> From web searching, North America is 120V but two phases. So you could redo the socket to 240V without running new cable through your walls.

You'd need an extra conductor of wire than is there in the cable for a standard 240V outlet. (Hot1, Hot2, Neutral, Gnd).

You can run a Hot1/Hot2 240V circuit with existing 14/2 romex, but odds are that whatever feeds the outlet in your kitchen also feeds many other outlets, and you don't want them all to be 240V.

Note North America doesn't use ring circuits like the UK.

The Japanese hot water things basically solve this problem too by maintaining hot water all the time.

Convenient but inefficient.

So water heaters are inefficient? I’m confused, they save time.

I mean the thermal (and hence, electrical) inefficiency of keeping hot water warm all the time.

Similarly, even with good insulation, a combi boiler is more efficient than having a hot water tank.