It may be where you live but I am down south temperatures don’t get that cold for the most part. I have know a few people who have Generac generators and they can run for about 6 to 7 days on 200 gallons of propane. I had thought about a 500 gallon tank but the longest we have ever been out was 7 days. For me a 250 propane tank seems logical for a short outage. I live in the country and don’t have access to NG that would be the best way to go if you have access, I assume you don’t
No mine is on NG but it’s dual fuel capable and I was thinking as a backup to that. I had a 250 that I sold because they said it was too small for how cold it gets here. Anyway good luck with your project. Be safe.