Noob Question - Equal Number of Panels per String?

[tom3096]

Thanks for all the great info. I'm going to drum up at least one more solar panel of the same model so I can keep the number equal per string. Luckily I'm seeing used Kyocera gt200 panels on craigslist, etc. Just might have to take a little drive. Currently have one additional panel wired up on each string and seems to be working fine.

[foggysail]

Agreed. But adding the 13th panel will not increase power production.

In a series string the current available is determined by the current available from ALL the panels. If you make one string 12 panels long and one string 13 panels long, your MPPT algorithm will do one of two things:

-It will search for the maximum power point, and find it at the place where the current is highest - which will be at the Vmp of the string of 12 panels. Thus you will see the same power as if both strings had 12 panels.

-It will do a search and get stuck on the local maximum caused by the string of 13, in which case you will see the power from the string of 13 and almost no power from the string of 12 (since you are now operating at a voltage that's 8% higher than that string's maximum power point.)

So if you are lucky, you will still see the power from the 12x2 panels, and no new power.

BTW that is why modern higher power string inverters have more than one MPPT channel - so if you do have two do, say, three strings of 10 and two strings of 13, it will still work and accurately track the maximum power point for both sets. If tom3096's inverter has a spare MPPT channel, then he will have no problem with mismatched strings (again as long as he heeds current and voltage limits.)

Sounds good and maybe you are right. I had no desire for DC for numerous reasons including arc extinguishing, dangerous voltages, metal tubing, and satisfying NEC shutdowns which to my understanding require optimizers to do that. I upgraded my inverters (IQ8+ to IQ8A) about 5 days before the installation to hopefully eliminate power clipping. My system which has 28 Canadian 400W black panels was completed on September 9th. Tomorrow the Town's buiding inspector I expect to be here at 8AM and then I have to wait for the electrical guy. I will be surprised if I am grid tied by mid month. I am not a Greenie, I just want to get away from exorbitant electric bills.

[jflorey2]

Sure, parallel strings should have an equal number of panels. But even with an odd panel creating a difference in Vmp/string, power will still be produced.

Agreed. But adding the 13th panel will not increase power production.

In a series string the current available is determined by the current available from ALL the panels. If you make one string 12 panels long and one string 13 panels long, your MPPT algorithm will do one of two things:

-It will search for the maximum power point, and find it at the place where the current is highest - which will be at the Vmp of the string of 12 panels. Thus you will see the same power as if both strings had 12 panels.

-It will do a search and get stuck on the local maximum caused by the string of 13, in which case you will see the power from the string of 13 and almost no power from the string of 12 (since you are now operating at a voltage that's 8% higher than that string's maximum power point.)

So if you are lucky, you will still see the power from the 12x2 panels, and no new power.

BTW that is why modern higher power string inverters have more than one MPPT channel - so if you do have two do, say, three strings of 10 and two strings of 13, it will still work and accurately track the maximum power point for both sets. If tom3096's inverter has a spare MPPT channel, then he will have no problem with mismatched strings (again as long as he heeds current and voltage limits.)

[foggysail]

As long as you don't exceed the maximum voltage, current and wattage limits for the inverter - nothing at all. But that still doesn't let you use that 13th panel.

Yes, as long as you keep all the above in mind. If you are going to a shorter string you have to calculate MPPT range, and make sure that even in hot weather (which reduces output voltage of the panels) you do not go below the minimum MPPT tracking voltage. If you do you will lose power.

Sure, parallel strings should have an equal number of panels. But even with an odd panel creating a difference in Vmp/string, power will still be produced. If I was trying to squeeze as much power as I could get from a system I would not hesitate to add an odd numbered panel in a series/parallel system while keeping the parallel panel count as equal as possible.

Other than my above comment I do agree with the suggestions in your post. I just had a new AC system installed and am waiting for activation.

[jflorey2]

What prevents 12 panels from being split into two 6 panels in parallel?

As long as you don't exceed the maximum voltage, current and wattage limits for the inverter - nothing at all. But that still doesn't let you use that 13th panel.

any number of panels as I see it connected in series/parallel arrangements

Yes, as long as you keep all the above in mind. If you are going to a shorter string you have to calculate MPPT range, and make sure that even in hot weather (which reduces output voltage of the panels) you do not go below the minimum MPPT tracking voltage. If you do you will lose power.

[foggysail]

Only if it can handle the excess input wattage. Some inverters can't and burn up.

Absolutely!!! Again, right on!!! That earned a +1

[SunEagle]

AAHHhhhhhh!!!! RIGHT ON but I thought the problem was exceeding the inverter's voltage rating. But even with the maximum power being exceeded, it would overall provide greater output during times with less sun and until clipping occurs

Only if it can handle the excess input wattage. Some inverters can't and burn up.

[foggysail]

You still have a maximum wattage input allowed by the inverter. So paralleling them may not be a great answer.

AAHHhhhhhh!!!! RIGHT ON but I thought the problem was exceeding the inverter's voltage rating. But even with the maximum power being exceeded, it would overall provide greater output during times with less sun and until clipping occurs

[SunEagle]

And in my above post the count does not have to be limited to 12 panels, any number of panels as I see it connected in series/parallel arrangements that avoid the maximum inverter input voltage

You still have a maximum wattage input allowed by the inverter. So paralleling them may not be a great answer.

[foggysail]

And in my above post the count does not have to be limited to 12 panels, any number of panels as I see it connected in series/parallel arrangements that avoid the maximum inverter input voltage

[foggysail]

If your inverter has two separate MPPT channels - AND your worst case Voc is lower than the limit of the inverter - then no worries.

