Do you figure the water heater at 100% of it's full load KW rating? I have a customer with a 200 amp service. Present load calcs come to 123 amps without water heating. They want to add a Tempra 24KW tankless water heater. I know code doesn't care about occupancy but as a side note there is only one occupant.
That tends to be the most generous with big FIP loads so you might be looking at a service upgrade. That is not unusual with these electric tank less water heaters. Chances are they would never have a problem and an analysis of the other loads might give you better guidance but we tend to design for what might happen, not what usually happens. Think Christmas eve, everything in the kitchen is going, "Griswald" lights in the yard, heat is blasting, someone is in the spa and they turn on the hot water.
Re: Load Calcs and tankless water heater
The solution up/over here has been a service upgrade to 400/320 amp. Most of the design professionals on new/reno SFDs with the instant/tankless units.
We had one McMansion, where the owner was asking for three (3) of the 24KW units, and had a POCO request for 600 amp, 3 phase, 4 wire 120/208 volt. There is a lot of POCO line work required, and that equals $$$$
IMHO, 24KW added to a 200 amp, with 123 amp calc load is a potential liability on you, IF you are doing the calcs, instead of a design professional. No intent made to insult your ability, but watch the liability.