Thanks for all the replies. I think I'll have to go for storage after all. Hopefully, a 10/15l one should be good enough.
Do I have to have an expansion box with it?
I want to fit an undersink water heater for the kitchen sink.
My requirements are:
1. !3 amp electrical supply.
2. Using existing mixer tap.
3. As near as possible instantaneous hot water on turning the tap.
4. On demand heating of the water, rather than storage, to reduce wasted heat.
Is...