Variable Speed Furnace Fans A standard fan motor will typically use about 400 watts per hour while a variable-speed model will only use 75 watts per hour. This is the most direct way that the variable-speed units save electricity, but there are other means by which they affect energy usage as well.Click to see full answer. In this manner, how many watts does the average furnace use? Heating 600 watts Electric space heater (low) 750 watts Gas furnace (for the blower) 1100 watts Waterbed heater 450 watts Waterbed heater (avg. 10 hrs./day) Additionally, how many watts is a 20 box fan? 58 watts Also to know, is 200 watts a lot? So a 100watt light bulb left on for ten hours uses 1unit of electricity (100watts x 10hours = 1000 watt hours = 1kwh) as would a 2kw fan heater left on for half an hour. How much does it cost? Rating Time to use 1 unit 5 watts 200 hours just over 1 unit every 8 days 10 watts 100 hours just over 1 unit every 4 days How many watts does central heating use?Central Heating Furnace: 340 watts. Portable Electric Fan Heater: 2000 to 3000 watts. Central Air Conditioner: 1000 to 4000 watts.