No — leaving an immersion heater on all the time typically costs about £24 per day, whereas heating water only when needed (e.g., 2 hours/day) costs around £2 per day.
What happens if you leave the immersion on?
The immersion thermostat will continuously cycle on and off, which can wear it out and eventually cause the heater to fail.
When the heater stays powered, the thermostat repeatedly switches the element on to maintain the set temperature and off when that temperature is reached. Over many hours this constant cycling stresses the mechanical contacts, leading to premature failure. At the same time, the hot water tank loses heat through its walls, so the system uses more energy to keep replacing that lost heat.
Is it cheaper to leave immersion on?
No — leaving the immersion on all day usually costs more than heating water only when required.
Even with a well‑insulated jacket, the cylinder loses heat steadily; reheating that lost water consumes more electricity than the brief periods needed to bring cold water up to temperature. Based on a 3 kW heater and the 2026 UK average electricity price of about £0.34 /kWh, continuous operation costs roughly £24 per day, while a two‑hour daily boost costs only about £2. Switching on as required therefore saves money.
Source: Ofgem (electricity price data).
Should you turn your immersion heater off?
Yes — keep the immersion heater switched off at its isolating switch unless you need it as a backup.
The immersion heater is intended to supplement the main boiler, not to replace it. Leaving it on unnecessarily wastes electricity and can shorten the thermostat’s life. By turning it off at the dedicated switch when hot water is not required, you avoid standby losses and only use the heater when the boiler cannot meet demand.
What is the most efficient way to use an immersion heater?
The most efficient way is to run the heater only until the thermostat cuts off, using a timer to heat water during cheap‑rate periods and keeping the tank insulated.
Set the thermostat to the desired temperature (typically 50‑60 °C) so the element switches off automatically once that point is reached. Combine this with a timer that aligns heating with off‑peak electricity tariffs, such as Economy 7, to reduce cost. Finally, ensure the hot water cylinder has a good insulating jacket (minimum 75 mm thickness) to minimise standing losses between heating cycles.
Do immersion heaters use a lot of electricity?
Yes — a typical 3 kW immersion heater uses about £1 worth of electricity per hour at 2026 rates.
Power consumption is calculated as voltage × current; most residential immersion heaters are rated at 3 kW. At the UK average price of £0.34 per kWh (Ofgem, 2026), one hour of operation draws 3 kWh, costing roughly £1.02. Running the heater for several hours each day can therefore add a noticeable amount to your electricity bill.
Source: Ofgem.
Is it cheaper to have heating on low all day?
No — according to the Energy Saving Trust, leaving heating on low all day costs more than heating only when required.
Keeping the boiler running continuously means the system constantly replaces heat lost through walls, windows, and ventilation. The Energy Saving Trust notes that the energy wasted through these standing losses exceeds the small extra effort needed to reheat the house after a setback. Using a programmable thermostat to lower the temperature when the house is empty or asleep typically saves 10‑30 % on heating bills.
Source: Energy Saving Trust.
Should I turn radiators off in unused rooms?
Yes — shut off radiators in rooms you’re not using and keep doors closed to save on heating bills.
Turning off the radiator valve stops hot water from circulating through that emitter, so the boiler does not need to maintain its temperature. As long as the room door remains closed, the cooler air stays contained and does not draw heat from the rest of the house. This approach is especially effective in larger homes where many rooms are vacant for extended periods.
How long does it take an immersion to heat water?
About 30 to 40 minutes for a standard tank to reach the set temperature.
The exact time depends on the heater’s power (usually 3 kW), the volume of water in the cylinder, and the starting temperature. A 120‑liter tank heated from 15 °C to 60 °C with a 3 kW element requires roughly 0.6 kWh, which translates to about 30‑40 minutes of continuous operation. Larger tanks or lower inlet temperatures will extend this period proportionally.
Does turning the heat on and off cost more?
No — turning the heat off when not needed saves energy because the reheating cost is lower than the standing losses.
When the heating is off, the house loses heat at a steady rate; restarting the boiler only needs to replace that lost heat, which is less than the energy required to keep the system hot continuously. Studies by the Building Research Establishment show that night‑time setbacks of 3‑5 °C can reduce annual heating demand by up to 15 % without compromising comfort.
Can I leave my immersion heater on overnight?
Yes — you can leave it on overnight, but it’s more economical to set a timer to heat the water shortly before you need it.
If you require hot water first thing in the morning, leaving the heater on all night will maintain temperature, but the standby losses over 8‑10 hours can add several pounds to your bill. Using a delay‑start timer to begin heating 30‑60 minutes before use cuts those losses while still delivering hot water on schedule. Ensure the thermostat is functional to avoid overheating.
How long will water stay hot in water heater without power?
Water will usually remain hot for about one to two days after a power cut.
The duration depends on the tank’s size, insulation quality, and ambient temperature. A well‑insulated 150‑liter cylinder can retain usable warmth for 24‑48 hours, after which the temperature drops below comfortable levels. If the tank is poorly insulated or located in a cold garage, the hold time may be shorter, so consider using a thermal blanket for extra protection.
Can you turn off the water and keep the central heating on?
Yes — turn off the water supply and keep the heating on a low setting to prevent pipes from freezing.
Closing the main stopcock isolates the domestic hot‑water circuit while allowing the boiler to circulate water through the radiators at a reduced temperature (often 10‑15 °C). This frost‑protection mode keeps the pipework above freezing without wasting energy on unnecessary heat output. Remember to open a tap briefly after turning the water back on to release any trapped air.
Can you adjust the temperature on an immersion heater?
Yes — the temperature is adjusted using the thermostat dial under the protective cap.
Most immersion heaters feature a screw‑ or nut‑held cap that, when removed, reveals a dial marked in degrees Celsius. Turning the dial clockwise raises the set temperature; counter‑clockwise lowers it. After adjustment, replace the cap securely to maintain the unit’s ingress protection rating. Always verify the new setting with a thermometer at the tap after a heating cycle.
What is the ideal temperature for an immersion heater?
The recommended setting is 50 °C in hard‑water areas (55‑60 °C for soft water) to balance safety and efficiency.
At 50 °C the water is hot enough for domestic use while minimising scale formation in hard‑water regions, which can coat the element and reduce efficiency. In soft water areas a slightly higher temperature (55‑60 °C) helps prevent legionella growth without excessive energy use. If your cylinder has twin elements, set the upper thermostat to 50 °C and the lower to 60 °C to prioritize hot water draw‑off from the top.
Source: Wikipedia.
Is it cheaper to have hot water on constant?
No — keeping hot water constantly heated costs more than heating only when needed.
Maintaining a constant temperature means the tank continuously replaces heat lost through its insulation, leading to steady energy consumption. Heating on demand incurs energy only during the actual heating period, which is far shorter than the cumulative standby losses over a day. For a typical household, switching from constant to timed heating can cut water‑heating electricity use by 40‑60 %.
