What Size Space Heater Do I Need? Watts by Room Size
Part of the project planner: Planning heating, cooling and ventilation.
An average room needs about 10 watts of heater per square foot, so a 12 × 12 ft room (144 sq ft) needs 1,440 watts: one 1,500 watt heater. That is 4,913 BTU per hour. Rooms with high ceilings or poor insulation need more. The space heater size calculator works it out for your own room and tells you the cost to run it.
Space heater watts by room size
For 8 ft ceilings, at three levels of insulation, with the number of 1,500 watt heaters for an average room:
| Room | Well insulated (8 W/sq ft) | Average (10 W/sq ft) | Poorly insulated (12 W/sq ft) | BTU/hr (average) |
|---|---|---|---|---|
| 100 sq ft | 800 W | 1,000 W (1 of 1,500 W) | 1,200 W | 3,412 BTU/hr |
| 144 sq ft | 1,152 W | 1,440 W (1 of 1,500 W) | 1,728 W | 4,913 BTU/hr |
| 200 sq ft | 1,600 W | 2,000 W (2 of 1,500 W) | 2,400 W | 6,824 BTU/hr |
| 250 sq ft | 2,000 W | 2,500 W (2 of 1,500 W) | 3,000 W | 8,530 BTU/hr |
| 300 sq ft | 2,400 W | 3,000 W (2 of 1,500 W) | 3,600 W | 10,236 BTU/hr |
| 400 sq ft | 3,200 W | 4,000 W (3 of 1,500 W) | 4,800 W | 13,648 BTU/hr |
A taller ceiling means more air to heat: a 10 ft ceiling raises the 144 sq ft room to 1,800 watts.
Choosing a heater
- Size: match the watts to the room. A heater that is too small never gets the room warm, and one that is too big cycles on and off.
- Type: ceramic and fan heaters warm up fast. Oil-filled radiators are quiet and give steady heat. Infrared heaters warm people rather than the air.
- Safety features: tip-over and overheat protection, a thermostat and a UL listing.
- Circuit: a 1,500 watt heater uses 12.5 amps of a 15 amp circuit, so nothing else big should share it.
Use it safely
- Plug it directly into a wall outlet: never into an extension cord or a power strip.
- Keep it at least 3 feet from anything that burns: curtains, beds, paper and furniture.
- Put it on a flat, hard surface, and never leave it on when you sleep or are out.
- Test your smoke and carbon monoxide alarms.
- If the plug or the cord feels hot, stop using the heater.
Electric resistance heat is expensive: compare it with other fuels using the heating cost comparison, and see the space heater running cost guide.
Frequently asked questions
How many watts do I need to heat a room?
About 10 watts per square foot of floor in an average room with 8 ft ceilings: 1,440 watts for 144 sq ft. Use 8 watts if the room is well insulated and 12 if it is drafty.
Will a 1500 watt heater heat a 300 square foot room?
Only partly: 300 sq ft needs about 3,000 watts, so two 1,500 watt heaters or a larger heater on its own circuit. A 1,500 watt heater is good for about 150 sq ft.
How many BTU is a 1500 watt space heater?
About 5,118 BTU per hour (each watt gives 3.412 BTU per hour).
Are electric space heaters expensive to run?
A 1,500 watt heater uses 1.5 kWh an hour, which costs about 26¢ an hour at 17¢ per kWh. Heating a room for 8 hours costs a few dollars.
Can I use an extension cord with a space heater?
No, if you can avoid it. A heater draws a lot of current, and a thin cord can overheat. Plug the heater straight into the wall, and not into a power strip.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.