Why correct Voc to the coldest temperature?

A panel's open-circuit voltage rises as temperature falls, and the highest voltage of the day occurs on the coldest morning at first light with no load. NEC 690.7 requires sizing the maximum string voltage at the record-low expected ambient so the string never exceeds the inverter or equipment voltage rating.

Why correct Vmp to the hottest temperature?

The maximum-power-point voltage falls as the cells heat up. On a hot day the string Vmp drops, and if it falls below the inverter's minimum MPPT voltage the inverter stops tracking and production collapses. Sizing the minimum string length against the hot Vmp keeps the string inside the tracking window.

How is the temperature correction calculated?

Corrected voltage equals the rated voltage plus the temperature coefficient times the difference between the cell temperature and 25 C standard test conditions. The Voc coefficient is negative, so a lower temperature raises Voc. Coefficients are given per degree C, either in volts or as a percent of the rated value.

What cell temperatures should I use?

For the cold case use the extreme record-low ambient for the site, often from the ASHRAE extreme minimum design temperature. For the hot case use a high cell temperature of roughly 65 to 75 C, which accounts for ambient plus solar heating of the module above air temperature.

What sets the absolute maximum string voltage?

The cold-corrected Voc times the number of modules must stay at or below the inverter maximum DC input voltage, which is also the system voltage rating, commonly 600 V for residential or 1000 to 1500 V for commercial. Exceeding it can damage the inverter and violates NEC 690.7, so the cold Voc almost always sets the maximum string length.

PV String Sizing Calculator (Panels per String)

Sizing a PV string means finding how many panels can be wired in series so the string voltage stays below the inverter’s maximum on the coldest morning while still staying above the inverter’s minimum tracking voltage on the hottest afternoon. This calculator does both temperature corrections and returns the valid range of modules per string.

How it works

Module voltages shift with cell temperature relative to 25 °C STC. The tool applies the datasheet temperature coefficients:

Voc(cold) = Voc_stc + βVoc × (T_cold − 25)
Vmp(hot)  = Vmp_stc + βVmp × (T_hot  − 25)
max modules = floor( inverter max DC voltage / Voc(cold) )
min modules = ceil ( inverter min MPPT voltage / Vmp(hot) )

Because the Voc coefficient is negative, a cold temperature raises Voc and sets the maximum string length per NEC 690.7. The hot-corrected Vmp sets the minimum length so the inverter keeps tracking. Any whole number between the two is a valid string size.

Example and notes

A module with 40 V Voc, 32 V Vmp, a −0.12 V/°C Voc coefficient and a −0.10 V/°C Vmp coefficient, at a −15 °C record low and 70 °C hot cell temperature, gives a cold Voc of about 44.8 V and a hot Vmp of about 27.5 V. Against a 600 V max input and a 200 V minimum MPPT voltage, the string can hold up to 13 modules (max) and needs at least 8 (min). Always use the record-low extreme, not the average winter low, and confirm the inverter datasheet voltage limits.