If your hot water comes from an electric immersion, it is probably one of the biggest single uses of electricity in your home. The good news is that it is easy to cost out and easy to cut. This guide gives you the numbers from Electric Ireland, a formula you can use with your own rate, and the changes that matter most.
What an immersion actually costs
Electric Ireland’s guidance puts the cost of heating a full 120-litre cylinder at about €1.30 a day, with the caveat that it depends on cylinder size and how much hot water you use. The page gives costs and times but not kWh figures, so the examples below use a simple calculation of our own.
A typical immersion element is rated at 3 kW (check the label on yours). Running it for the 2.5 hours Electric Ireland quotes uses about 7.5 kWh. The formula is simple:
Cost per tank = kWh used × your unit rate
| Unit rate (illustrative) | Cost to heat a tank from cold (7.5 kWh) | Cost over 30 days |
|---|---|---|
| 15 c/kWh (a night rate) | €1.13 | €33.75 |
| 25 c/kWh | €1.88 | €56.25 |
| 35 c/kWh (a day rate) | €2.63 | €78.75 |
A tank heated from cold every single day at a day rate is a real cost. In practice, a tank that is topped up rather than heated from cold uses less, which is why timing and insulation matter more than the headline number.
Heat only what you need: the sink switch
If you only need a little hot water, Electric Ireland says the sink switch heats about 40 litres for about €0.45 in around 50 minutes. For a household that needs a couple of basins of hot water in the morning, that is far cheaper than heating the whole cylinder. Mount the sink and bath switches and the time switch outside the hot press, so they are easy to reach and you are more likely to use them.
Use a timer, not a habit
Electric Ireland’s advice splits in two:
- Occasional hot water: a countdown timer that starts the immersion with one button press.
- Regular use: a programmable time switch, so the immersion runs only when you need it. Choose a model with a timed boost for manual top-ups.
If you are on a night-rate or smart tariff, a timer lets you heat the cylinder in the cheap window. Our guide to night-rate electricity explains how to tell whether your usage pattern makes that worthwhile.
Insulate the cylinder and the pipes
Electric Ireland says to fit a good-quality 80mm (3 inch) lagging jacket, follow the manufacturer’s instructions, and check for international approval standards before buying one. It says this can save up to 30% on your water heating costs, which is a supplier estimate. Insulating the pipes in the hot press area also helps, and if you ever replace the cylinder, Electric Ireland suggests considering a factory-insulated model.
Using the table above, a 30% saving on a tank heated daily at 25 c/kWh would be roughly €17 a month (30% of €56.25), if that estimate holds for your cylinder. A jacket is cheap, so it is usually the first fix to try.
Element maintenance
If lime build-up keeps killing your elements, Electric Ireland notes that an Incoloy-sheathed element costs more than a standard copper one but lasts longer. That is worth asking a plumber about in hard-water areas.
Other ways to cut the cost
- Solar PV: if you have panels, diverting surplus solar to the cylinder can beat exporting it. See solar export or hot water.
- Heating controls grant: if your home was built and occupied before 2011, SEAI offers a €700 grant for smart controls that schedule heating and hot water together. See our SEAI heating controls grant guide.
- Measure it: our seven-day appliance energy audit shows how to see what each item in your home really uses.
Before you change anything
Read your latest bill for your unit rates, note the wattage printed on your immersion element, and work out your own cost per tank with the formula above. This article is general information, not personal financial advice, and the Electric Ireland figures are the supplier’s own estimates.
