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Watts for any room

Space Heater Size Calculator

What size space heater do you need? Set the room size and how you will use the heater. We give the watts, how many square feet one 1,500 W heater covers, and the best-selling models that fit.

sq ft
ft
How will you use it?
Insulation
Heat needed –watts
How we got this number

    100% private: the math runs in your browser and nothing is sent anywhere. Estimates for one room or open space up to 2,500 sq ft.

    Best sellers on Amazon

    Space heaters for your room

    Watts, coverage and features as stated by each manufacturer, checked October 2026. Only models with at least 200 Amazon reviews. As an Amazon Associate, ComfortSizer earns from qualifying purchases.

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    How many watts does a space heater need?

    Two numbers decide it: the size of the room and whether the heater is the only heat or just extra heat on top of your central system.

    • Extra heat for one room (the usual case): the common rule is 10 watts per square foot with an 8 ft ceiling. A 1,500 W heater, the most you can buy for a standard outlet, covers about 150 sq ft this way.
    • Main heat for the room: on the coldest days the heater has to replace the whole heat loss, so we use the same climate-zone method as our heating BTU calculator. In a cold climate a 1,500 W heater may cover only about 90 to 110 sq ft.

    We then adjust for ceiling height and insulation, and convert to watts (1 watt = 3.412 BTU per hour).

    Why every heater says 1,500 W. Plug-in heaters on a standard 120 V outlet top out at 1,500 W, about 12.5 amps. That is why almost every model on the market has the same maximum output, whatever its size or shape, and why a 15-amp circuit can run only one of them.

    Ceramic, tower, oil-filled or infrared?

    Every electric resistance heater turns its watts into the same amount of heat: a 1,500 W oil-filled radiator puts out no more and no less than a 1,500 W ceramic heater. The type changes how the heat reaches you:

    • Ceramic and tower heaters blow warm air with a fan. They heat a room fast and are small and cheap; towers spread the air with oscillation.
    • Oil-filled radiators have no fan. They are silent and slow to warm up, then hold steady heat. Good for bedrooms.
    • Infrared heaters warm people and objects in front of them, which feels quick even in a drafty room.
    • Mini and desk heaters (500 to 800 W) warm one person at a desk, not a room.

    What a space heater costs to run

    Cost per hour = kilowatts × your electricity price. A 1,500 W heater on high uses 1.5 kWh per hour: at 17 cents per kWh, about 26 cents an hour, or about $2 for an 8-hour evening. The thermostat cuts that once the room is warm.

    Safety

    Plug the heater straight into a wall outlet, never into an extension cord or power strip. Keep it at least 3 feet from anything that can burn, such as bedding, curtains and furniture, and turn it off when you leave the room or go to sleep. Choose a model with a tip-over shut-off and overheat protection, and a safety listing from a recognized lab such as UL or ETL.

    FAQ

    Questions

    How many square feet will a 1,500 watt heater heat?
    About 150 sq ft as extra heat, using the common 10 watts per square foot rule with 8 ft ceilings. As the only heat on cold days it covers less: roughly 150 to 160 sq ft in a hot climate and about 90 to 110 sq ft in a cool or cold one, with average insulation.
    What size space heater do I need for a 300 sq ft room?
    About 3,000 W as extra heat, so one 1,500 W heater will warm part of it or take the chill off. To heat all of it you would need two heaters on separate circuits, or a permanent heater.
    Do oil-filled heaters use less electricity?
    No. A 1,500 W oil-filled heater uses the same power and gives the same heat as any other 1,500 W heater. It is quieter and holds heat longer, and its thermostat may cycle a little more gently.
    Can I plug a space heater into an extension cord?
    No. Fire safety agencies advise plugging space heaters directly into a wall outlet. Extension cords and power strips can overheat at the 12.5 amps a 1,500 W heater draws.
    How much does it cost to run a space heater?
    Multiply kilowatts by your electricity price. At 1,500 W and 17 cents per kWh, a heater on high costs about 26 cents per hour. Using the low setting or the thermostat lowers that.