Natural gas is about 1000 btus per cubic foot
Electricity is 3413 btus per hour
Electricity is almost 100 per cent efficient in heating water in a new hot water heater. I would guess a natural gas hot water heater is only 80 to 90% efficient.
I don't know how long your heater would run in 24 hour but if it ran for one hour then you could use:
1 kilowatt of electricity for every 3.8 cubic feet of natural gas you burned assuming a 90% efficient heater. If the heater is only 80% efficient then the gas burned would be 3.9 cubic feet.
Now, having said all of that, the easiest way to compare the cost of operating a natural gas hot water heater and an electric hot water heater is to go to a website like the one below and compare. The one below has a place where you can click on the spec sheet for their electric or gas heaters and on the far right hand side the cost per year is shown. For electric heaters it is based on 8.6 cents per killowatt hour and for gas heaters it is based on 91 cents per therm which is 100,000 btus.