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Immersion heater 

Water heating by electricity is usually done by an immersion heater mounted in the top of the hot water cylinder. The heater contains an insulated electric resistance heater and a temperature sensor. Domestic immersion heaters (usually rated at 3 kilowatts in the UK) run on the normal domestic electricity supply. Electric shower and tankless heater also use a immersion heater shielded or naked which is turned on by the water passing and turned off when the tap is closed. A group of heaters working each one or together provide different heating levels. Electric showers and tankless heaters usually have since 3kilowatts to 7.5kilowatts according the voltage supply. Industrial immersion heaters (such as those used in electric steam boilers) may be rated at 100 kilowatts, or more, and run on a three-phase supply.