I have a pole mounted array that was formerly set up as 16 Siemens M-75 , 48 watt panels (19.8 Volts short circuit, 15.9 volts rated). They were previously wired in a series of 16 panels running into a Fronius IG2000. The system is located in Northern NH and the memory on the inverter indicates that at some point the voltage was 348 volts. The panels are all 1995 vintage but a lot of them were stored for an unknown period of time. It can and does reach minus 30 F on rare occasions and generally the coldest days are the clearest.

I have recently added 4 panels to the array and wired them in two series strings of 10 as I was concerned that if I ratio the 348 volts for 16 panels, I get 435 volts peak for the new array if wired in one series string, which is out of the inverter tracking voltage range of 150 to 400 volts (max input rating is 500 volts). The two series strings arrangement seems to work but the operating voltage in warm weather has gotten very close to the lower end of the MPPT tracking lower limit of 150.

So the question of the day is if there is a significant difference in the MPPT tracking efficiency between operating on the low end of a MPPT range or operating near or at the top end of the range (if I rewired for a 20 panel string)?

I do realize that operating with one series string would halve the voltage and reduce the drop through the approximately 100 foot lead between the pole and the inverter but it is somewhat oversized to begin with, so it may not be significant . I expect that the winter time peak voltage probably occurs near sunrise and the panels will warm up somewhat in the sun before there is any significant generation so most likely the inverter will be near the 400 volt high end of the MPPT range.

Any opinions or suggestions?

efficiency is better with the lower voltage differentials, but the heat of summer may shift the array too low and no production to be had if it does slip too low. got at least 2 more you can throw on with at least 1 per string to up the minimum voltage? if not then find 2 pvs with a similar current rating and use them to add a bit to it.

I'm guessing you mean Voc (open circuit) as Vsc on a PV panel is always zero.