A gas fired furnace in most homes does not require many watts to operate. Small amounts of electricity are needed to ignite the appliance and spin the fan motor. On average most gas furnaces require a 15 amp breaker and use less than 600 watts.
Electric furnaces on average can use 18,000 watts of electricity to heat a 2,500 square foot home. Depending on the size of resistance heaters installed which range from 10-50 kilowatts of electricity.