Borderline Should I Do it ?

[Nobadays]

My plan is to add 3 more panels to my current system.... right now I have 6 x 245 watt in a 3s2p configuration. My plan is to add 3 more 250 watt panels that have almost exactly the same specs. It is the top end I am concerned about.... possible max output. If I understand mixing panels correctly, I will see the output of the lower wattage/amperage panels.... I read the lower panels will bring down the output of the others. For reference, the current 6 are combined on the roof and brought down to a breaker, the additional 3s will be brought to a second breaker in the same box, so there combined with the other array. So 3s3p.

I'm using a Schneider 60/150 SCC 48v system
The SCC manual has these statements: "Although the solar charge controller can harvest a maximum of 3500W, the PV array size can be as high as 6720 W (based on 48A× 140 Voc = 6720 W)." And: "Do not allow the PV array voltage to exceed 150 Voc (open circuit voltage) under any conditions. The array Isc (short circuit current) must not exceed the 60 A input current rating of the solar charge controller at any time."

Specs....
245: VOC = 37.5 VMP = 30.2 ISC = 8.68 IMP = 8.13
250: VOC = 37.6 VMP = 30.3 ISC = 8.85 IMP = 8.27

Using a great simple formula "onemorebattery" gave me on another question, here is what I get FOR THE MAX. Normal output is easily within spec...

250 watt panels: 3 * 37.6 * 3 * 8.85 =2,995 watts - 2,995/48 = 62 amps

If I am correct about mixing panels... I could expect max output equal to the 245 watt panels.

245 watt panels: 3 * 37.5 * 3 * 8.68 = 2,930 - 2,930/48 = 61 amps

I can't find a Schneider string calculator (apparently you can no longer download it from their site) so I used the one online by Midnite Solar and according to that calculator I am actually way lower. Their numbers hardly resemble my other calculations at all! Here is what I get using the specs off the 250 watt panels:

Rated power: 2250
Anticipated power @ 93F = 2160
Current: 24.81A
Battery charging current @ 57.6 = 39.1A
VMP = 90.9V
VOC = 112.8V
VMP @ -33F = 115.8V
VOC @ -33F = 135.6V

Any thoughts by those smarter than me when it comes to solar math ( not hard to do!) ??
If my first numbers are correct... would you hook them up anyway?

Thank you!

 

[MichaelK]

You're making several math errors. First, is that you are using the open circuit voltage to do your power calculations. The open circuit voltage is the "potential" voltage when it's not connected to a load. The second mistake is that you used the open circuit amperage, and not the load amperage (IMP)
You should be using the following per string...

250 watt panels: 3 * 30.3 * 8.27 = ~752 watts 752/48 = 15.66 amps

So, expect no more then 30-31 amps with your two arrays. BTW, the panel specs for the third string are insignificantly different from 1 and 2. There's likely to be more voltage difference from the array that's the longest wiring distance to the controller.

Here's one more parameter I can throw out to add to your calculation. In the real world, your panels are not likely to produce STC specs. I routinely de-rate my panels by 85 percent to get a real-world value. To add to your formula, it looks like this...

250 watt panels: 3 * 30.3 * 8.27 * 0.85= ~639 watts

So, with three strings, you will be very lucky to exceed 2000 watts at noon on a really good day.

 

[Nobadays]

Thank you! I don't doubt your math... I like it actually, but I was under the impression that you should use the VOC and ISC for planning purposes as this would represent the Maximum possible output. I had done the math using the VMP and IMP and actually arrived at the ~same numbers you had. I did not derate the output. If this is in fact the correct way to calculate the strings.... by VMP/IMP, I'm good to go!

 

[MichaelK]

I'm sorry, but I still don't think you understand. VOC and ISC are NOT maximal PRODUCTION values. They are the maximal values reached when there is NO flow of power. Think of the voltage as being like the pressure in a water pipe. If the valve is completely shut, then water pressure is at maximum. When you suddenly open the valve, the pressure of the flowing water will drop. Electrons flowing in current behave the same way.

Why VOC is important is when a unit first switches on, for the faction of a second all the electrical traces on the circuit board are exposed to the higher than normal voltage, but in a fraction of a second the voltage drops down. It is in this first blink of an eye that the electronics get damaged. The reason why there are voltage calculators is because the voltage a panel puts out is weather dependent. The colder a panel gets, the higher the voltage. The morning first ray of light falling on a panel in the middle of winter is when voltage gets the highest, and that is what your system is supposed to be designed for.

 

[Nobadays]

You write: "Why VOC is important is when a unit first switches on, for the faction of a second all the electrical traces on the circuit board are exposed to the higher than normal voltage, but in a fraction of a second the voltage drops down. It is in this first blink of an eye that the electronics get damaged."
I guess you are right that I don't understand. I do think I understand the difference between production... power flowing and open circuit voltage/current. Isn't it that fraction of a second spike to the SCC what should be planned for? Right when the first sunlight on a cold winter morning hits the panels? This is why I used VOC and ISC. Are you saying then that this spike would not reach the SCC!

Thank you for helping me understand this!

 

[Craig]

Voltage spike needs to be accounted for extra watts produced by that spike should not hurt anything.

As long as Voc is not higher than SCC max input you will be ok.

 

[Nobadays]

Craig.... the VOC should not be above 112.8V with a 3s3p array.

Interesting doing the math on the quote from Schneider's manual... "the solar charge controller can harvest a maximum of 3500W... " 3500W/48V = 72.91A so apparently the SCC can handle a pretty big spike?

Thank you!

 

[Craig]

Craig.... the VOC should not be above 112.8V with a 3s3p array.

Interesting doing the math on the quote from Schneider's manual... "the solar charge controller can harvest a maximum of 3500W... " 3500W/48V = 72.91A so apparently the SCC can handle a pretty big spike?

Thank you!

Yes that is what it can harvest although you can feed it more than that. How much more is a question. I currently have plans to send double in the summer due to bad winter conditions. If things get hot or unstable I will just add another SCC. But going over by 25% in watts to the SCC I dont think will matter. Always as long as Volts are not over.

 

[Nobadays]

Yes... they indicate you can feed it up to 6720 watts... Crazy! That would be 140A @ 48V if I'm getting of this math correct. That is more than double the rated 60A for the SCC. I am guessing the circuitry chops the amps off at a specified point, not allowing them to come through. That's a guess though!