[poizan42]

But the more powerful kettle should be slightly more efficient[0] because there is less time for heat to escape from the kettle while the water is being heated.

[0] Energy efficiency at boiling the water. A kettle is always 100% effective at making heat.

[throwaway198846]

You could use vacuum like in a vacuum flask. In fact to my surprise the product seems to already exists although its selling point is how long it holds water hot after boiling and not its efficiency.