Get a 2 - 2.5 K, run each for a few hours and keep the lids closed as much as possible. also, the fuller the freezer, the more efficient it is.
For emergency running with the load in rotation that is enough. The generator will be far lighter and easier to use and you will find lots of other uses for it. get bigger and it will have just one use as a back-up.
Run your freezers as cold as possible. Stuff in it will keep better, the cost to run it will be lower, and in an emergency the lower starting temperatures will help food remain frozen longer and more easily.