If your inverter has only one MPPT channel then go with 2 new panels instead of 3 - add one to each string. Again assuming your worst case Voc is lower than the limit of the inverter.

You cannot just parallel them. That will do nothing.

Note that to calculate worst case Voc you cannot just use the panel's STC Voc rating. You have to figure out the coldest it will ever get, then correct the Voc rating for that. Add them all up. If you are still under the inverter's limit you are good to go.

From my initial calculations you are pretty close on maximum voltage. 12 panels at STC will be 396 volts. If the coldest you get is 0C (which we get here in San Diego) AND you have high tempco panels that will mean 443 volts, cold day / grid outage. 14 panels in a string would be 517 volts and you risk blowing the inverter. 13 panels in a string would be ok UNDER THOSE CONDITIONS.

But you have to do the math.

What prevents 12 panels from being split into two 6 panels in parallel? At most as far as I can see is that Vmp may be missed but so can it be with unlike panels placed in series. Those panels will still provide power even if operated at a point that does not satisfy Vmp

[jflorey2]

I have an older system with two strings of 12 200w panels each. I bought three used panels off a disassembled install that are the same model. I would like to connect to them to the existing system. Is it possible to connect one panel to existing string 1 and two panels to existing string 2? Thanks for any advice...

If your inverter has two separate MPPT channels - AND your worst case Voc is lower than the limit of the inverter - then no worries.

If your inverter has only one MPPT channel then go with 2 new panels instead of 3 - add one to each string. Again assuming your worst case Voc is lower than the limit of the inverter.

You cannot just parallel them. That will do nothing.

Note that to calculate worst case Voc you cannot just use the panel's STC Voc rating. You have to figure out the coldest it will ever get, then correct the Voc rating for that. Add them all up. If you are still under the inverter's limit you are good to go.

From my initial calculations you are pretty close on maximum voltage. 12 panels at STC will be 396 volts. If the coldest you get is 0C (which we get here in San Diego) AND you have high tempco panels that will mean 443 volts, cold day / grid outage. 14 panels in a string would be 517 volts and you risk blowing the inverter. 13 panels in a string would be ok UNDER THOSE CONDITIONS.

But you have to do the math.

[foggysail]

The typical limitation of most charge controllers is voltage. Current will only be pulled from the panels if the controller can process it. Therefore Wattage of panels can exceed the Wattage of a controller in most cases without damage, Best to verify with your charge controller. Clipping does not damage controllers, it is just the process a controller uses to limit current. DC to AC ratios in excess of 1 to 1 are common. In the OPs case 9 panels per string should be no problem. His existing DC ratio is 1.2 to 1 and no clipping reported. The new panels will bring it to 1.35 to 1. At that ratio it is not likely that he will see any clipping because the peak he has seen with 4800 Watts of panels is 3600 Watts. The inverter does say a maximum of 4800 Watts DC but neither the voltage or current limits have been exceeded. He will get more overall production because the output will ramp up quicker and ramp down later in the day.

Help me with this. DC systems I know little about so my questions could be in left field or someplace. I see no reason why a configuration has to exceed the inverter's upper voltage limits when the panels are all connected in parallel. The inverter should IMHO force operation to a voltage that provides the maximum output power. All solar panels are current sources..........they will output an almost constant current depending of course on the sun and a panel's electrical features regardless of whatever the output voltage is. Personally I would avoid series connections because I would want to avoid current impairment caused by a single panel's restrictions. But again, I am not familiar with DC and there maybe reasons for series connections

EDIT: If a DC system has a grid connection most likely optimizers (for shut down) and they should be able to satisfy an inverter's voltage limitations.
Shut down features are necessary by code.

[Ampster]

The typical limitation of most charge controllers is voltage. Current will only be pulled from the panels if the controller can process it. Therefore Wattage of panels can exceed the Wattage of a controller in most cases without damage, Best to verify with your charge controller. Clipping does not damage controllers, it is just the process a controller uses to limit current. DC to AC ratios in excess of 1 to 1 are common. In the OPs case 9 panels per string should be no problem. His existing DC ratio is 1.2 to 1 and no clipping reported. The new panels will bring it to 1.35 to 1. At that ratio it is not likely that he will see any clipping because the peak he has seen with 4800 Watts of panels is 3600 Watts. The inverter does say a maximum of 4800 Watts DC but neither the voltage or current limits have been exceeded. He will get more overall production because the output will ramp up quicker and ramp down later in the day.

[SunEagle]

Thanks for the info. As I mentioned, I currently have 3 additional panels of the same model that I wish to add to my 24 panel array (new total would be 27 panels). It looks as if I have some room to increase DC input. The max power generation I've ever seen from the array is approx. 3600w. Am I correct that I could feasibly increase DC input by up to 1200 additional watts?


My inverter specs are:

Fronius Model Number IG 4000
AC output 4000 watt
Max. DC Power (STC) 4800 W
Max DC Voltage 500 V
MPPT Voltage Range 150V - 450V
Max. Input Current 26.1 A
Inverter Efficiency 95.2%


My panel specs are:

- Kyocera 200GT (24 panels)
- Max power: 200w
- Max voltage: 26.3v
- Open circuit voltage: 32.9v
- Max power current: 7.61a
- Short circuit current: 8.21a

The choice is yours to make. Adding wattage can either burn up an inverter or cause clipping.