Ian Jackson wrote:

Assuming it survives long enough, a 120V kettle run on 240V will boil in
less than a quarter of the time. [Note 'quarter', not 'half', as twice
the voltage will provide four times the power.]

Surely the manufacturers of kettles will use an element that has an
appropriate wattage, irrespective of the supply voltage it's designed for.

I used to have a 12V DC kettle and it managed to boil two cups of water
quite quickly. It pulled about 30A!

Bill