Solar & DC

PV String Voltage Calculator (Cold Voc & Max Modules)

Find the worst-case cold open-circuit voltage of a PV string and the maximum number of modules that stays within the inverter’s DC voltage limit. Cold mornings — not hot afternoons — set the highest string voltage, and exceeding the inverter limit can destroy it. Use the module and inverter datasheet values.

ⓘβ Voc is the negative temperature coefficient of open-circuit voltage from the module datasheet (typically −0.25 to −0.35 %/°C for crystalline silicon).
Cold Voc per module
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Max modules per string
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String cold Voc (entered)
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Against inverter limit
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Why cold weather sets the limit

A module’s open-circuit voltage rises as temperature falls. The coldest expected cell temperature — a clear, freezing dawn before the array warms — produces the highest Voc, and that is the voltage the inverter and all DC components must withstand:

Voc(cold) = Voc,STC × [ 1 + (β / 100) × (Tmin − 25) ] Max modules = floor( Vdc,max / Voc(cold) )

β is negative, and Tmin is below the 25 °C reference, so the correction raises the voltage above the datasheet figure. Sizing a string on the STC voltage instead of the cold voltage is a common and dangerous error — it can push the array past the inverter’s maximum on the first cold morning.

⚠Use the lowest credible cell temperature. Standards base this on the site’s record low ambient. The string must stay under the inverter maximum at that temperature with the string as designed, including any future extension.

Frequently asked questions

Do I use record low or average low temperature?

The lowest expected cell temperature, normally derived from the site’s record low ambient. A milder figure understates the peak voltage and risks exceeding the inverter limit.

Which voltage does the inverter limit refer to?

The absolute maximum DC input voltage — not the MPPT window. The cold-temperature string Voc must stay below this absolute maximum; the operating voltage should also sit within the MPPT range for good yield.

Does the same voltage size other DC parts?

Yes. The cold string Voc sets the minimum voltage rating for the DC isolator, fuses, SPD (Ucpv), and cable — not just the inverter.

Build the DC side

With the string voltage fixed, size the combiner, isolator, fuses, and SPD to the same cold Voc.

Explore PV combiner boxes Get a free quote

Related products: PV combiner box · DC MCCB · SPD

Standards referencedIEC 60364-7-712 (PV systems) · IEC 62548 (PV array design) · module & inverter datasheet limits.

Estimate for guidance only. Final string sizing must use the module and inverter datasheet values, the site’s lowest expected temperature, and be verified by a qualified person.