Why the low power?

[Vic]

Hi z,

YES,  with PV that are not loaded,   that IS Voc for the Irradiance and cell temperature at that time.

With PVs not connected to the system,   you must have shorted the PV cables together  --  a Short.   This is Isc  --  Short Circuit current.

Voc is not too useful in guessing what power might be produced,  because  at that voltage there can be no power produced.

Also,   it is important to orient the PVs at right angles to the sun to get the best power production at that moment.

Vic

[ClassicCrazy]

Something bothers me about the calculated power...

What is meant by "Open circuit power" 

There is NO power when the PV is open circuited.  I don't suppose that you are calculating expected
power by multiplying some measured current by measured  Voc ?

I took readings on each panel when they were not connected....in bright sun...... Voltage and Amperage.......I guess the Voltage when  not connected is Voc ....or 'open circuit' I think.......I don't know what you call the amperage when you measure it when they're not hooked to the system......but I was getting around 6-7 amps or so.....

Since watts is V*A, I calculated about 37v x 6 amps is about 222 Watts.....

I'm just ball parking here, but when they were connected to the controller, they only get 60% or so of that....

See the spreadsheet I made.....


Edit. I used a VC890C volt/ohm meter to measure the amps..........
z

I see Vic just replied but I will finish this
If you have just your panel in the sun and not connected to anything and then put a wire between its positive and negative terminal - that is a short  - and if you measure the current on that wire that will be the Isc  which is short circuit current.
If you take that short circuit wire off and just measure the voltage with nothing connected that is the Voc  voltage open circuit.
What I believe others are saying is that you can't take the Voc and multiply it by the Isc to get your maximum watts.
The Imp current maximum power is at a voltage much lower than the Voc .  The Classic software searches for this point  which can change - it is the maximum power point MPPT  . 
So from what I think you are saying is that you are multiplying Voc by Imp and using that as your PV power rating . That is not correct .

Larry

[KyleM]

Larry, 

I am totally stealing this explanation    just wanted you to know I might take credit for it too 

Kyle

[FNG]

To add to this, if you measure the VOC generally speaking a good rule of thumb is VMP will be 75% of VOC, So now with VOC you can calculate VMP and multiply it by ISC and get a close value to what you may expect for wattage.

In a nutshell in my opinion the issue, in this case, is low voltage, if this was my system I would wire two of those modules in series, With homebrew modules or mismatched modules I also would use Legacy P&O

[Resthome]

Something bothers me about the calculated power...

What is meant by "Open circuit power" 

There is NO power when the PV is open circuited.  I don't suppose that you are calculating expected
power by multiplying some measured current by measured  Voc ?

I took readings on each panel when they were not connected....in bright sun...... Voltage and Amperage.......I guess the Voltage when  not connected is Voc ....or 'open circuit' I think.......I don't know what you call the amperage when you measure it when they're not hooked to the system......but I was getting around 6-7 amps or so.....

z

With the amp meter connected across the terminals you are measuring Isc. and you are correct when the meter is in Voltage mode you are measuring Voc. Those measurements are not what is used to calculate typical system panel Wattage.

The Pmax is Vmp * Imp.  See the specs on a typical manufactured solar panel and check out these values and the Wattage rating of their panels.

And observed panel output is typical 80% of Pmax at a nominal temp of 72-75 degrees of Vmp * Imp or the rating the manufacture specs as Pmax. With higher temp on the panels the wattage decreases even more.

[Zardiw]

Thanks Guys for those explanations.......looks like I was making some assumptions..........lol

z

[boB]

Thanks Guys for those explanations.......looks like I was making some assumptions..........lol

z

That's OK  !   Been done before and will be done again in the future by many.

We'll help one at a time, no problem.

boB

[Zardiw]

To add to this, if you measure the VOC generally speaking a good rule of thumb is VMP will be 75% of VOC, So now with VOC you can calculate VMP and multiply it by ISC and get a close value to what you may expect for wattage.

In a nutshell in my opinion the issue, in this case, is low voltage, if this was my system I would wire two of those modules in series, With homebrew modules or mismatched modules I also would use Legacy P&O

Thank You!

I built another panel, and now have 5 pairs in series: 1+2, 3+4, 5+6, 7+8, and 9+10

Here are the readings at High Noon:

Pair, Voc, Isc

1+2.......74.4v........7.30a
3+4.......74.7v........7.68a
5+6.......73.5v........8.02a
7+8.......72.1v........7.34a
9+10.....71.8v........7.63a

So if I take 75% of Voc, and multiply that by Isc, I should get a close value of what power the panels should put out.....is that right?

Because say on average, the Voc is 72v, times 75% is 54v times say avg Isc of 7.5a is 405watts/pair, times 5 pairs is 2025 watts.

So in theory I'm supposed to get 2025 watts...........is that correct?

But here are the readings in the Status Panel......with a good load on the system:

System is in Legacy P+O mode (what exactly is that?)

Input Volts - 48
Watts - 1614
Amps - 58.9
With the battery at 26.6v

SO I guess the power output is in the ballpark of what it's supposed to be......

Is that correct?

And thanks to everybody for helping with this.......hopefully it will help others to understand..

Time to build more panels.......more power!!........lol

z

[ClassicCrazy]

I would say you are doing pretty good with the 1600 watts .
Remember also that the Classic is not 100% efficient in conversion .
Larry

[Westbranch]

Now you can compare those estimated watts to the actual harvest as recorded by the classic, but you will have to start the day with batteries that have been working after the sun went down and no loads while you are testing the panels...
have fun....

[boB]

The measurement will be closer than before for sure but still not perfect....

The Isc  will be kind of close to the Imp but will still be a bit higher than actual current at max power point (75% of Voc)

BUT there are more gothchas by using 75% of Voc and that might be from partial shading or possibly other factors.

One factor would be how well the panels match as far as Imp and Vmp.

Irradiance and PV panel temperature will be different from one time to another.  i.e. the Isc you measure will more than
likely be higher or lower than when you actually place the panels back in circuit with the charge controller.

Best measurement is to use an ammeter and voltmeter from the PV into the charge controller WHILE it is operating.
Then you will be measuring the actual power into the controller.  Assuming your current probe or clamp meter is
fairly accurate.

That is another aspect of this type of measurement...  Measurement accuracy.  It's not perfect.

Sounds like you are on your way to getting more meaningful results though !

boB

[Resthome]

Of course we know boB is correct and is what I stated in my previous post. Power max = Vmp * Imp. You are using Isc and that is going to be a higher current than Imp. So NO your 2025 Watts is not the Power you are going to get from your panels it is going to be lower than that because Imp is going to be a lower value than Isc. And because your panels are all slightly different you are going to have slight differences.

Your best way is to measure your total panel output is while all your panels are connected to the controller and battery bank and have a large load on your system to get the max out of the system including all your wiring and connections. The controller can measure your Vmp and Imp. One of the status screens shows the total amps coming from you panels when at max power point.

You could put a clamp on amp meter around the negative PV lead going to the controller that will be your total Imp. Then measure the Vmp across you positive and negative PV leads while connected to the controller. 

Again go back and look at how a typical manufacture specifies these values. I posted a sample spec for you to look at but all panels have this specifications and you can see how they get their panel wattage. Again manufacturers spec their wattage at a nominal temp of approx. 75 degrees. Panels in direct sun light are usually warmer. What most people see out of an off grid system is about 80% - 85% of the wattage specified by a manufacture.

I think your 1614 watts is a pretty good output for your system. Without a WBjr and shun you can't see how much of this is going to your batteries vs your loads.