When the furnace blower motor is running the natural gas furnace typically uses around 600 to 750 watts of electricity depending on the size of the furnace and blower motor.
Also it will use a little electricity for the igniter and transformer but that would be very little.
Still it's cheaper to heat your home using a natural gas forced air furnace instead of an electric furnace.
Electric furnaces can use as much as 18,000 watts per hour or around $15.00 to $25.00 per day depending on how long the electric furnace has to run to maintain the temperature in the house.