OK, now take a look at the specifications for even a quite low capacity residential tankless electric water heater. For example, got to SETS Systems and look at their model 110 "point of use" unit.

They describe that model as "ideal for secondary point of use application in warmer climates or in northern climates in the summer months". By "secondary point of use," they mean a single sink or low flow shower. See their chart for some idea of what one could expect with respect to input temperature, maximum output temperature, and rate of flow.

They're talking 60 A, 240 VAC for this. It takes a lot of power to heat up water "on the fly"!