Guides

How to check string voltage for cold weather

Published 8 October 2026. About 7 minutes to read.

A string that is comfortably inside the inverter's limits on a summer afternoon can go over them on a clear winter morning. Panel voltage rises as the cells get colder, and the coldest morning of the year is the one that matters.

Why voltage goes up when it is cold

A solar cell's voltage depends on temperature. Every datasheet gives a temperature coefficient for the open-circuit voltage (Voc), usually around −0.25 to −0.30 % per °C for modern panels. The minus sign means voltage falls as temperature rises, so it also means voltage rises as temperature falls below the 25 °C used for the datasheet values.

At sunrise on a cold, clear day the cells are close to the air temperature, there is already enough light to reach almost full open-circuit voltage, and the inverter has not started drawing current yet. That is when a string is at its highest voltage. If it is above the inverter's maximum DC input voltage, the inverter can be damaged and the warranty is usually void.

The calculation

For one panel:

Voc(cold) = Voc(STC) × (1 + coefficient ÷ 100 × (T_min − 25))

Multiply by the number of panels in series to get the string voltage, and compare it with the inverter's maximum DC voltage.

Use the lowest temperature the site is likely to see, not the average winter low. Local weather records or the record minimum from the nearest weather station are a safer basis than a regional average. Your local wiring rules may also say which temperature to use.

A worked example

Take a panel with Voc 49.5 V, Vmp 41.5 V, a Voc coefficient of −0.27 %/°C and a Vmp coefficient of −0.35 %/°C. The inverter allows up to 600 V DC and tracks between 150 V and 500 V. The coldest morning on site is −5 °C.

• Cold Voc per panel: 49.5 × (1 + −0.0027 × −30) = 49.5 × 1.081 = 53.5 V.
• Most panels in series: 600 ÷ 53.5 = 11.2, so no more than 11.

The MPPT window sets the other limits. On a hot afternoon the cells can run around 20 °C above a 40 °C air temperature, so take 60 °C:

• Hot Vmp per panel: 41.5 × (1 − 0.0035 × 35) = 36.4 V. To stay above 150 V you need at least 150 ÷ 36.4 = 4.1, so 5 panels.
• Cold Vmp per panel: 41.5 × (1 + 0.0035 × 30) = 45.9 V. To stay below 500 V when cold, 500 ÷ 45.9 = 10.9, so 10 panels.

So this inverter takes 5 to 10 of these panels per string. Eleven would be safe for the hardware but would push the operating voltage above the tracking range on cold days, which costs energy.

Don't forget the current

Each MPPT input also has a current limit, usually given twice: a maximum operating current and a maximum short-circuit current (Isc). Strings connected in parallel on the same MPPT add their currents. Two strings of a panel with Imp 9.65 A and Isc 10.4 A give 19.3 A and 20.8 A. An input rated for 16 A operating and 20 A short-circuit would clip the first and is over the second.

Bifacial panels and bright, cool conditions can push current above the datasheet values, so leave some margin.

Checking it in Lekker Sun

On the Equipment page, enter the panel and inverter values from their datasheets, then set the panels in series, strings per MPPT and the site's minimum and maximum temperatures. Lekker Sun works out the cold Voc, hot and cold Vmp and the MPPT currents and marks each check as passing, a warning or a fail.

This guide is general information to help you plan and compare options. It is not engineering, electrical or financial advice. Always follow the equipment manufacturer's instructions and your local wiring rules, and have a qualified installer check the design.