If you have an electric heater, don't bother. A gas heater (conventional) has a significant storage heat loss; an instantaneous, or "flash" heater will save on that. Most, though,won't supply as much hot water as us folks in the effete US are used to. (Electric instantaneous heaters will provide a goodly flow of barely warm water or a stingy trickle of hot water---your house wiring just won't permit a current high enough to heat water at more than a moderate rate.)