It is important to know the maximum voltage that a string of solar modules can produce at any given time.
This is mostly important, because if it goes over your string inverter's maximum input voltage, it can void the warranty on the inverter.
In order to calculate this important figure, look for the open circuit voltage (Voc) on the module's data sheet.
Multiply this voltage by the number of modules on the string, and then you have your base Voc. From here, find the temperature coefficient (often labeled K), and the historical LOWEST temperature in your zip code.
Multiple the temperature coefficient by the difference between the nominal temperature on the data sheet (almost always 25 degrees C) and the historical LOWEST temperature. Then, multiply that number by the number of modules on the string. The resulting number is the voltage to add to your string's base Voc to determine the maximum voltage.
If it is above the maximum input V on your inverter, it is too high!