Just a nit pick but I have never seen such a waterhetater and doubt they even make them even if someone manufactured one I would never consider it standard to have such a large 120V load.
I'd say 'standard' for a residential / light commercial storage type resistance electric water heater the would be 50 gallon 4500W 240V, or 80 Gallon 5500W 240V, and yes that would be a continuous load, 10AWG wire 25A breaker and 10 AWG 30A breaker respectively.
They do make 1500 watt 120 volt elements though and I suppose could be possible to have a unit that is capable of running both upper and lower element at same time. Probably not common factory configuration though.
A place I am working on has a WH on site though not plumbed or wired yet. It is 80 gallon tank, can't recall exact watt rating but it is @ 277 volts and both upper and lower elements operate independent of one another, so both can be on at same time. However it will be connected @ 240 volts. Converting from 277 volts, again can't recall exact watt rating but do recall it will need a 50 amp circuit @ 240 volts, so it is like 9600 watts or a little less.
I wasn't convinced this is the way to go, but that's what the plumber thought we should use. I think he will be putting in a circulating pump as well
If it were my place I would have put typical 4500 watt in the basement where this one is going, (maybe still 80 gallon) and another 4500 watt in mechanical room that is on the second story. Still looking at similar overall capacity but putting some of it closer to point of use and possibly can eliminate the circulating pump.