Why does cold weather raise PV voltage?

Crystalline-silicon modules have a negative temperature coefficient of Voc, meaning voltage rises as cell temperature falls. On a cold clear morning before the array warms up, open-circuit voltage can exceed the STC rating by 10 to 25 percent, which is why NEC 690.7 requires correction to the lowest expected temperature.

What is the NEC 690.7 correction method?

NEC 690.7 lets you either use a table of crystalline-silicon correction factors or the module's published temperature coefficient. This calculator uses the coefficient method: corrected Voc equals rated Voc times (1 plus coefficient-per-degree times the temperature difference below 25 degrees C).

Which temperature should I use?

Use the extreme annual mean minimum design dry-bulb temperature for the site, available from ASHRAE data or the manufacturer's table. It represents the coldest expected ambient temperature the array will see, which produces the highest Voc.

What happens if the corrected voltage exceeds the maximum?

If corrected string Voc exceeds the inverter or combiner maximum input voltage, you must remove one or more modules from the string. Exceeding the rating can permanently damage equipment and violates code, so the string must be resized.

Does the temperature coefficient sign matter?

Yes. Enter the coefficient as a negative value in percent per degree Celsius, exactly as the datasheet lists it (for example minus 0.29). The tool uses the magnitude to raise voltage below 25 degrees C, so a more negative coefficient yields a higher corrected voltage.

PV String Voc Temperature Correction Calculator

When the sun first hits a solar array on a freezing morning, the modules sit at ambient temperature with no load, and their open-circuit voltage spikes well above the nameplate rating. NEC 690.7 requires you to size series strings so this cold-temperature Voc never exceeds the maximum input voltage of the inverter or other equipment. This calculator applies the correction and tells you whether your string is safe.

How it works

The coefficient method from NEC 690.7 corrects rated Voc to the coldest expected temperature:

deltaT          = 25 - Tmin        (degrees C below STC, positive when cold)
correctedVoc    = Voc_stc × (1 + (|coeff %/°C| / 100) × deltaT)
stringVoc       = correctedVoc × number of modules

The module datasheet lists Voc at STC (25 degrees C cell) and the temperature coefficient of Voc, usually a small negative percentage such as minus 0.29 percent per degree Celsius. Because the coefficient is negative, falling below 25 degrees C raises the voltage. The string voltage is then compared with the equipment maximum input voltage.

Example and tips

A module rated 49.5 V Voc with a coefficient of minus 0.27 percent per degree Celsius, at a site minimum of minus 10 degrees C, corrects to about 54.2 V per module. A string of 12 modules reaches roughly 650 V, which must stay below a typical 600 V or 1000 V inverter limit. Always design to the ASHRAE extreme minimum, not the average winter low, and round the module count down if the corrected string voltage lands near the limit. Leave a margin so manufacturing tolerance and unusually cold mornings do not push you over.