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.
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.
throwaway198846|2 months ago