With generators other than propane generators, the time frame becomes extremely crucial because of the source of fuel. If your generator runs on gasoline and your fuel capacity is to a maximum of twelve hours, it means that your generator will need to be refueled before the twelve-hour deadline. Even if you have a 35 gallon reserves gas, the generator will have to be refueled after 48 hours.
A propane generator does not require this frequent and time bound refueling because it is built to last and hold up even with continuous 24/7 usage of any number of appliances.