wattbench
Engineering Tool

Solar String Sizing Calculator

Work out how many panels can go in series on a string, within the inverter's MPPT window and under the maximum system voltage, using temperature-corrected module voltages.

Sources listedReviewed July 2026Our methodology

Inputs

View result

From the module datasheet. The STC value. No default is assumed.
From the module datasheet. The STC value. No default is assumed.
From the module datasheet. Type it as printed. The sign is normalised. No default is assumed.
From the module datasheet. The Vmp coefficient, not the power coefficient. Type it as printed. No default is assumed.
From your design for this site. Your minimum cell design temperature. No default is assumed.
From your design for this site. Your maximum cell design temperature. No default is assumed.
From the inverter datasheet. No default is assumed.
From the inverter datasheet. No default is assumed.
From the inverter datasheet. No default is assumed.

Live result

Edit inputs
Enter the module, temperature and inverter values to size a string.
Key takeaways
  • Cold mornings raise a module's open-circuit voltage, so the coldest expected temperature sets the maximum panels you can safely put in series.
  • Hot afternoons lower the operating (Vmp) voltage, so the hottest expected temperature sets the minimum string length to stay within the inverter's MPPT window.
  • The result is a calculated range of modules per string, an estimate to sanity-check a design, not a substitute for a licensed system designer.

How it works

A module's voltage moves with temperature. The colder it gets, the higher the open-circuit voltage (Voc) climbs, so the maximum string length is set by the coldest moment, when Voc peaks:

Voc(cold) = Voc × (1 + βVoc × (T_cold − 25)), where βVoc is the module's documented Voc temperature coefficient (negative, in %/°C).

  • Maximum modules is the largest count whose cold-temperature string voltage stays under both the inverter's maximum system voltage and its MPPT ceiling: floor(V_max ÷ Voc(cold)).
  • Minimum modules keeps the hot-day operating voltage (Vmp) above the inverter's MPPT floor: ceil(MPPT_min ÷ Vmp(hot)).

Vmp uses its own documented temperature coefficient for the MPPT checks; the tool no longer substitutes the Voc coefficient. Enter the applicable equipment limits and design temperatures, then confirm the final design against AS/NZS 5033 and the exact product documentation with a licensed system designer.

Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.

Source ledger3 sources

Best next action

AS/NZS 5033Do this next

Did this help you move the job forward?

Tell us what is missing
Good to know

Frequently asked

Is this calculator free to use?
Yes. The string sizing calculator is free and needs no sign-up. It runs entirely in your browser.
Is my data stored or sent anywhere?
The calculation runs in your browser. Keep with this job stores a dated snapshot on this device without an account. Nothing is sent to Wattbench unless you sign in and deliberately select Save result. Local snapshots are not a backup.
Which temperature coefficient does it use?
It uses the entered Voc coefficient for open-circuit voltage and a separate entered Vmp coefficient for MPPT operating voltage. Confirm both against documented module data. The result is an estimate, not a substitute for a licensed designer.
Keep calculating

More tools

Design Check

Multi-Face MPPT Allocation Planner

Allocate strings from several roof faces across one inverter's MPPT inputs, and check temperature-corrected voltages and summed currents against the limits you enter.

Cold Voc · Vmp window · summed Imp and Isc · DC:AC ratio

Design Worksheet

Module & Inverter Compatibility Worksheet

Compare entered string voltage and current against documented inverter input limits at the design conditions you supply.

String voltage · voltage headroom · current check

Identification Tool

Single or Three-Phase Supply Check

Use visible main-switch or meter clues to identify a likely single- or three-phase supply without opening the switchboard.

Likely supply · visible clue · safety check