Well, you rate vehicle motors by miles per gallon, but since a generator just sits there, I assume they rate fuel usage by the hour or something. I have an Onan 4.0 Kilowatt Generator. Yesterday and today I ran it for about a total of 14 hours. I am unable to tell how much gas it used during that time, since my fuel guage doesn't work. Can anyone who might know give maybe a general idea of how much fuel a generator like that might suck down per hour under a medium load?