There is no backup power answer that will be a 100% assurance against everything. I have a whole house unit with 5-7 days of propane. If that were to run out I can revert back to the portable that would run for a further 5 days from the gasoline I keep on hand and in the cars. Anything beyond 10 days seems unlikely and Armageddon stuff when I’ll probably have bigger worries than the tank LOL.