as a electric unit what unit would you suggest to a 45 min shower girl?
Assuming a 45F worst-case incoming water temp from your wells in the dead of winter and a low-flow 2 gpm (=~1000lbs/hr) shower, 105F at the showerheatd you're looking at a 60F rise x 1000lbs/hr= 60,000 BTU/hr. Converting the units to watts that would be 60,000/3.412= 17,585 watts for
just the shower.
The very smallest that just covers the shower would be something like an
EcoSmart ECO-18 or
Rheem Rtex-18, and it would just barely cover it. If someone else uses hot water for any other purpose the person in the shower would get a jolt of tepid or cold water. That requires a pair of #8AWG wiring runs and a pair of 40A double-pole breakers. Do you have that much power to spare on the panel or service drop to the house?
At about 18 kilowatts a 45 minute shower uses 13.5kwh. Even at Ohio's average
12.7 cents/kwh that adds up to well over a buck per shower.
If there is at least 5' of vertical drain downstream of the shower you can buy some margin on that
using a drainwater heat recovery heat exchanger, can cut the peak power draw in half or more, and cut the total power use/cost by the same fraction. The biggest one that fits is the "right" one, the marginal cost of the taller units is more than paid back in higher efficiency, and offers more margin for other uses, to help mitigate against the shower girl isn't screaming curses every time somebody else washes their hands during the 45 minute interval.
Even though the heat exchanger could set you back the better part of a grand (or more) as a DIY this solution has a
very decent return on investment in households that like to take long showers. For tub-bathers or 5 minute showerers, not so much.
With any tankless there will be liming issues with the heat exchanger if your well's water hardness isn't pretty low. Installing it with isolation valves and ports for pumping white vinegar through it on a periodic basis is a good idea. (A half hour of de-liming solution pumping every year is good enough for most.)
For higher flow applications there are bigger electric tankless units out there, but when you're in the 36kw range most homes don't have enough spare service capacity.