One of the most upvoted "You should know" posts on Reddit, with 570+ comments, is simple: every 1,500-watt electric heater puts out the same amount of heat. The expensive one and the cheap one turn the same 1,500 watts into the same heat. Once you know that, picking a space heater gets a lot easier.
Short answer: plan on about 10 watts per square foot of room in an average home. A 12 × 12 ft bedroom (144 sq ft) needs about 1,450 W. A standard plug-in heater tops out at 1,500 W, so one heater covers a room up to about 150 sq ft.
Watts by room size
| Room | Area | Watts (average insulation) | Poorly insulated (+30%) |
|---|---|---|---|
| 8 × 10 ft | 80 sq ft | 800 W | 1,050 W |
| 10 × 10 ft | 100 sq ft | 1,000 W | 1,300 W |
| 11 × 11 ft | 121 sq ft | 1,200 W | 1,550 W |
| 12 × 12 ft | 144 sq ft | 1,450 W | 1,850 W |
| 12 × 15 ft | 180 sq ft | 1,800 W | 2,350 W |
Our Space Heater Size Calculator adjusts for insulation and gives the BTU equivalent (1 watt = about 3.4 BTU per hour).
Anything over 1,500 W means two heaters on different circuits, or a hardwired 240 V heater.
Do the different heater types matter?
They all produce the same heat per watt. What changes is how they feel:
- Ceramic fan heaters: warm a room quickly and are small. Some fan noise.
- Oil-filled radiators: slow to warm up but give steady, silent heat. Popular for bedrooms.
- Infrared/radiant heaters: warm you and the objects in front of them, not the air. Good for a desk or a drafty spot.
As one r/AskPhysics answer put it, resistive heaters are basically 100% efficient. The "energy-saving" claims are about thermostats and timers, not magic.
Is a lower wattage cheaper?
Only if it keeps you comfortable. A 750 W heater costs half as much per hour as a 1,500 W heater, but if it runs twice as long to reach the same temperature, you save nothing. Where low-wattage heaters shine is small rooms and shared circuits. Several r/homeowners posters use 500–750 W heaters in bedrooms to stop tripping the breaker.
Safety rules that actually matter
- Plug it directly into a wall outlet. Never a power strip or a thin extension cord.
- One 1,500 W heater per 15 A circuit, with not much else running on it. A 15-amp circuit is rated for about 1,800 W total, and continuous loads should stay under 1,440 W.
- Keep 3 feet of clearance from beds, curtains and furniture.
- Buy one with tip-over and overheat shutoff, and a safety certification mark (UL, ETL or CE).
- Don't leave it running unattended or overnight unless it's designed for that, like many oil-filled radiators with thermostats.
Make the room easier to heat
A heater works much better in a room that holds heat. Close the door, use thick curtains at night, and put a rug down on hard floors. In bedrooms, a good duvet and a heated mattress pad are often cheaper than running a heater all night, a point raised in a 90+ comment r/Frugal thread.
For more on making a room feel warm and comfortable, see cozy bedroom ideas.
Frequently asked questions
What size space heater do I need for a 12x12 room?
About 1,450 watts in an average home, so a standard 1,500 W heater is enough. In a drafty room, you may need a little more.
How many square feet will a 1500 watt heater heat?
About 150 square feet with average insulation, or about 115 square feet in a poorly insulated room.
Are bigger space heaters more powerful?
No. Heat output depends on wattage, not size. A small 1,500 W heater makes the same heat as a large 1,500 W heater.
Can I plug a space heater into an extension cord?
It isn't recommended. Plug it directly into a wall outlet. If you must use a cord, use a short, heavy-duty one rated for the heater's wattage.