We had one (Stiebel Eltron 36KW) installed earlier this year which required a service upgrade to 320A service. AMA, though responses may be delayed a day or two.
Our all-electric home's max instantaneous draw over the last a couple months peaked around 44 kW.
Just saw this, I would recommend going the heat pump route.
In retrospect, it's insane to get an electric tankless hot water heater from a climate perspective. Gas tankless makes infinitely more sense if you need limitless hot water. If you don't need limitless, heat pump would be the most emissions efficient.
I'll try to get my usage data together and update this comment.
Edit: Cost of usage electric and gas over the last five years. The big spike last winter was due to our remodel and heating the whole house while it was neither sealed nor insulated in the middle of winter with just the 10kWh furnace backup heater which ran almost non-stop. Units are USD. The two gaps in data are due to meter replacements, I think. https://imgur.com/AVTFwpP
It's kind of hard to compare the data since we re-insulated, went from gas furance to heat pump, and added a hot tub.
Our all-electric home's max instantaneous draw over the last a couple months peaked around 44 kW.