I noticed some whole house 4 bath electric tankless require 300A service...so your 200A would not be enough for those (3x60A). I have decided that the ones that work on 2x50 and 2x60 circuits would not be all that great due to my cold ground water (very cold deep well water).
I am however considering a couple of 30A tankless as boosters for a couple of my showers (or one 40 or 50 amp one since they are close and on same line). Its a long run and takes longer to get really hot water from other end of house than to take a shower, so its either take a lukewarm or cool bath or wait. What do you guys think of them for that application (that is localized boost heating)?