On demand electric heaters will save you money in the short term but are not as good for the environment, and are not as good for the efforts of the utilities managing the supply of power. That is why these water heaters are falling out of favor in places such as Japan where until recently they were very popular.
The main reason on-demand heaters save money is that in a tank water heater, you produce heat throughout the day for use later, and some of that heat flows from the tank to the surrounding living space (or wherever your hot water tank is located). Read More>>