this post was submitted on 21 Nov 2025
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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:
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.
Oops, my bad, I really thought it used a regular 240v plug. Turns out its hooked into my kitchen's high voltage (400v) circuit and uses 21kW.
Thanks for explaining why physically my assumption could not be possible.
400V in residential sounds exciting. That's one way to cut down on wire cost I guess.