Immersion Heater vs Combi Hot Water: Which Is Cheaper in 2026?
If you still have an immersion cylinder, is it costing you more than switching to a combi? A UK engineer breaks down the real 2026 numbers.
Immersion heaters run on electricity. Combi boilers run on gas. On today's tariffs that alone dictates who wins on running cost — but the picture changes if you have solar PV, an Economy 7 tariff, or a cylinder that's already there and paid for.
Cost per litre of hot water — the real comparison
Heating 150 litres of water from 10°C to 60°C needs ~8.7 kWh of energy input.
| Source | Cost per 150L tank |
|---|---|
| Immersion — standard electric (27.5p/kWh) | £2.39 |
| Immersion — Economy 7 night rate (13p/kWh) | £1.13 |
| Immersion — surplus solar PV | ~£0 |
| Combi boiler — gas (7.9p/kWh, 90% efficient) | £0.76 |
Combi gas is roughly a third the cost of standard-rate immersion. Economy 7 closes the gap but still loses. Solar PV surplus beats both — but you can't rely on it in a British winter.
When keeping the immersion makes sense
- You have solar PV with a diverter (Solic 200, Eddi) — free hot water on sunny days.
- You're off-grid (no mains gas) — heat pump + immersion backup is often cheaper than LPG or oil.
- Your combi is dead and replacing the whole system would exceed £4,000 of disruption.
When switching to a combi wins
- You're on mains gas and your cylinder needs replacing anyway.
- You have a bigger family and hate waiting 45 minutes for the tank to reheat.
- You want to free up cupboard space.
The middle option: keep the cylinder, upgrade the immersion
If replacing everything isn't practical, a modern high-efficiency immersion element (Cotherm, Backer) with a smart timer costs £180 fitted and pays back in a year through better scheduling. Pair it with a heat-pump-ready cylinder and you've future-proofed for a heat pump swap later.
Watch out for scale
Hard-water areas (Home Counties, East Anglia) crust immersion elements in 2–3 years. Efficiency drops 20% before failure. Change the element proactively; don't wait for it to burn out.
Cover for immersion cylinders
Our home emergency cover includes emergency plumbing response for cylinder leaks and immersion failures. Not every provider does — check your policy wording. Compare cover tiers on the pricing page.
FAQ
Can I use both — combi and immersion?
Only in specific setups (thermal stores, or combi-with-cylinder installs). A standard combi doesn't work with a stored hot water cylinder.
How long does an immersion element last?
Soft water: 10–15 years. Hard water without a softener: 3–5 years.
Summary
- Standard-rate immersion costs ~3× as much as gas per tank in 2026.
- Solar PV + immersion diverter is genuinely free hot water — for the months it works.
- Off-grid? Keep the cylinder. On mains gas? Combi almost always wins.