We have run a 1500 watt heater for years, usually in the morning when we first wake up. We make coffee, toast and have the heater on all at the same time. The coffee pot and toaster are on the same breaker. The heater on another breaker. By the time Ginny starts making bacon and eggs, the place is usually so warm, the vent and window are usually open and the heater off.
Byron's comment about the voltage of AC is absolutely correct. I note the time to make toast seems to vary. I attribute this to differences in the AC voltage.
By the way, we rarely find it necessary to run on 1500 watts at least in our little Scamp
. We found the point where the heater thermostat cuts off at 70 F and usually set it there and usually on 800 watts.