The problem with electric Tankless (Flash) water heaters is that to have sufficient water flow at a high enough temperature it takes a really large power input. 30A service would most likely not be enough to do what you want the water heater to do and still use other appliances in your unit. My experience with a Bosch 1500 watt unit is the water is barely warm enough to wash your hands with a small flow and if you increase the flow the water keeps getting colder.
Oh yes, one in the home, especially when fired by LP or natural gas work great but they are pricey. They are particularly good in remote areas or in buildings where the need for hot water is sporadic.