Quick Answer
A combi boiler is significantly cheaper to run than an immersion heater for most UK homes — gas costs roughly a quarter of the price per unit compared to electricity. However, if you have a heat pump, solar panels, or live in a small flat without a gas supply, an immersion heater can make sense.
The choice between an immersion heater and a combi boiler is one that UK homeowners face when replacing a hot water system — or when a property has no mains gas supply. In 2026, with energy prices continuing to affect household budgets, the running cost difference between the two systems is substantial, but the right choice depends on your circumstances. This guide gives you the full comparison.
In this guide
How Each System Works
Immersion Heater
An immersion heater is an electric heating element, similar to a kettle element, installed inside a hot water cylinder. When electricity passes through it, it heats the surrounding water. The cylinder stores hot water until it is needed. Most cylinders have a thermostat set at 60°C (to prevent Legionella bacteria) and a timer to control when the element heats the water.
Combi Boiler
A combination (combi) boiler burns gas to heat water directly from the mains on demand — there is no storage cylinder. When you turn on a hot tap, the boiler fires and heats water in seconds. For most UK homes, combi boilers are the standard because they are compact, relatively efficient, and produce instant hot water.
Running Cost Comparison
The biggest difference between the two systems is the cost of energy. In 2026, the Ofgem price cap has electricity at approximately 24p per kWh and gas at approximately 6p per kWh. This four-to-one cost ratio is the dominant factor in any immersion vs combi comparison.
Heating a 120-litre hot water cylinder from cold to 60°C requires approximately 5 kWh of energy. At current rates:
- → Immersion heater: 5 kWh × 24p = £1.20 per full cylinder heat
- → Gas boiler (combi): 5 kWh equivalent × 6p (at 85–90% efficiency) ≈ 35–40p per equivalent heat
Over a year, a family of four might heat the water equivalent of 365 full cylinder loads. The annual cost comparison is approximately:
- → Immersion heater: £438/year (365 × £1.20)
- → Combi boiler (gas): £128–£146/year (365 × 35–40p)
This is a simplified illustration — actual costs depend on water usage, cylinder size, insulation, and tariff. But the directional difference is consistent: gas combi boilers are substantially cheaper to run for hot water than electric immersion heaters on standard tariffs.
Installation Costs
For full combi boiler installation costs, see our dedicated guide on how much a new boiler costs in the UK. Summary comparison:
| Installation Type | Typical Cost (2026) | Notes |
|---|---|---|
| Replace immersion heater element only | £120–£200 | Plumber or electrician; 1–2 hours |
| Replace hot water cylinder + immersion | £600–£1,200 | New cylinder and element, installed |
| Install new combi boiler (like-for-like) | £1,800–£3,000 | Gas Safe registered installer required |
| Convert from immersion to combi (no gas) | £3,000–£5,000+ | Includes gas pipe installation to property |
| Install heat pump + hot water cylinder | £8,000–£18,000 | Eligible for Boiler Upgrade Scheme grant |
Immersion Heater vs Combi Boiler: Full Comparison
| Factor | Immersion Heater | Combi Boiler |
|---|---|---|
| Running cost (hot water) | High — electric at 24p/kWh | Low — gas at ~6p/kWh |
| Hot water availability | Limited by cylinder size; runs out | Unlimited (mains pressure) |
| Hot water speed | Must pre-heat; slow if cylinder empty | Instant on demand |
| Space required | Requires cylinder + airing cupboard | Compact unit, no cylinder needed |
| Installation cost | Low (element only £120–£200) | Moderate to high (£1,800–£3,500) |
| Gas supply required? | No | Yes |
| Best with solar/heat pump? | Yes — use off-peak/renewable electricity | No particular advantage |
| Reliability | Very reliable (few moving parts) | Reliable; more complex |
| Environmental (current grid) | Depends on grid mix | Direct gas combustion |
When an Immersion Heater Makes Sense
Despite the running cost disadvantage, an immersion heater can be the right solution in these circumstances:
- ✓ No mains gas supply — properties not on the gas network have no practical alternative to electric hot water (unless switching to oil, LPG, or heat pump)
- ✓ Solar panels — an immersion heater paired with solar PV allows you to heat water free during daylight hours using excess generation, dramatically cutting running costs
- ✓ Heat pump — heat pumps heat water more efficiently than electric resistance (3–4x more efficient at typical UK temperatures), making electricity a viable hot water source
- ✓ As a backup — if you have a combi boiler, an immersion in a cylinder provides backup hot water during a boiler breakdown
- ✓ Economy 7 / time-of-use tariffs — heating a well-insulated cylinder overnight on a cheap-rate tariff can bring costs closer to gas
For a future-looking comparison, also see our guide on heat pump vs gas boiler which covers the broader decarbonisation context. And for finding a plumber or heating engineer, see our guide on how to find a reliable plumber in the UK or post your job on GetQuickHelp.
Switching from Immersion to Combi: What's Involved
Switching from an immersion heater system to a combi boiler is a significant project if your property is not already connected to the gas mains. It typically involves:
- 1. Checking whether a gas supply is available at your street
- 2. Arranging a gas connection from the street to your property (cost: £1,000–£2,000+, depending on distance)
- 3. Installing the combi boiler and running new pipework
- 4. Removing the hot water cylinder and any cold water feed tanks
- 5. Upgrading the radiators if necessary for the new system
Total cost for a full conversion from immersion to combi: £3,000–£5,000+ in most cases. This is significant, but the ongoing gas savings of £300–£400/year may make the payback period around 8–12 years — worthwhile for a long-term owner.
Frequently Asked Questions
Is it cheaper to heat water with gas or electricity?+
In 2026, gas is approximately 4x cheaper per unit than electricity on standard tariffs (6p vs 24p per kWh). A combi boiler is therefore significantly cheaper to run than an electric immersion heater for most households. The exception is if you are using off-peak Economy 7 electricity or generating your own solar electricity.
How long does an immersion heater element last?+
An immersion heater element typically lasts 8–12 years, though hard water areas can reduce this to 5–8 years due to limescale build-up. Replacing the element costs £120–£200 including labour. Fitting a limescale reducer or using a cylinder thermostat set to 60°C helps extend element life.
Can I use an immersion heater as a backup for my combi boiler?+
Yes, but only if you have a hot water cylinder in your system. Combi boilers do not use cylinders — so a pure combi setup has no immersion heater. Properties with a system or regular boiler + cylinder often have an immersion element as a backup, which is useful during boiler breakdowns.
What is an Economy 7 tariff and does it help with immersion heating?+
Economy 7 is an electricity tariff that provides cheaper electricity for 7 hours overnight (typically midnight to 7am). If you set your immersion heater timer to heat the cylinder overnight, you can reduce the running cost significantly — typically to around 10–12p/kWh instead of 24p. However, you need a well-insulated cylinder to retain the heat through the next day.
Which is better for the environment — electric immersion or gas combi?+
Currently, it depends on the electricity grid mix. As the UK grid becomes greener (more renewable generation), electricity becomes cleaner than gas. In 2026, the carbon intensity of grid electricity is around 200g CO₂/kWh, while gas is around 240g CO₂/kWh for direct heating. The comparison is becoming more favourable for electricity over time, but gas still has the running cost advantage.
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