The 3.5 kW heater will have a tank reservoir that will have time to heat up the water when it’s not being used. Tankless means it has to be heated instantly.

It takes ~ 4 kJ to heat 1 kg of water for each 1°C. If you want to do that in 1 second, you need 4 kW of heating power.

So if a shower uses, say, 9 litres per minute, i.e. 0.15 litres per second, heating that water from 10°C (typical cold water temperature) to 40°C (comfortable shower temperature) is:

4 • 0.15 • 30 = 18 kW

Anything less heats less water per second, or to a lower temperature.

Like this 5.1 l/min unit at 9.5 kW will be able to heat that flowrate ~28°C above its inlet temperature.

I appreciate the work :)

1 kWh/s (with you there) = 3600 kWs / s = kJ/s = kW!

= 3600000 J/s (=W) = 3.6 MJ/s (=MW)

3.6 MW even. :)

3.6 kW is what a 50cc internal combustion engine typically produces

Different films. I guess this is the Jackie chan movie:

https://en.wikipedia.org/wiki/Crime_Story_(1993_film)?wprov=sfla1

https://champagne.pages.dev/docs/getting-started/entertainment/live-sports