It would be enough said if I knew what you were talking about.
I have 240 run to the existing electric water heater tank. No clue on the amps amount from the breakers.
One 240 volt circuit won't be enough. That's one of the issues with the tankless water heaters. It could require 2 if not more 240 circuits. And when your electric bill is calculated, it is based not only on usage, but on demand. When the tankless kicks in, your meter registers the surge and that is what you pay for. I'll say a bit more about the regular electric or gas water heaters.....they don't run all day to keep the water hot. The insulation is very good and more than likely they will keep the water hot all day without coming on. At least mine does (did).