The top-rated Generac XT8500EFI 8,500-watt generator has an 8-gallon tank and a run time ranging from 9 to 15 hours. That means running it for two days straight can take more than 40 gallons of ...

Generac estimates that the machine will run for 5.7 hours at 50 percent of its rated load. At 25 percent of that load, its run time is rated at 7.7 hours.