We ran into this recently. Home totally renovated, 200 amp service already existing. Customer wants electric heat (not central) and he had already bought an on-demand water system. The hot water alone was 24kw (100 amps @ 240 volts)and because the hot water had to be calculated at 100% demand, that left us with 100 amps for the rest of the calculated load. We reduced the total heating to 7kw - not enough (in my opinion) for a 2-storey (600 ft² per floor)with a full basement which also needs to be heated.
Owner is convinced that the on demand system will save him money and that the electric heating for the house will be adequate. We also tested the on demand system and at full volume, it heated the water to luke warm from 36ºF. Owner had to cut back the input valve in order to get the output temperature to an acceptable level. There's no way this tankless heater will be able to keep up with a shower, dishwasher and washing machine at the same time, and space heating had to be sacrificed to boot.
This tankless heater needed a feed of #2 copper (125 amp breaker), which cost the homeowner over $500 for the wire alone (and wire prices have gone up since then) because the tank was closer to the bathroom and kitchen and quite a ways from the panel. This is something else to consider when calculating the payback period.
If a customer wants a tankless system I will recommend a gas unit, not electric. But not everyone has that option when gas isn't available.
