How Many Watts Does a Space Heater Use? And What That Does to a Circuit

A space heater is the appliance most likely to be the biggest thing plugged into a bedroom, a home office or a garage — and it is the one most often plugged in badly. The good news is that the number is simple. Almost every plug-in electric space heater sold in the US is built to one ceiling: 1,500 watts.

The figure on the label

Look on the rating label, usually on the back or underneath. It will state a wattage, an amperage, or both, at 120 V.

Amps are watts divided by volts, and a heater makes that easy: a heating element is a pure resistance with a power factor of one, so there is no correction to apply. The watts to amps calculator gives the same answer and carries it through to a breaker size.

Why 1,500 watts? Because that is roughly the most a single appliance can reasonably take from an ordinary 15 A household receptacle, and manufacturers build to the outlet people have. Larger electric heaters exist, but they are permanently wired 240 V units, not plug-in ones.

The type does not change the heat

Oil-filled radiators, ceramic heaters, infrared panels and plain coil-and-fan heaters are marketed as if some were cheaper to run than others. At the same wattage they are not. An electric resistance heater turns essentially all the electricity it draws into heat, so 1,500 W is about 5,100 BTU per hour from any of them.

What differs is delivery:

• Oil-filled units warm up slowly and keep radiating after the element cycles off — gentle and quiet.
• Fan and ceramic units move warm air quickly, which heats a small room faster.
• Infrared units warm the objects and people in front of them rather than the air.

Those are comfort choices. None of them makes 1,500 W of electricity into more than 1,500 W of heat.

What it does to the circuit

This is where a space heater earns its reputation. A branch circuit that feeds several outlets has two limits worth knowing, both covered in detail in how many outlets on a 20 amp circuit:

• NEC 210.23(A)(1): one cord-and-plug appliance may use no more than 80 percent of the circuit's rating — 12 A on a 15 A circuit, 16 A on a 20 A circuit.
• A heater running for hours is a continuous load, the kind a circuit is sized at 125 percent for. 12.5 A × 1.25 is about 15.6 A.

Either way you read it, a 1,500 W heater on high is more than a shared 15 A circuit is meant to carry, before the lamp, the TV and the laptop charger are counted. On a 20 A circuit it fits under the 16 A limit with a few amps to spare. Two heaters on one 20 A circuit are 25 A — that trips, every time.

If the breaker for one room keeps tripping in winter, the heater is almost always the reason. The fixes are, in order: use the low setting, move the heater to an outlet on a different circuit, or have a dedicated circuit added.

Plug it into the wall — and give it room

The Consumer Product Safety Commission's advice is short and worth following exactly:

1. Plug the heater directly into a wall outlet. Not an extension cord, not a power strip. Both are common points of failure at 12.5 A.
2. Keep it at least three feet from drapes, bedding, furniture and anything else that can burn.
3. Check the plug. If it fits loosely in the outlet, or the plug or faceplate feels hot, stop using that outlet — a worn receptacle heats up under exactly this kind of load.

Tip-over and overheat shutoffs are worth having, but they do not fix a bad outlet or a crowded circuit.

240-volt heaters are a different case

Baseboard and wall heaters that are permanently wired often run on 240 V. At that voltage the same 1,500 W is only 6.25 A, which is part of why larger heaters are built that way. One wrinkle: in an apartment building with a 208 V supply, a 240 V resistance heater delivers only about 75 percent of its rated output, because its power falls with the square of the voltage. The volts to watts calculator works that out for any rating.

What it costs to run

Energy is kilowatts times hours:

• 1.5 kW × 8 hours = 12 kWh at full output
• At an illustrative 17 cents per kWh, that is about $2.04 for the night

Your rate is on your bill and varies a lot by utility and season, so use your own. The thermostat usually brings the real figure down, because the element is not on the whole time — the watts to kWh calculator takes a duty cycle for exactly that reason.

During an outage

A space heater is the load that surprises people planning backup power. A portable power station has to be able to supply 1,500 W continuously just to start it, and at full output a 1,000 Wh station is empty in well under an hour once inverter losses are counted. A whole-home battery can carry a heater, but at 1.5 kWh every hour it is often the first thing to come off the list — the critical load panel planner shows why, circuit by circuit.

If you are weighing what a home battery should keep running through a winter outage, a heater belongs in that conversation early. Talk to Seamless Home and the load list gets built from your actual appliances rather than guesses.

A space heater is a simple load with a simple number. The trouble always comes from where it is plugged in, not from what kind it is.