PV String Sizing Calculator

🔌 PV String Sizing Calculator

Maximum and minimum modules per string for the inverter voltage window.

Module Voc (V)

Module Vmp (V)

Voc temp coeff (%/°C)

Site min temperature (°C)

Inverter max DC (V)

MPPT minimum (V)

Max cell temperature (°C)

Vmp temp coeff (%/°C)

Max Modules / String

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Min Modules / String

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Voc at Min Temp

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Max String Voc

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Vmp at Max Cell Temp

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⚠️ Max set by Voc at the record low temperature vs inverter max DC; min set by MPPT lower limit at high temperature. Always use the site record low and the module datasheet coefficient. Verify before professional use.

Standards & method

✓ Formula independently verified 12 July 2026

Governing standard

NEC 690.7

Clauses applied

V_oc at record-low ambient sets the maximum modules per string
V_mp at high cell temperature sets the minimum

Independently verified

12 July 2026 — Formula re-derived from the standard and checked numerically against worked reference cases.

Why this matters: The minimum is computed from hot-temperature V_mp, not the STC value. Using STC V_mp lets the string fall out of the MPPT window on a hot day.

Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.

PV String Sizing — Method

A string is a series chain of modules; its voltage must stay inside the inverter window in all weather. The worst case for high voltage is the coldest temperature, which raises open-circuit voltage: Voc_cold = Voc × [1 + (temp-coefficient ÷ 100) × (T_min − 25)]. The maximum modules per string is floor(inverter max DC ÷ Voc_cold). The minimum is set so the string still sits above the MPPT lower voltage when hot: ceil(MPPT min ÷ Vmp). Design between these two limits.

Frequently Asked Questions

How many panels can I put in a string?⌄

Maximum = floor(inverter maximum DC voltage / Voc at the coldest site temperature). Minimum = ceil(MPPT minimum voltage / module Vmp). Design the string length between these limits.

Why does cold weather matter for string size?⌄

Open-circuit voltage rises as temperature falls (negative temp coefficient). At the record low, a string can exceed the inverter maximum DC voltage and damage it, so the max length is set at the coldest temperature.

How do I correct Voc for temperature?⌄

Voc_cold = Voc x [1 + (temp coefficient %/C / 100) x (Tmin - 25)]. With Voc 49.5 V, coefficient -0.27%/C and Tmin -10 C, Voc_cold is about 54.2 V.

What sets the minimum string length?⌄

The inverter MPPT lower voltage. When modules are hot their voltage drops, so the string must have enough modules to stay above the MPPT minimum and keep tracking.

What temperature should I use?⌄

Use the site record low (ASHRAE extreme minimum) for the maximum-voltage check and a high cell temperature (e.g. 70 C) for the MPPT-minimum check, per the inverter manufacturer guidance.

PV String Sizing Explained

Modules wired in series form a string whose voltage must live inside the inverter MPPT window across the full temperature range of the site. Two limits define the valid string length.

The cold limit (maximum)

Open-circuit voltage rises as it gets colder. At the record low temperature the string Voc must stay below the inverter maximum DC input, or the inverter can be damaged. That fixes the maximum modules per string.

The hot limit (minimum)

When panels are hot their operating voltage falls; the string must keep enough modules to stay above the MPPT lower voltage so the inverter can still track. Combine with inverter sizing and array sizing for the full design.

Related: Inverter Sizing, Panel Sizing, Battery Bank.

PV String Sizing Calculator

PV string sizing — Quick answer