I currently have a standard 40 gallon electric water heater on a 30
amp breaker. The panel is 100A service which also services an electric
dryer. I was thinking about getting a tankless water heater to save
space, but after reading some electrical specs on these units, it
seems I would have some issues with the electric.

I have 1 bathroom and 1 kitchen and a washer. From the calculations I
would need at least a 5 gpm unit.
If you look at the specs for the Rheem units, the RT18 needs 2
seperate 40A circuits, with maximum power being 75 amps.

http://globalimageserver.com/fetchDo...2-5934e3043c7d

Obviously my panel cannot support that. I could maybe drop to the RT9
unit that requires 1- 40A breaker, but I'm concerned it might be
undersized at 3 GPM, and also concerned that it uses 38 amps. If my
dryer is on, thats another 26 amps, and my 220V water pump using 5
amps, totaling about 70 amps.
That leaves me with about 30 amps to spare.
So I could either upgrade my panel, or just stick with what I have.
This is for a seasonal home in the summer. Just wondering what
everyone else thinks I should do.