How much does my immersion heater cost to heat my water?

Following on from my post about the cost of running an oil boiler to heat the average house, see here. A reader contacted me and asked how much it costs to heat the water in their immersion heater. They remembered when they were young that the biggest sin possible was to leave on the immersion heater by accident, thereby incurring unnecessary costs. The answer is approximately €1.20 or 120 cent, when left on for two hours to initially heat the water. But once hot the amount of heat required to keep it hot will depend on the insulation surrounding the water tank.

Background. The water in an immersion heater is heated by a 3kW element and usually takes around two hours to raise the temperature to the maximum throughout the whole tank. The average cost of electricity, day-rate, in Ireland is around 20 cent/kW hour and therefore its an easy sum; 3kW X 20 cent X 2 = 120 cent or €1.20.

Once the water in the immersion heater is heated to the maximum, which is approx. 60 degrees Celsius, a thermostat kicks in and turns off the element. If the immersion heater is left on, when the water starts to cool the thermostat will keep turning the heater on when necessary to maintain the water at a high temperature. The rate at which the water cools will depend on the quality of the insulation surrounding the tank and of course whether any of the hot water is used. I hope that answers the question.

A poorly insulated immersion heated hot water tank

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5 Responses to How much does my immersion heater cost to heat my water?

  1. Naomi says:

    Is it cheaper to leave the immersion on constantly or turn it on every second day or so to heat enough water to have a shower? also how long would you need to turn it on for in order to have 3 showers? Or to fill a bath?

    • Frank Mulrennan says:

      It take about 8kw of water to heat a cylinder from cold. ( 150 litres tank from 15 Degrees to 60 degrees). A bath takes approximately 60 litres of hot water so it will take about an hour to reheat the cold water that comes in. Showers use about 6 litres a minute so a 10 minute shower is like running a bath.
      Great work with the website.

  2. Aidan says:

    I have a solar system provision installed, with 300l tank. No solar panels installed yet. Should my tank be insulated or are these type tanks insulated?

    • The Helpful Engineer says:


      Typically these large tanks are already insulated. But you should simply contact the manufacturer/ supplier to check.

      Kind Regards,


  3. Padraig says:


    I have a larger that usual immersion tank. Perhaps 4.5 feet high.

    The heating “element” was recently replaced but I do not seem to have a much hot water with the new element.

    QUESTION: Since I have a bigger tank should I have a longer element? Is it likely that the new element was not as “long” as the old and therefore only the top of my tank is heated.

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