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Electric Immersion Water Heater Running Costs Calculator

Hot water immersion heater elements are usually rated at 3 or 6 Kilowatts (kW). Hot water cylinders/tanks come in many shapes, sizes and capacities, ranging from 40 to over 400+ litres in larger homes. Running a bath will use approximately 60 - 80 litres of water.

To estimate* how much a tank of hot water costs to heat simply enter your tank size in litres and the immersion heater element power rating (usually 3kW). If your heater is 6kW it will cost the same amount but take half the time.

*calculation criteria

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Immersion heater power rating:
Tank capacity: litres
Cost of electricity: per tank per year*
Time to heat water: minutes per tank

Cost of heating a full water tank using an electric immersion heater

Tank Capacity Size of Home Per Tank Per Year*
120 litres 1 bedroom flat/apartment £0.97 £705.57
150 litres 1/2 bedrooms with shower/bath £1.21 £881.96
180 litres 2/3 bedrooms with shower/bath £1.45 £1,058.35
210 litres 3/4 bedrooms with shower/bath £1.69 £1,234.74
250 litres 4/5 bedrooms with shower/2x baths £2.01 £1,469.93
300 litres 5/6 bedrooms with showers/3x baths £2.42 £1,763.92
400 litres 6+ bedrooms with showers/4x baths £3.22 £2,351.89

*Based on heating mains water from 15° to 60° (recommended for hot water storage) using the UK Average rate of 15.41p/kWh (March 2017 tariff). Totals do not include keeping the tank at the required temperature between heating cycles and assume 100% efficiency. Per year costs are based on 2 heating cycles per day.

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Should you leave your immersion heater on 24/7?

Heating water with electricity is expensive when compared to gas or even oil. Leaving your immersion heater on all the time can be a waste of energy. We would recommended having a timer fitted – making hot water available to suit your lifestyle. Having a correctly sized hot water tank for your needs could help reduce your bills.

What can affect the cost of electric water heating?

Check that your immersion heater is thermostatically controlled – this is very important – otherwise it will be overheating your water which will cost more. If your water is scalding hot, then it may not be working correctly. Having an insulated cylinder is important too, most modern hot water cylinders are pre-insulated. If yours isn't, consider buying an insulated jacket to keep the water warmer for longer, so you’ll use less kilowatts (money!) to bring the water back up to the required temperature. Remember you need to heat the water in your immersion heater to above 50°C to kill off bacteria.

Can you get energy efficient immersion heaters?

Unfortunately not, electric immersion heaters will consume the same amount of electricity and produce the same amount of thermal heat, relative to power input.

Something that can affect their performance is limescale in hard-water areas. A lining of calcium carbonate (limescale) grows thicker over time on the heating elements of the immersion rod. If this becomes too thick, the heat generated by the heater element can’t be transferred into the surrounding water. This trapped heat builds up in the immersion heater rod causing it to burn-out and ultimately fail.

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