Good point isn't it? I got fooled by that sticker number for a long time. The fact is when the electric tankless water heater claims 95-99% that was based on half truth..
Electricity---(1)-->Heat at coil heating exchanger---(2)---flowing water
The efficiency claimed by the manufacturer mean converting electricity to heat energy at heat exchanger coil. The flaw of resistance water heater is how to effectively transfer that heat to the mass of flowing water heater. That is the part that they do not mention. Please take a look at the following post and the last three paragraph of the attached PR, your similar question got addressed there. PM me, if you need further explanation. Go WDRP!!!!
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