These things aren't mutually exclusive. Once upon a time, batteries were generic and fit some standard form-factor. You could swap batteries between devices and often did! You could even connect your device to a pack of batteries, and swap out the batteries within the pack.
reply240W max is very little when it comes to hearing up water, and most powerbanks don't even do more than 100W output. That's more in the range of those swappable tool batteries.
replyThe Ikape Cera+ can heat water from room temp but it can’t do this many times.
replyBut in many environments you don’t have to heat from cold. There’s often a Zip tap or kettle to get you most of the way.
But maybe the internal battery can deliver more power directly to the heating element.