Would a manual transfer switch also work? When we had our turkey operation we had a big manual transfer switch to hook our PTO 25kw generator to in case power went out and the turkeys needed feed and ventilation. The switch completely disconnects from the power line and transfers to the generator input.

We had to use the generator a few times. One time in an ice storm the better part of a week. 

Seems like that would also work for "the world is ending" situations but certainly not as handy if a person had regular outages with an automatic standby generator of in residential case a power wall battery..