Worked problem

Solar PV String Voltage on a Cold Day

Correct a module open-circuit voltage for a −10 °C cell temperature and check how many modules in series stay under a 1,000 V inverter limit.

Electrical & Power · Last reviewed October 17, 2026

Given

A module has an open-circuit voltage Voc of 49.5 V at standard test conditions (25 °C cell temperature). Its Voc temperature coefficient is −0.27 %/°C. For this exercise take the lowest expected cell temperature as −10 °C. The inverter accepts a maximum DC input voltage of 1,000 V. A designer proposes 20 modules in series.

Find

Whether the proposed string is acceptable, and the largest number of modules that is.

Solution

Temperature change from the test condition: −10 − 25 = −35 °C.
Voltage correction factor = 1 + (−0.27 ÷ 100) × (−35) = 1.0945. A negative coefficient means the voltage rises when it is colder.
Cold open-circuit voltage per module = 49.5 × 1.0945 = 54.18 V.
Proposed string: 20 × 54.18 = 1,083.6 V, which is above 1,000 V. It fails.
Maximum modules = 1,000 ÷ 54.18 = 18.46, so 18 modules. Their cold voltage is 18 × 54.18 = 975.2 V, a margin of 2.5%.

Answer

20 modules: 1,084 V (too high)  |  18 modules: 975 V (acceptable)

Modules in series	At 25 °C (Voc)	At −10 °C
18	891 V	975 V
19	941 V	1,029 V
20	990 V	1,084 V

Why open-circuit voltage is the check

A module produces its highest voltage when it is cold and no current flows, for example at sunrise on a clear winter morning with the inverter not yet drawing power. A string checked at 25 °C (990 V for 20 modules) looks fine and still exceeds the limit when it is cold.

Common mistakes

Checking the string at the 25 °C test voltage.
Applying the coefficient with the wrong sign, which lowers the voltage in the cold.
Using the maximum power voltage instead of the open-circuit voltage for the maximum-voltage limit.
Forgetting the other end: the string also needs enough voltage on a hot day to stay inside the inverter MPPT window.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

Which temperature should I use?

The lowest cell temperature expected at the site. The code allows specific methods for this, so confirm the method and the edition that apply. The −10 °C here is an exercise value.

Does a larger voltage limit change the answer?

Yes. With a 1,500 V inverter, 27 modules would give 1,463 V cold, so the number of modules depends strongly on the equipment rating.

Does the coefficient apply to current too?

Current has its own small positive coefficient. It matters for the maximum-current checks, not for this voltage check.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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