Solar panel reads A&V much higher than printed on the back,not sure what specs are

[Rixsta]

Hi
I was just wondering if this is normal
I have solar panels which on the back printed read as follows.

1580x808x35

210w

36v working voltage

5.83A working current

15.5kgs

this is all the info I have as I found them second hand 3x of them for £160, the problem is.... they read the following on the multimeter

7.1A
47V

Is this normal ? as I was planning on using them with the Morningstar MPPT 60A charge controller with 12v system untill I realised with the mutlimeter measurements they would be outputing around a total of 900w of power!! instead of 630w which means ill have to choose a 24v system to Handle the 900w ?

[SunEagle]

Is that 47v open circuit voltage and the 7.1a short circuit amp? Both the amps and voltage reading will be higher then "working" values if the panel is not under any load.

If the panel is connected to a charge controller and it is connected to a battery that needs charging then in most cases under the Noon day sun both the voltage and amp reading at the CC will be very close to the "working" ratings or 'MP" max power ratings listed on the panel.

[Rixsta]

Is that 47v open circuit voltage and the 7.1a short circuit amp? Both the amps and voltage reading will be higher then "working" values if the panel is not under any load.

If the panel is connected to a charge controller and it is connected to a battery that needs charging then in most cases under the Noon day sun both the voltage and amp reading at the CC will be very close to the "working" ratings or 'MP" max power ratings listed on the panel.

Im connecting multimeter to one panel with nothing else connected, multimeter direct to panel, it actualy produced little sparks, but i was shown on another website its ok to read the amps with a multimeter.

[Rixsta]

I presume that the amps X volts = watts of the panel ? which in this case means they are 333.7W!! yet state 210 on the back.

[SunEagle]

I presume that the amps X volts = watts of the panel ? which in this case means they are 333.7W!! yet state 210 on the back.

Using a meter to measure the voltage and amps on the wires coming from the panel without the panel connected to a load will result in much higher voltage and amp readings.

The panel wattage is determine by multiplying the Vmp x Imp = Watts. In the case of your panel you could use the working voltage x the working current or 36v x 5.83A = 209.98 or 210 watts.

[Rixsta]

Ok thanks a lot, with the load it seems the readings are diferent I guess.

so would you say that 3 of those panels in series = 210 x3 = 630w
Theres no way they could produce many more watts than rated ? just freaks me out that I read 47v and 7.1A when the panels are actualy 36v and 5.83A .

If the open circuit voltage dosent matter then I have no problem with the Morningstat MPPT 60A ? as it can only take 150V max and 800W with my intended 12v Battery bank.

If those panels are producing over 800W the controller will fry of corse, Hence why im asking all the questions

[inetdog]

The current versus voltage curve for the panel (any silicon panel) has two endpoints where the power is zero, namely open circuit where V = Voc and I = 0 and short circuit where V=0 and I = Isc.
0 times anything is zero and you cannot pull Isc and get Voc at the same time.
So to get maximum power you increase the currrent draw to decrease the working voltage from Voc until the product of the current and voltage reaches a maximum and pulling any more current gives less power. That point is the Maximum Power Point, Vmp and Imp.
Vmp is very weakly dependent on the amount of sunlight coming in while Imp is closely proportional to the amount of light.

[SunEagle]

Ok thanks a lot, with the load it seems the readings are diferent I guess.

so would you say that 3 of those panels in series = 210 x3 = 630w
Theres no way they could produce many more watts than rated ? just freaks me out that I read 47v and 7.1A when the panels are actualy 36v and 5.83A .

If the open circuit voltage dosent matter then I have no problem with the Morningstat MPPT 60A ? as it can only take 150V max and 800W with my intended 12v Battery bank.

If those panels are producing over 800W the controller will fry of corse, Hence why im asking all the questions

You are correct 3 of those 210 watt panels wired in series or in parallel = 630watt total.

When you wire panels in series you add up the voltage. To make sure you do not exceed the maximum DC voltage input rating of your CC you should use the open circuit voltage which is higher then the working voltage. If your panel is measuring 47v as the open voltage then you multiply that by 3 to get the total DC input voltage which comes to 141volts. Just be aware that sometime the open voltage may exceed the 47volts that you measured which could result in exceeding the Morinigstar DC voltage input rating.

[Rixsta]

You are correct 3 of those 210 watt panels wired in series or in parallel = 630watt total.

When you wire panels in series you add up the voltage. To make sure you do not exceed the maximum DC voltage input rating of your CC you should use the open circuit voltage which is higher then the working voltage. If your panel is measuring 47v as the open voltage then you multiply that by 3 to get the total DC input voltage which comes to 141volts. Just be aware that sometime the open voltage may exceed the 47volts that you measured which could result in exceeding the Morinigstar DC voltage input rating.

Thanks, yes so the 47v is the highest voltage ive seen the panels produce and ive had them outside a few days and tested each day. so together 141v which is close to the max rating of 150v of the controller, but not too close ? as I doubt they will rise much more. ok so thats all good
So n regard to wattage it looks like 630w of panel even though my open circuit ampage is 7.1 is still 630w of panel and nowhere near the max rating of 800w for the Morningstar.

Ok thanks seems like im understanding it all a bit.

[Rixsta]

The current versus voltage curve for the panel (any silicon panel) has two endpoints where the power is zero, namely open circuit where V = Voc and I = 0 and short circuit where V=0 and I = Isc.
0 times anything is zero and you cannot pull Isc and get Voc at the same time.
So to get maximum power you increase the currrent draw to decrease the working voltage from Voc until the product of the current and voltage reaches a maximum and pulling any more current gives less power. That point is the Maximum Power Point, Vmp and Imp.
Vmp is very weakly dependent on the amount of sunlight coming in while Imp is closely proportional to the amount of light.

Thanks I think I just about follow
My panels lack actual ratings and only have working voltage/current. altho if I look hard on the internet I may find the exact same panels, which I purchased second hand.
The figures must be out there somewhere

[Sunking]

Your panels only list Pmax, Vmp and Imp which you cannot measure. All you can measure is Voc and Isc which are significantly higher than Vmp and Imp.

[Rixsta]

I just had a look elsewhere on this forum and I found this ...

You need to also add temperature coefficient to the Voc as it gets cold there and Voc will go up
Multiply VOC x 1.25 for worst case or calculate it by the given temp coefficient from the panel specs


If I did that on top of my Voc I'd get 176 volts which would fry controller right ? Although I don't plan on being any colder than England which in the morning would not generally go below 0 deg.

[Rixsta]

I just thought. My panels will be facing upright on the roof of the van this means morning sun will not be hitting them and by the time the sun does hit them it will no longer be as cold outside.

[Sunking]

I just had a look elsewhere on this forum and I found this ...

You need to also add temperature coefficient to the Voc as it gets cold there and Voc will go up
Multiply VOC x 1.25 for worst case or calculate it by the given temp coefficient from the panel specs


If I did that on top of my Voc I'd get 176 volts which would fry controller right ? Although I don't plan on being any colder than England which in the morning would not generally go below 0 deg.

With a prime number of panels (3 is a prime number) only gives you two configurations of all in parallel or al in series. There is no other option. If 3 panels in series is too high of a voltage for the controller you only have one other option of all in parallel.

Not too big of a deal in a RV with a 12 volt system using 24 volt battery panels. It just means larger wire, 3 times larger. You will need 6 mm^2 wire or 2.6 mm diameter cable. AWG = 10 AWG