No semiconductor site has enough backup generators to run continuously and you can only have so much diesel onsite to run those generators (a few hours or maybe a full day if you are lucky). They are there for basic safety systems, air handling and lighting. For example Samsung was down for many weeks after the February storm that caused power outages and fuel delivery issues. They literally ran out of diesel to run the generators and their cleanroom air stopped circulating. It took weeks to get things back to normal and my understanding is getting the air quality back was the hardest part.

https://www.zdnet.com/article/samsung-reopens-austin-chip-pl...