String voltage calculator

How many panels can you safely wire in series? A panel's open-circuit voltagerises as it gets colder, so a string that measures fine in summer can exceed an inverter's PV input limit on a January morning and destroy it. This corrects Voc for your record low and checks the result against the inverter.

PanelInverter or charge controllerPanel Voc (V)Inverter max PV input (V)Voc temperature coefficient (%/°C)Negative. From the panel datasheet; typically −0.24 to −0.35.Record low temperature (°C)The coldest it has ever been at the array, not the average winter low.

How the number is worked out

Panel specifications are measured at 25°C. Open-circuit voltage has a negative temperature coefficient, so below 25°C it goes up:

Voc(cold) = Voc(STC) × [ 1 + (T_min − 25) × coeff / 100 ]

At −20°C a 37.2V panel with a −0.27%/°C coefficient reaches about 41.7V — 12% higher. Over a 12-panel string that is 54V of headroom you did not know you needed, and it is why strings sized on nameplate Voc fail in winter rather than at commissioning.

The calculator leaves the inverter's stated maximum as the hard ceiling and does not add a safety margin of its own. NEC 690.7 permits either this temperature-coefficient method or a table of correction factors; some AHJs expect the table. If you are pulling a permit, check which your inspector wants.

Exceeding an inverter's PV input voltage destroys it instantly and is not covered by warranty. This tool is a planning aid, not a substitute for the panel datasheet and the inverter manual. Verify both before wiring anything.