Note that the Zojirushi and @kevin1’s InSinkErator are doing the initial heat cycle in the 700W realm. Keeping a larger volume of water hot, “a big kettle”, is most efficiently done with a kind of mid-range element I suppose. Half way between a Keurig keep-warm and a regular kettle that aims for maximum speed.
So, I don’t expect we’re necessarily looking for a 1,500W heat cycle from a primary element.