And for the other point of view, I find Designers to be overly conservative with VOC. Irradiance will be quite low when these low temps occur. I dont know why everyone will count every volt from a 100 year cold event, but not count the volts gained from low irradiance. Go measure voltage some really cold morning and tell me what it is compared to the standard computed value - a few percent is all you need to get that extra 20 volts you were fretting over to get that extra module in. furthermore it is highly unlikely an inverter will smoke at +20,+40 volts even if it happens at all. Even if an inverter does have a warranty issue, and even if the inverter records the voltage, and even if the voltage has been slightly exceeded, the manufacturer still may not automatically disqualify the warranty. Even if they do, I imagine the labor and hassle are more of a concern than the actual repair/replacement cost....Maybe I am biased as I have been installing these systems in the 35 degree rain in mud 1 foot deep :rant:
It's pretty safe, and conservative, to assume you will have STC Voc at 20% insolation. Current is very dependent on insolation, open circuit voltage much less so. I've heard the argument that the daily low occurs before sunrise so why worry about it, or the insolation is so low in the morning why worry about it. There is also cold soak to consider, and supercooling due to dark sky radiation that many people don't know about much less consider. Someone can go get a Ph.D. in physics and know down to the degree what is happening and still get it wrong because temperatures fluctuate and are not always what we think they are going to be. Yesterday's record low is broken today.
Many people use the ASHRAE extreme annual mean minimum temperature. But notice the "mean" in there. That is the average of the low temperatures which means
half the time the temperature will go below the number being used in the design. I usually use the mean minus one standard deviation, also given in the ASHRAE tables, that means about 80% of the time the temperature will be above my design low temperature. Belt and suspenders.
It's one thing to make a boo boo when designing a 5kW residential system and popping an inverter, it's a little different when one is designing a 1MW+ systems and popping inverters will cost millions of dollars and the EPC will be dragging the designer into court to cover that loss.