Harold in a medical unit I installed a 440 V instantaneous electric heater.
Of course this person could always go for a 220 commerical unit BUT the flow rate would be much slower (GPH) demand unless they have enough storage.
About the size of the electirc heater bein so small I did a 6 story one family brownstone with a 4 gallon 110 V heater under the vanity.
The origional plumber on the job failed to bring the return circulation line to the top floor thus this family had to wait a full 22 seconds for hot water to reach them.
What I did was install a 4 gallon H/W point of use heater hooked up to the hot water line by the time this electric heater did run out of hot water the main hot water was already in this tank so there were no loss of time.
Hey for the right price almost anything can be accomplished.