Florida Solar Design Group

PV String Sizing Calculator

Works out how many modules go in a string, and whether that string is safe on the MPPT channel you are putting it on. Pick a module from the full CEC approved list, enter your site temperatures, and Voc, Vmp, Isc and Imp are checked against every channel on the inverter.

  • All 22,138 CEC approved modules, searchable by manufacturer, model and wattage
  • Cold weather Voc correction to NEC 690.7, from your own site temperatures
  • Save a design, reopen it later, and hand over a PDF with a wiring schematic
22,138Modules
291Manufacturers
690.7Cold correction

Create a free account

The calculator is free to use. An account is what saves your work.

Screens from the live calculator
01 / 04
The string sizing calculator with a module and a Tesla Powerwall 3 selected.
Module search filtered to Qcells, showing the selected module's specifications beside the inverter.
String configuration table with pass, warn and fail results, and a design alert naming the failing string.
String schematic showing five strings wired into the inverter's MPPT inputs with a paralleled pair.
Library

The whole CEC list

All 22,138 approved modules, searched on the server so the entire list stays reachable without loading it into the page.

Checks

Per channel, not per inverter

Every MPPT channel carries its own voltage and current limits, including paralleled pairs like the Powerwall 3's tied inputs.

Temperature

Corrected for your site

Voc at your record low ambient, Vmp at cell temperature under load, using the module's own temperature coefficients.

Arrays

A different module per MPPT

Run one panel on one channel and a different panel on the next, the way a re-roof or an add-on usually lands. Every string on a given MPPT stays the same module.

Projects

Saved and shareable

Name a design, reopen it, duplicate it for the next roof, and send it to someone with a link.

Output

Ready for the plan set

A one page report and the string schematic, exported together as a PDF you can hand straight over.

PV String Sizing Calculator

Module

Inverter

Site Conditions

Look up ASHRAE design temperatures for this site Use the Extreme Annual Mean Minimum Dry Bulb for Tmin, and the 2% Design Dry Bulb for Tmax.

String Configuration

Str Inverter MPPT Channel Module Mods/Str Par Voc max (V) Vmp min (V) Isc tot (A) Imp tot (A) W/str V Vmp Isc Imp Result
Design Alerts: Select an inverter and configure strings.

System Summary

Total Modules 0 kW DC 0.000
kW AC 0.000 DC/AC Ratio -
% of Max DC - Status -

String Layout

Calculation Basis
  • Voc_max = Voc_STC × [1 + (B_Voc / 100) × (T_min - 25)] × modules per string. Per NEC 690.7 correction for minimum ambient.
  • Vmp_min = Vmp_STC × [1 + (B_Vmp / 100) × (T_cell_max - 25)] × modules per string. T_cell_max = max ambient + mount adder.
  • Isc_tot = Isc_STC × [1 + (a_Isc / 100) × (T_max - 25)] × strings in parallel. For NEC 690.8(A)(1) wire and OCPD sizing, multiply by 1.25 separately.
  • Imp_tot = Imp_STC × strings in parallel. Conservative for MPPT current check.
  • Per-string checks: OK (green) = comfortable, WARN (amber) = within 5% of limit, FAIL (red) = over limit, "?" = channel not in library.
  • Result priority: FAIL > DUP (duplicate channel) > "?" (missing channel) > WARN (margin tight) > PASS.
  • Paralleled MPPTs (Tesla PW3 MPPT1+2, MPPT5+6) appear as separate channel entries with doubled current limits. They are excluded from the MPPT count shown in the inverter specs.
  • B_Voc and B_Vmp should be NEGATIVE. a_Isc should be POSITIVE. Sign errors in the module library silently invert the correction.