Don't full size water heaters almost always run on 240v, even in the 120v riddled US? 120v is just what our wall outlets are at, houses all tend to have 240v split phase for the water heaters, ovens, washer/driers etc. I have seen some 120v units/ replacement heaters, but they all tend to be <5kw units, which is "fairly weak" by modern US standards. /AFAIK
Also, if the unit heats with intensity proportional to demand, adding insulation to the tank may solve the problem, as well as saving money.