Noob Question - Equal Number of Panels per String?

[tom3096]

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...

[SunEagle]

It really depends on if the inverter can handle the additional wattage or not. Usually there is a maximum allowed DC wattage for each input that is stated in the Specs for each inverter and what orientation each string is positioned to capture the sun.

[bcroe]

Yes check inverter capacity. Does it go into clipping in good sun?

Here I have 12 panel strings, because more would produce too
much voltage for the inverter under some situations (open circuit
in very cold weather). You may have the same restriction, check
it out. If so, you could run 3 strings of 9 panels each. In good sun,
I would use a clamp on DC ammeter to compare the strings, to
assure you have not connected a weak panel in a string.

Bruce Roe

[tom3096]

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

[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.

[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.

[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.

[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]

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

[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

[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]

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]

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]

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

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

[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.