We have had on demand water heaters in the UK for more than 30 years now. When i moved to San Diego i had a Takagi wall heater installed and cut my utility bill in half the very next month.
The size of the unit will dictate how many showers you can run at the same time, But the unit i purchased for around a grand would supply more than enough hot water for two showers running concurrently. The downside was that few plumbers in San Diego 20 years ago had the expertise to install them correctly, so if i remember correctly it cost me around $1500 to have them fit the gas unit.
Ran 16 years without a single issue. I would have one in my new house but my solar system supplies all the hot water i need for free even in -5F temperatures, as long as the sun shines
