Note that heating element ratings will be rated at nominal 120 volts. Heat output varies as the square of the voltage. *If* the kill-a-watt is accurate and no losses in any of the circuit wiring, 286 watts would theoretically indicate the voltage was 113.
Also, heating elements have an inrush current, kind of like motors and capacitors. Nichrome heating elements can take several seconds for the resistance to stabilize because the resistance goes up slightly as it reaches an operating temp. Unimportant factoid: Can be an issue for power utility companies after the power goes out for a period and then they bring a large geographical area back online. Vancouver Island used to be like that when many homes & buildings were heated by electric before a natural gas line to the Island was built.
