An immersion heater is an electric resistance element screwed directly into a hot water cylinder, typically rated 2–3 kW in domestic sizes. Convection means it heats only the water above its own level, so its mounting position decides how much of the tank it serves. At 3 kW, reheating 200 litres by 50 °C takes about 3.9 hours and 11.6 kWh of electricity.
How does an immersion heater work?
An immersion heater converts electricity to heat at 100% efficiency through a sheathed resistance element immersed directly in the stored water, controlled by its own rod thermostat and backed by an over-temperature cut-out. Domestic elements are typically 2–3 kW single-phase; commercial cylinders take larger or multiple three-phase elements. Heating water needs 1.16 Wh per litre per degree, so a 3 kW element raises 200 litres by 50 °C in roughly 3.9 hours (11.6 kWh).
One behaviour defines everything about immersion use: heated water rises, so an element warms only the water above itself. The layer below it stays cold — a direct consequence of stratification.
Where should an immersion heater be positioned?
Position follows purpose, because the element's height sets the volume it can heat:
| Mounting position | Water heated | Typical use |
|---|---|---|
| Bottom entry / low side entry | Whole cylinder | Primary electric heating, off-peak tariff charging, PV surplus diversion |
| Mid-height side entry | Upper half | Everyday backup giving a usable hot top without heating the full volume |
| Top / high side entry | Top portion only | Fast boost for the next shower, anti-legionella duty on the standby zone |
Many cylinders fit two elements — one low for cheap-rate full charges, one high for quick daytime boosts — a layout that also suits solar systems, where the lower tank zone is deliberately left cold for the collectors to charge.
What role does an immersion heater play in heat pump and solar systems?
In renewable systems the immersion is the auxiliary, not the workhorse. Three jobs dominate. First, legionella control: heat pumps often store water at 48–55 °C for efficiency, and a periodic immersion boost takes the cylinder to 60 °C or above to sanitise it — legionella bacteria are killed within minutes at 60 °C. The strategy is detailed in what temperature kills legionella with a heat pump. Second, backup: a 3 kW element guarantees hot water during heat pump servicing or extreme weather. Third, PV surplus diversion: a solar diverter modulates the element to soak up export-bound photovoltaic power, storing it as hot water at zero marginal cost.
In solar thermal cylinders the immersion doubles as the cloudy-day top-up, mounted above the solar coil so it never robs the collectors of cold water to heat.
How much does an immersion heater cost to run?
An immersion delivers exactly 1 kWh of heat per kWh of electricity — a COP of 1.0 — which makes it one of the most expensive ways to make hot water at standard tariff rates. A heat pump moving the same heat operates at a COP of around 3, cutting the electricity needed by roughly two-thirds; the IEA identifies this multiple as the core running-cost advantage of heat pump water heating. The immersion earns its keep on timing and flexibility instead: charged from an off-peak tariff or from surplus PV, its effective cost per kWh can undercut daytime rates several-fold, and as a boost device it runs so few hours that efficiency barely matters. Whether time-shifting storage pays is worked through in is it cheaper to leave the hot water on all day.
Frequently asked questions
How long does an immersion heater take to heat a tank?
Time equals litres × temperature rise × 1.16, divided by element wattage. A 3 kW element heats 100 litres by 50 °C in about 1.9 hours, 200 litres in about 3.9 hours, and 300 litres in just under 6 hours — assuming the element sits low enough to heat the full volume.
Why is only the top of my cylinder hot on immersion?
Because an element only heats the water above its own level. A mid- or top-mounted immersion leaves the lower tank cold by design. If you need the whole cylinder hot on electricity, use the bottom element or check that a lower element hasn't failed.
Is it cheaper to heat water with an immersion or a heat pump?
Per kWh of heat, the heat pump wins clearly: at a COP around 3 it uses roughly a third of the electricity. An immersion only competes when powered by surplus PV or a deep off-peak tariff, or when used briefly as a boost.
Do immersion heaters scale up in hard water?
Yes. Limescale coats the element sheath, slowing heat transfer, encouraging local boiling noise and shortening element life. In hard water areas choose low-watt-density or coated elements and check them when the cylinder is serviced.
Can an immersion heater run the anti-legionella cycle?
Yes — that is one of its standard roles in heat pump systems. The controller schedules a periodic boost, typically weekly, lifting the stored water to 60 °C or more to sanitise the tank while letting the heat pump run at efficient lower temperatures the rest of the time.