Noob Question - Equal Number of Panels per String?

[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]

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]

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.

[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.