The standby heat for an elec. tank heater at 15 cents/kwh may cost you $65/year. I assume 50w for 8700 hours.
How much you pay for NG, per CCF?
I pay $1.60 per CCF = $1.60 per therm and so my standby for NG would be $23/year.
A therm = 100,000 BTU = 29.3 kwh.
Here's some prices -
Figure on 100 gals per person per day and a bathtub may fill at 6 GPM.