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Solar String Sizing Calculator

Works out how many panels can safely go in series, using the cold-weather voltage rise rather than the rated figure. This is the calculation that protects the charge controller, and the one most often done wrong.

V
V
%/°C

From the datasheet, usually around −0.25 to −0.35%/°C. Negative: voltage rises as it gets colder.

°C

Record low for your site, not the average winter temperature. This is a safety limit.

V

From the charge controller or inverter datasheet. Exceeding it destroys the unit.

V

Optional. MPPT controllers need the array meaningfully above battery voltage to start.

Maximum panels in series
— panels

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

Voc per panel when cold — V
String voltage when cold — V
String voltage at max power — V
Minimum panels to start charging — panels
Why cold matters —

The formula

Voc(cold) = Voc × (1 + tempCoeff × (Tmin − 25)) max panels = controller max input ÷ Voc(cold), rounded down

Panel specifications are given at 25 °C. Open-circuit voltage rises as temperature falls, so the dangerous condition is a freezing, sunny morning with the array unloaded — exactly when a string sized on the rated Voc will exceed the controller limit.

What to watch out for

The failure mode is specific and avoidable:

  • Voc rises as it gets colder, at roughly 0.3% per °C below 25 °C. At −10 °C that is about 10% above the rated figure.
  • The worst case is cold, sunny and unloaded — a clear winter morning before the controller starts drawing. That is when the string is at its absolute maximum voltage.
  • Exceeding the input limit destroys the controller, and it is not covered by warranty. Round the panel count down, never up.
  • Use the record low for your site, not the average winter temperature. This is a limit, not an estimate.

The minimum figure matters too: an MPPT needs the array voltage comfortably above battery voltage to begin charging, so too few panels in series means poor performance in low light.

Frequently Asked Questions

How many solar panels can I put in series?

Divide the controller's maximum input voltage by the panel's cold-weather Voc and round down. For a 150 V controller and panels reaching 55 V when cold, that is 2 panels — not the 3 the rated 49.5 V would suggest.

Why does cold weather raise panel voltage?

Semiconductor physics: as cell temperature falls, open-circuit voltage rises. Typically about 0.3% per °C below the 25 °C rating, so a −10 °C morning adds roughly 10%.

What happens if I exceed the controller voltage limit?

The controller is destroyed, usually immediately and usually outside warranty. It is the most common and most expensive DIY solar mistake.

What temperature should I use?

The record low for your site, not the winter average. The calculation is a safety limit, and it only has to be exceeded once.

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