What is the downside of an electric tankless water heater?

What is the downside of an electric tankless water heater? The primary drawback of an electric tankless water heater is that it may be constrained by the electrical supply and may not be able to meet high demand for hot water.

An electric tankless water heater (often called an instantaneous electric water heater in the UK) heats water just when you turn on a tap or shower. So there is no stored hot water cylinder and no standing heat loss from it. However, the water heater requires a lot of electrical power to heat cold mains water fast.

Electric tankless water heaters are frequently sufficient for a hand basin, cloakroom, kitchen sink or low-flow outlet and can be large enough for baths, numerous outlets or high-flow showers. In winter the water coming from the mains is colder, and the heater has to work harder to bring the water temperature up, which might result in a lower available flow rate.

Another drawback can be the cost of installation. Larger instantaneous electric water heaters may need a dedicated electrical circuit, improved wiring, appropriate circuit protection, checks on the capacity of the consumer unit and even a three-phase electrical supply. These requirements can make the installation more complicated and expensive.

Running costs can also be a factor. Electric tankless water heaters are efficient at the point of use, but electricity is usually pricier per kWh than mains gas. For regular large-volume hot water use, a gas water heater, unvented cylinder, or an air source heat pump water heater (with a COP of 3.7 to 1, meaning they are as cheap to run as a gas-fired water heater) may be more practical.

They are also sensitive to the flow rate and the water pressure. Too much flow and the water could not get to the proper temperature. A flow that is too low causes some units to not switch properly.

Simply put, the downside of an electric tankless water heater is that, while it is compact and efficient, it typically has a limited hot water flow rate and the electrical demand it requires, making it unsuitable for significant hot water demands unless used in parallel installation configurations.