Bosch makes their smaller unit 117000 btu which is fine for 1 person.
They have pilotless, battery ignition or mini hydro generator ignition.

The larger unit is a Takagi 188000 btu ? which is for 2 showers running
at once.

Both may require a larger gas line.

I have the 117000 Bosch battery ignition it works great..

Will it vent to a chimney, you dont want to buy the blower kit extra,
if you need a blower order it that way.