Boilers are much convenient and less consuming, for instant heaters of that kind one needs a much powerful electrical wiring as long they take about 5-6 kwh i think instead 1.2 or 1.5kwh needed for a boiler, moreover they need a strong and secure grounding and many unpleasant things happened with those, a woman here was shocked during her daily showering bussiness falling out from the cabinet and getting a broken leg, arghhhh..