I have seen a lot of big house's in my day that have every gadget under the sun, electric heat, multiple refrig's electric gates, hot tubs, etc,etc, and have never even approached using 200 amps all at once. That on demand water heater sounds like it use's way too much, and would cost too much just to run it . Put it on e-bay and get a gas unit or better yet some solar stuff.
Mike
Mike