Open-Circuit Voltage (Voc)
The voltage a solar module or string produces with nothing connected to it and no current flowing. It is the highest voltage the array will ever present, and it rises as cell temperature falls.
Voc is the number that sets the maximum number of modules per string, because it must stay below the inverter's maximum DC input voltage in the worst case — a clear, freezing morning before the array has warmed up. Use the module's temperature coefficient of Voc together with the site's record low temperature, not the annual average, and never the datasheet value at 25 °C on its own. This calculation is the single most common cause of field failures in otherwise competent designs. Voc is also why an isolated string is dangerous to handle even when the inverter is off: the voltage is present whenever there is light.