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How to size solar strings safely

The check that matters happens on the coldest morning of the year, before anything is drawing current.

Panels in series add their voltages. That total must stay inside the inverter's operating window at all times — and the dangerous moment is not the hottest day.

Step-by-step

  1. Enter the panel's Voc and its temperature coefficient.
  2. Enter your record low temperature.
  3. Enter the inverter's maximum input voltage.
  4. Read the maximum panels per string.

Cold increases voltage

Panel voltage rises as temperature falls, typically around 0.3% per degree below 25°C. On a clear morning at −10°C, open-circuit voltage can be 10–12% above the rated figure.

Open-circuit voltage — Voc — is what the panels produce before the inverter starts drawing current, so the highest voltage of the year occurs at first light on the coldest clear morning, at the moment when nothing is loading the string. That is the number the design must survive.

Exceeding it destroys inverters

Going over the maximum input voltage damages the inverter, and typically voids the warranty because the fault is a design error rather than a defect. It may work for years and fail on one exceptional morning.

Use the record low for your location, not the average winter minimum. This is a worst-case calculation and there is no benefit in optimism.

The lower bound too

Strings must also stay above the inverter's minimum tracking voltage on a hot day, when voltage is at its lowest. Too few panels means the inverter drops out of its range in summer and produces nothing at exactly the wrong time.

There is a window, not a maximum, and both ends need checking.

This calculation must be verified by whoever signs off the installation. Getting it wrong is expensive and it is their professional responsibility — but understanding it means you can ask whether the record low was used.

Frequently asked questions

Why does cold weather matter for solar?

Panel voltage rises as temperature falls. The year's highest voltage occurs at first light on the coldest clear morning, before the inverter draws current, and that is what the string must not exceed.

What happens if a string is too long?

The inverter's maximum input voltage is exceeded and it is damaged. The warranty will not usually cover it, because it is a design error rather than a component failure.

Can a string be too short?

Yes. On a hot day voltage falls, and a string below the inverter's minimum tracking voltage stops producing. Both ends of the window need checking.

Open the string calculator →