Unless you have a big tank, if the grid goes down, you won't have fuel of any kind. Propane is probably the easiest to store and it won't go bad. You need to get some kind of estimate on how many gal. of fuel it takes to run the gen for a given length of time at full load so you can guess how big of a tank you need. The gens that I've looked at tell you how long they'll run on a tank at 1/2 load. That's a starting point, anyway.
