Why Do Solar Panels "Secretly" Gain Voltage in Winter?

Had a customer trip overvoltage protection on a cold winter morning. Turned out panel Voc hit 470V, vs 430V in summer.
The reason is simple:
Solar cells are semiconductors. Lower temperature = electrons move less = need higher voltage to drive them. It's the temperature coefficient at work.
Quick numbers:

• At standard 25°C: Voc = 43V
• At -10°C winter: same panel hits 47.5V
  

Ten in series = 475V, dangerously close to the 500V inverter limit.
Design tip:

Size your strings for the coldest local temperature, not summer conditions.