Solar Panel Labels: What Happens at −40°C?
The biggest number on the label isn’t the whole story.
A panel’s wattage is rated under standard test conditions, including a cell temperature of +25°C. In comparable sunlight, colder cells generally produce higher voltage and can produce more power.
That extra voltage must be allowed for when designing the array.
Start with these label values:
- Pmax: rated maximum power in watts.
- Vmp / Imp: voltage and current when producing maximum power.
- Voc: open-circuit voltage, before the controller draws power.
- Isc: short-circuit current, used for electrical sizing.
Check the manufacturer’s data sheet for the Voc temperature coefficient. It tells you how open-circuit voltage changes with cell temperature.
For example, take a panel with a 50 V Voc and a coefficient of −0.25%/°C. At a cell temperature of −40°C—65 degrees below the test temperature—its calculated Voc is about 58.1 V.
Ten panels in series would reach approximately 581 V, rather than the 500 V suggested by adding their label values. That would exceed a 550 V input limit.
Use cold-corrected open-circuit voltage when checking the inverter or charge controller’s absolute maximum PV input voltage. Also follow its operating-range, current and sizing requirements. Design for the applicable local minimum temperature, which may be colder than −40°C, and include required tolerances and margins.
More winter watts can be useful. Exceeding an input-voltage limit can damage equipment.
Your array may work beautifully in July. Have you checked a clear January morning?
Unsure? Send Solarwyse your panel label and controller model. Advice is free—ask before you spend a dollar.