A gas furnace uses a negligible amount of electricity for lighting the flame. The major amount of electricity used with gas heating is in connection with the blower, and the watts required depends on the type of blower.
Older gas furnaces may use one-speed fan motors, and they use approximately 400 watts per hour of activity. More recent technology has led to the development of variable-speed blowers that use as little as 75 watts per hour when operated at the lowest speeds. The watts required to operate an electric furnace vary based on the age, design and heating capacity of the unit.