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.

What is the PV String Voc Temperature Correction Calculator?

Multiply module Voc by the NEC 690.7 cold-temperature correction factor at the lowest expected ambient temperature, then check the corrected string Voc against your inverter and equipment maximum input voltage. Prevents cold-temperature Voc exceedance. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

PV String Voc Temperature Correction Calculator

Name: PV String Voc Temperature Correction Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Worked example: sizing a string for a cold climate

Say a module is rated at 49.5 V Voc with a temperature coefficient of −0.27%/°C. The installation site (for example, Denver, Colorado) has an ASHRAE extreme minimum design temperature of around −18 °C.

deltaT         = 25 − (−18) = 43 °C
correction     = 1 + (0.27/100) × 43 = 1.116
correctedVoc   = 49.5 × 1.116 ≈ 55.2 V per module

For a 1000 V inverter maximum:

Maximum modules in series = floor(1000 / 55.2) = 18 modules

For a 600 V inverter:

Maximum modules = floor(600 / 55.2) = 10 modules

If the same calculation were done using the STC Voc without temperature correction (49.5 V), one might incorrectly size a 12-module string for a 600 V inverter. At −18 °C that string would produce 12 × 55.2 = 662 V, exceeding the 600 V rating and risking equipment damage.

Why the ASHRAE extreme minimum, not the average winter low

A “typical” cold day or even the average January minimum understates the worst-case voltage. NEC 690.7 requires the extreme annual mean minimum design dry-bulb temperature because the array will sit unloaded at ambient temperature on these mornings — no electrical load warming the cells — and even one exceedance event can permanently damage inverter input stages. ASHRAE tables provide this value by location; it is also available in manufacturer correction-factor tables grouped by state/region.

Table method vs. coefficient method

NEC 690.7 offers two approaches. The table method uses pre-computed crystalline-silicon correction factors by temperature range — simpler but less precise and only valid for standard c-Si modules. The coefficient method (used here) uses the module’s published datasheet value, works for thin-film and other technologies with different coefficients, and gives a more accurate result. Always use the datasheet coefficient when it is available.

Common mistakes

Entering a positive coefficient: The coefficient must be negative (e.g., −0.29%/°C). A positive entry would subtract voltage at cold temperatures, which is physically wrong.
Using average winter temperature instead of the ASHRAE extreme minimum: This underestimates corrected Voc, potentially by several volts per module.
Ignoring manufacturing tolerance: Modules within a rated spec can exceed nominal Voc by a small margin. Leave a buffer — many designers target 90–95% of the equipment maximum rather than sizing right to the limit.