I was doing some research on this a few years ago. It was reported that a generator runs about $8 a 12 hour day with natural gas. People were standing in lines during Katrina for almost all day every day to keep enough rationed gasoline for a genny. They highly recommended nat gas conversion because it is somewhat perpetual in nature. It was reported that natural gas is pumped at the source with natural gas powered pumps. So they run without much intervention even when power is down.