the panel was in full sun. that is the rating for the panel.. 9amps. the wires on my cheap multimeter instantly smoked. have you tried this method?

He did, and probably damaged his meter because he had several panels in parallel. It was discussed several pages ago.

[almac]

Ok so I tested panels number one all day and these are my findings

10:15 am 35.43V 0.45A 15.9W at 69F

11am 34.70V 1.11A 38.5W at 69F

1:45pm 34.84V 1.24A 42.6W at 68F

2:40pm 35.09V 1.16A 40.06W at 68F

3:26pm 35.02V 0.96A 33.02W at 69F

4pm 35.02V 0.83A 29.02W at 67F

These reading below I am not quite sure of what they are I just wrote them down at the end of the day I will have to consult the operation manual. sorry for the edit. Wh=watt hours, Vm=minimum volts, Ah= Amp hours, Ap= Amp peak, Wp= Watt peak
Wh 239.
Vm 29.81
Ah 6.899
Ap 5.32
Wp 180.02
panels specs below

Nominal peak power(Pmax) 240Wp 245Wp 250Wp
Maximum power voltage(Vmp) 30.65V 30.8V 30.91V
Maximum power current(Imp) 7.83A 7.96A 8.09A
Short-circuit current(Isc) 8.36A 8.49A 8.62A
Open circuit voltage(Voc) 37.8V 37.98V 38.11V
Optimized cell efficiency(η） 17.00% 17.40% 17.60%

when i tested my panels for current i just used a $10 multimeter, put the switch to the 10amp position then put the red pin to the +ve on the panel and the black pin to the negative on the panel. the meter read 9amps when the panel was in full sun. that is the rating for the panel.. 9amps. the wires on my cheap multimeter instantly smoked. have you tried this method?

[Mike90250]

The old spinning disk meters will NOT work with DC. They use the AC to induce eddy currents in the aluminum disk which makes it rotate.