At 4.2kJ/kg*°C, and given a 40°C deltaT, 2.8KWh/hr (
) would be 60kg/hr of hot water, or about 15gal. But that's probably not right. I'm guessing the vast majority of the 6.46A is at a COP of 1, i.e. the resistive heating element.
Does it have a "first hour" rating?
Edit: I just thought of something. Does "normal" mode mean it uses the resistive heater every time you take a shower? Unless it has a frugal, intelligent controller, that mode would reduce your COP to maybe 1.2 all the time.