I looked at a tankless WH at Lowes yesterday. They want two 40a 240v circuits. I didn't see the KW rating but a max would be 15KW or so @ 80%.
I may be missing something but it looks like that translates to 110 degree/gallons a minute. If you need to bring it up 60 degrees that is lsss than two GPM but that is just counting on me fingers based on 3400BTU/KWH. Assuming I rounded them all up it is maybe a tad over 2GPM at 60 degree delta. That is not much hot water for the "whole house" that it is marketed for. It will put most people up into a 320/400a service too.
