Large storms regularly result in some customers with lack of utility power for more than 7 days for some customers. When storms take out major transmission lines and roads and bridges, you can end up with some pretty lengthy outages, and fuel deliveries will also be difficult.

Look at data center responses from Hurricanes Katrina and Sandy. This guy [1] was onsite for Katrina. Lost utility power on August 29. They did have some access to fuel on Sep 1, but pretty spotty until maybe the 3rd. Looks like power started coming back in some places Sep 8, and maybe widely restored on Sep 14th.

I would say, by 7 days in you'll probably have a good idea of if 30 days might not be enough.

[1] https://web.archive.org/web/20061206181753/http://interdicto...

7 days of unreliable electricity wouldn't be unheard of for a very large storm