Practically all on-demand water heaters, single or multi-point, are built pretty much the same way. They require 6-8kw, a restriction of flow & a minimum water pressure of around 35kpa/5psi. Their achilles? Won't last long with hard water. They require constant cleaning & are like CVTs...disposable. Though power consumption duration is just as long as the shower, it's 3-4 times more than a good tiny storage type Ariston(Italian...pic below, Wilcon is the Ph distributor).
The advantages of Ariston storage heaters? Requires just 2-3kw. Thermostat & heating element are serviceable/replaceable. Easy to clean....built like conventional A/Ts. Its lone disadvantage is just that lil inconvenience....You'll need to switch it on 5mins prior to taking a shower.
Many who've gone tired of the maintenance & replacement of on-demand heaters have gone Ariston's way. The 4 & 8gal models have the corrosion resistant plastic body. They can be mounted under the lavatory or outside for easier maintenance access.
From service & ownership experience, having tried most out there, I'd point the trusty Ariston. Rebuildable, parts aplenty.....Low maintenance......lesser consumption....way longer service life.[emoji106]

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