Ah. Indeed I did not take into consideration at all the actual flow-rate of a shower. Is 4 gallons/minute way more than is needed? I would imagine modern water-saving showerheads can help with this.
I just Googled it.. and indeed it would appear that 2 gal/min is sufficient.
My bet. 11 x 11 x 6" 2.5 gallons a minute. 30 amp breaker pulling 19 to 23 amps. Efficiency, well out of this worlkd says enough. GO WDRP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!