Solar String Sizing Calculator (NEC 690.7)

Find a feasible modules-per-string window by checking cold-weather Voc against inverter max DC voltage and hot-weather Vmpp against MPPT minimum.

  1. Enter module electrical data Provide module Voc and Vmpp at STC plus temperature coefficients from the datasheet.
  2. Set site temperature extremes Enter site record low ambient and expected hot-cell temperature for worst-case voltage checks.
  3. Enter inverter limits Input inverter maximum DC voltage and MPPT minimum tracking voltage.
  4. Calculate string window Run the tool to get minimum and maximum modules per string and verify that the window is feasible.

How many panels can I put on one string for a 1000 V inverter?

It depends on cold-weather Voc. The calculator computes cold-adjusted Voc and divides inverter max voltage by that value to set the safe maximum module count.

Why does winter temperature reduce max modules per string?

Module voltage rises as cells get colder. A string that is safe at mild temperatures can exceed inverter max DC voltage on very cold mornings if not checked.

Why do I also need a minimum module count?

Hot conditions lower Vmpp. You need enough modules in series so the hot-day string voltage stays above the inverter MPPT minimum and the inverter can track power.

Can I mix different module models in the same string?

It is usually avoided because mismatched electrical characteristics reduce performance and can violate inverter design assumptions. Use matched modules in each string.

Should I use design heating temperature or record low for Voc checks?

Use the site record low (or AHJ-required extreme) for conservative voltage safety checks, not a mild average design temperature.

Does this replace the inverter manufacturer string tool?

No. Use this as a first-pass sizing screen, then confirm in manufacturer design software and final permit documents.

Calculators