This question does not pertain directly to generators themselves. Alas, though, it remains critical if your intentions are to invest in a portable generator for power outages and other emergency situations. Depending on the severity of the situation, gas stations may be closed around you and also the roads for you to travel on. In this case, you may be left with no feasible way of running your generator (rendering it useless). Thus, making it highly recommended to store enough gasoline that would allow the generator to run for 24 hours. Of course, if the generator also runs off propane, store as many bottles as you need.
Unfortunately, this is going to require some mathematics on your end. Thankfully, it is simple as all you need to do is calculate how much gas or propane you would need for a 24-hour period. This is where the runtime and efficiency will come in handy.