Volts x Amps produces Killo Watts. This is power. A certain KW heater will produce a corresponding temperature rise at a certain flow rate. I found an equation for temperature rise per KW for a microwave water heater, and it is the same as a resistance electric water heater. Thus a 30Amp 120 volt microwave unit will heat the same as a 30Amp 120 volt tankless electric water heater. I think the only advantage they can hope for is a price improvement, and I think this will be hard to accomplish. A resistance heating element is pretty cheep compared to magnetrons and transformers and rectifiers. I hope I am wrong, but this is what I expect.