32 deg F rising to 96 or 140, that's 64 to 108 BTUs per pound, or 534 to
901 BTUs per gallon, or 4540 to 7660 BTUs per minute at 8.5 gpm, or 272K to
460K BTUs/hr, assuming 100 percent efficiency, which equates to 80 to 135
kilowatts, or 333 to 563 amps of 240 VAC.

Does this unit come with a hookup to Niagara Falls, or perhaps the Alaskan
pipeline?

See below for a more practical example and accurate calculation.

Who uses 8.5 gpm? Shower restricter inserts in the USA are
restricted to a maximum flow of 2.5 GPM.

Incoming water is at 42F not 32F, so a rise from 42F to 96F (a good
hot shower) is 54 BTU per pound.

If one gallon of water weighs 8.33 lbs, that means a one minute shower
uses 20.82 lbs of water requiring a rise of 54 BTU or 1124 BTU's per
minute. This x60 is 67440 BTU/hour.

One kiloWatt is 3412 BTU so:

A one hour shower would consume 19.77 kwh
A one minute shower would consume 0.33 kwh

And a realistic 10 minute shower would consume about 3.33 kwh

For the instantaneous power load, a one hour 2.5 gpm shower with a 54F
rise consumes 19.77 kwh, which at 240 volts = 82 amps whether you use
the shower for 1 minute or 60 minutes.

82 amps is still pretty high, but it is nowhere near 333 amps of 240
VAC as erronously stated above.

Few residential pass through DHW heatingunits will be rated at 80 amps
but 40 amp 240V units are quite common. In fact, if you are going to
go with two electrical units like this, the manufactuer recommends
putting the plumbing in series.

Only problem is if every time you turn on the shower your neighbors'
lights dim (because of the 80 amp demand draw, your neighbors and the
power company may give you a hard time.

Beachcomber