Temperature coefficient of solar panels

Solar panels are rated at a 25°C cell temperature, but roofs get much hotter and much colder than that. The temperature coefficient tells you how the panel behaves away from 25°C — and it cuts two ways: heat robs power, while cold raises voltage. Both matter, for different reasons.

Three coefficients on every datasheet

Datasheets list temperature coefficients for three values, each in percent per degree Celsius:

• Coefficient of Pmax (power) — how output power changes with temperature. Always negative.
• Coefficient of Voc (open-circuit voltage) — how voltage changes with temperature. Also negative.
• Coefficient of Isc (short-circuit current) — small and positive.

The reference point is 25°C. A negative coefficient means the value falls as the cell heats up and rises as it cools down.

Why hot panels lose output

The power coefficient is the one homeowners feel in summer. If a panel's Pmax coefficient is around -0.3%/°C and the cell reaches 60°C — common on a hot roof — that is 35°C above STC:

• 35°C × -0.3%/°C = about -10.5% power relative to the nameplate.

This is normal and expected, not a defect. It is why array design uses realistic cell temperatures rather than assuming STC output all day. Good airflow behind modules and lighter roof surfaces help keep cells cooler.

The Voc side: cold sets the ceiling

The subtler risk is voltage. Because the Voc coefficient is negative, open-circuit voltage climbs as temperature drops. On the coldest morning of the year, before the array warms up, a string can sit well above its STC voltage.

That matters because every inverter has a maximum DC input voltage, and the system has a voltage limit (600 V for most one- and two-family dwellings). NEC 690.7 requires string voltage to be calculated at the lowest expected ambient temperature for the site.

Working a Voc calculation

The basic steps a designer follows:

• Find the record low temperature for the location (ASHRAE extreme-minimum data is common).
• Take the temperature difference below 25°C.
• Multiply by the Voc coefficient to get the voltage increase per module.
• Add it to the STC Voc, then multiply by the number of modules in series.

Keep that corrected string Voc under both the inverter maximum and the system voltage limit. This is exactly the math walked through in how to size PV strings step by step.

Why reviewers check it

A string sized only at STC can pass in summer and overvoltage the inverter on the first cold snap. Plan reviewers look for cold-temperature Voc correction because it protects equipment and reflects code.

The power coefficient explains summer losses; the Voc coefficient governs how many modules you can safely put in a string. Pull both numbers straight from the datasheet, and have a licensed electrician or PE confirm the string sizing. New to the process? Try the DIY workflow or browse the engineering reference.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.