Most portable space heaters in the US are about 1,500 watts on high. That is not a rumor. It is the usual plug-in ceiling for a 15-amp circuit. One hour on high is 1.5 kWh, every hour, until you switch it off.
Low or eco modes are often half that, or a cycling thermostat that averages less than the nameplate. Oil-filled radiators feel “gentler” but still use similar watts while the element is on; they just stay warm after it cycles off.
Using a space heater to avoid running central heat in one room can be cheaper or more expensive depending on how sealed that room is and how long you leave it on. “Just for a bit” that becomes all evening is a central-heat-sized line hiding in a small box.
Whatt defaults many space heaters near 1,500 watts. Change to the low setting or a mini personal heater if that is what you own.
At 18.8¢ per kWh, 1,500 watts for 1 hour is about 0.28. Four evening hours is about 1.13. Eight hours overnight is about 2.26 — before any other heat in the house.
Circuit
Most 15-amp rooms cannot run two 1,500-watt heaters. Whatt estimates dollars, not whether the breaker will trip.
Related
Usage estimate only — not a utility bill, load calc, or advice.