If you see the spec clearly, it didn't save much or pretty non. It selling point is the heating element.

Joven take 30min at 3kw to boil the water to 70°.

Hydro1 take 36min at 1.5kw to boil to 44°c. See the temp different.

I don't see the Hydro1 save in electricity.

Wrong.

It takes more energy and time if want to go higher. It is not a linear relationship.

Energy loss  to surrounding due to temp difference will be more.

The lesser the temp difference between heating element and the water, the lower the heat transfer rate. Hence again, will take more time and energy.

So such calculation cannot be used. Any fluid thermodynamic engineer here that can shed more light ?

Hydro1 have 30L, 50L and 80L. Check at the Specification.

Base on the Eco30 L, 36min - 44°. If reach to 70° will take  - 70/44x36= 57min.

Base on Eco50L, 45min - 44°. If reach to 70° will take - 70/44x45= 71min.

Base on the Joven JH35L, 60min - 70°.

Base on the Joven JH50L, 90min - 70°.

If I just need hot water for shower in 1 bathroom, does it still make sense to go for storage water heater? Currently I'm using Joven instant water heater. I'm quite satisfied with the temperature as I don't really need very hot water for shower but I want to change to big rain shower for personal enjoyment. So far when I read in this thread seems like instant water heater is not recommended to be use big rain shower? or do you guys know any instant water heater that I can use with big shower head maybe like below type?