What type of electrical device is often fitted to hot water storage vessel supplement the hot supply from a boiler?

An immersion heater is often fitted to a hot water storage vessel to supplement the hot supply from a boiler. This electrical device heats water directly within the tank, providing an additional source of hot water when needed, especially during peak demand or when the boiler is not operational. Immersion heaters are typically used in electric water heaters and can serve as a backup heating element.