I know you guys have already moved past this discussion, but since we have a natural gas whole house generator, I wanted to share my experience. We have natural gas for our central heat furnace, range, fireplace, and water heater also. Our gas bill is generally around 30-35 dollars a month, and in the winter, it will go up because of the gas used to heat the house. It starts to fall off as it warms up outside and we don't run the heat as much. Our highest bill last winter was $78 in Feb. March dropped to $60. April saw it fall another $4 to $56. In May we had our longest generator run time we have had. It ran for 16 hours. The bill that month went back up to $61. The temperatures were still coming down, so I would have expected the bill to drop again that month. I can't say for sure how much it would have been, but I know by the time the June bill hit, we were down to our normal 30-35 dollars. I think its safe to say it costs us no more than $45 to run for a whole day. That is assuming it cost us $30 to run for 2/3 of a day.