The main drawback of an instantaneous electric water heater is its high electrical power requirement for efficient operation. To supply multiple outlets simultaneously, such as a shower and a sink, the unit must be sufficiently powerful. Consequently, its electricity consumption is significant. This power requirement may also necessitate modifications to your electrical installation.

Furthermore, the water temperature can vary depending on the flow rate. The higher the flow rate, the less hot the water is likely to be, as the unit must heat a larger volume of water at once.