If you have just one shower and are looking to vent out of the sidewall, Bosch currently offers two tankless water heater solutions.
The Bosch AquaStar 125FX Propane tankless water heater is a 125,000 BTU input tankless water heater (130,000 BTU for Natural Gas model) that has an electronic ignition system. The 125FX comes with a side-wall vent kit called the FX Hood. This heater is designed to be vented out of the side wall and it cannot be vented up through the roof. The 125FX requires electricity to power the electronic ignition and FXHood.
The Bosch AquaStar 1600H is a 117,000 BTU input tankless water heater that is designed to be vented up through the roof. It uses Bosch's patented Hydro ignition system that automatically starts the ignition process when it senses the flow of water. The Bosch 1600H does not require electricity nor does it have a battery. It also has NO PILOT LIGHT. It does require a 5" diameter Type B double-walled vent of at least 6 feet in height when vented vertically.
The Bosch 1600H can be vented through the side wall using the AQ4 Vent Kit. However, please note that the AQ4 Horizontal Vent Kit DOES require electricity. (The AQ4 Vent Kit can also be used for the Bosch 1600P tankless water heater.)
In summary, Bosch offers two solid tankless water heater options for single shower homes requiring side wall venting.