Not a recommendation, but there are at least two sorts of these.
The ones that require a very fat cable going back to the mains and will provide lashings of hot water forever and others with a more modest power requirement that heat up a small highly insulated tank of water
15-20L at a time and deliver it as needed. The latter is OK for washing up small numbers of cups or infrequent hand washing provided that you are not one of those who runs the tap for five minutes "to get it warm".They tend to have specific needs in terms of input mains water pressure and safe venting to waste in case of overpressure or boiling. You could run into trouble with hard water killing a high power unit prematurely.
We have them in our VH toilets and the water stays hot enough overnight without power to still be useful first thing in the morning. YMMV.
The main kitchen has a conventional big hot water cylinder.