Solar Base

String voltage calculator

Cold-morning Voc against the inverter's maximum, hot-day Vmp against its MPPT minimum, and the module range that fits.

Use the Pmax coefficient if Vmp's is not listed
The coldest morning expected at the site
Cells on a roof run well above air temperature
Most modules per string 11 modules fit 11 5 to 11 modules
Voc per module, cold53.10 V
String Voc, cold584.1 V of 600 V
Vmp per module, hot35.98 V
String Vmp, hot395.8 V (MPPT minimum 160 V)

How it's worked out

Voc rises as cells get colder: module Voc × (1 + coefficient × (lowest temperature − 25 °C)). The most modules is the inverter's maximum DC voltage divided by that, rounded down. Vmp falls as cells heat up, so the fewest modules is the MPPT minimum divided by the hot Vmp, rounded up. 49.5 × (1 + -0.26% × (-3 − 25)) = 53.10 V

Figures from your module and inverter datasheets. Checked 21 October 2026. A check, not a design record: the design follows AS/NZS 5033 and the inverter maker's string sizing.

Solar Base keeps every job on one record, from the first enquiry to the meter alteration, and does the portal work for you. See how it works

Why the cold morning sets the limit

A module's voltage climbs as it gets colder, and it is highest at first light on the coldest morning of the year, when the cells are cold and the inverter has not started loading the string. That open-circuit voltage, summed along the string, must stay under the inverter's maximum DC input or the inverter can be damaged.

The other end matters too. On a hot afternoon the string's working voltage falls, and if it drops below the inverter's MPPT range the inverter loses output. The module count has to sit between the two.

Voc(cold) = Voc × (1 + coefficient × (Tmin − 25))

Worked example

Module49.5 V Voc, −0.26%/°C
Coldest morning−3 °C: 53.10 V per module
Inverter600 V maximum DC input
600 ÷ 53.10 = 11.311 modules: 584.1 V cold

Questions installers ask

Why 25 °C?

Datasheet voltages are measured at standard test conditions, with the cells at 25 °C. The coefficient says how far the voltage moves for each degree away from that.

What lowest temperature should I use?

The lowest expected at the site, from long-term weather records, and note where the figure came from in your design record.

Can I use the Pmax coefficient for Vmp?

When the datasheet does not list a Vmp coefficient, the Pmax coefficient is a reasonable stand-in for the hot-day check, and errs on the low side.

A tool does one sum. Solar Base does the whole job.

The STC count, the NMI, the meter alteration and the invoice, worked off one job record, with the last step left to you.

Register interest
11 modules max
5 to 11 modules
Show the breakdown