Use the max open circuit voltage of the PV string. You need to know the temperature coefficient of the cells and modules. If you use a string sizing guide, this is taken into account. It will ask you for your record low temperature in the area you are installing. This can be found at any weather site usually (like the Weather Channel). This is because the PV string VOC goes to its highest level during cold temperatures. It also decreases when the modules heat up. However, no damage is done when the string voltage falls below the inverter operating window. Damage could occur if the string voltage exceeds the safe limits of the inverter equipment.
