Heat Battery vs Hot Water Cylinder UK 2026
Sunamp and Tepeo are pitched as cylinder replacements. The cost premium is £2-4k. Here's when that pays off, when it doesn't, and how each tech actually pairs with a heat pump.
The two technologies explained
Hot water cylinder (HWC) — the traditional choice
An unvented stainless-steel cylinder (typically 200-300L for UK homes) stores hot water. The heat pump heats a coil inside via the primary flow circuit, transferring heat into the stored water. When you turn on a tap, hot water flows out at mains pressure.
- Cost: £900-£1,400 supplied + fitted (250L Megaflo, Telford, Joule, OSO typical)
- Footprint: ~600 × 600 × 1,800 mm (250L)
- Heat losses: 1.5-2.0 kWh/day standby
- Recovery: 90-180 minutes from cold (250L)
- Lifespan: 15-25 years
Heat battery — Sunamp Thermino or Tepeo ZEB
A heat battery stores thermal energy in phase-change material (PCM) — typically a sodium acetate compound. The PCM melts at ~58°C, absorbing energy; it releases that energy when re-solidifying as it transfers heat to incoming cold mains water at draw-off.
- Cost: £2,800-£5,500 supplied + fitted
- Footprint: ~370 × 540 × 950 mm (Sunamp Thermino 210, equivalent to ~210L of hot water output)
- Heat losses: 0.3-0.7 kWh/day standby (significantly lower)
- Recovery: similar to cylinder (90-180 minutes)
- Lifespan: 15-20 years (newer tech — limited long-term field data)
Direct cost comparison — 3-bed UK home
| Item | 250L cylinder | Sunamp Thermino 210 | Tepeo ZEB |
|---|---|---|---|
| Unit cost | £600-£900 | £2,400-£3,200 | £3,200-£4,500 |
| Installation labour | £300-£500 | £400-£700 | £500-£900 |
| Total installed | £900-£1,400 | £2,800-£3,900 | £3,700-£5,400 |
| BUS-eligible | Yes | Yes | Yes |
| Footprint (mm) | 600 × 600 × 1,800 | 370 × 540 × 950 | 700 × 600 × 1,200 |
| Standby losses/day | 1.5-2.0 kWh | 0.3-0.5 kWh | 0.5-0.7 kWh |
The cost gap is real: £1,900-£4,500 more for a heat battery vs a standard cylinder. That has to be justified by space or operational benefits.
When a heat battery actually wins
Scenario 1: Small flat or 1-bed home, no airing cupboard
A 250L cylinder needs 1.8m of vertical space — often impossible in flats. A Sunamp at 950mm tall fits under a kitchen counter or in a base cupboard. This is the main use case where heat batteries earn their keep.
Scenario 2: Loft conversion with no head room
Some loft conversions don't have ceiling height for a tall cylinder. A Tepeo ZEB at 1.2m fits where a cylinder won't.
Scenario 3: Multiple appliances competing for cupboard space
If you need to share the cylinder cupboard with washer/dryer/boiler controls/storage, the compact Sunamp wins.
Scenario 4: Listed building where cylinder location is restricted
Conservation officers sometimes object to large cylinder cupboards in listed properties. A compact heat battery is easier to hide. See Listed Buildings guide.
Scenario 5: Grid-flexibility tariff optimisation
Some heat batteries can charge from grid electricity directly (alongside heat pump heating), allowing you to use Cosy or Agile cheap windows to top up. This is a marginal benefit — most cylinders can also do this via immersion heater.
When a cylinder wins
For most UK installs:
- Standard 3-bed or 4-bed home with airing cupboard or utility room space — cylinder fits, costs 1/3 the price
- Tight budget — saving £2-4k makes a real difference to net out-of-pocket after BUS
- Long lifespan certainty — cylinders have 50+ years of field history; heat batteries have 5-10 years
- Repair simplicity — any plumber can service a cylinder; heat battery service is brand-specific
- Future flexibility — cylinder works with any heat pump brand; heat batteries are more integration-specific
Performance and SCOP impact
Both store thermal energy. The heat pump heats them to ~50-55°C in normal operation. SCOP impact is minimal — both perform similarly.
One subtle difference: heat batteries' lower standby losses (0.3-0.5 kWh/day vs 1.5-2.0 kWh/day for cylinders) save roughly 400-550 kWh/year of "lost heat". At 26p/kWh that's £100-140/year saving. Over 15 years that's £1,500-£2,100 — not enough to bridge the £2-4k purchase premium for most homes, but worth noting.
Legionella and cleaning
Cylinders need a weekly Legionella cycle (60°C for 1 hour) to prevent bacterial growth in stored water. Most modern controllers do this automatically.
Heat batteries don't store water — they instantaneously heat incoming cold mains water. No Legionella risk because the stored material is sealed PCM, not water. This is a small but genuine advantage for vulnerable or immunocompromised households.
Aesthetics and noise
Both are quiet (no moving parts during storage; minimal during charging). Cylinders are taller and more visually prominent; heat batteries are squat and easier to hide behind a cupboard door.
Verdict — when each is right
- Choose a cylinder if: you have airing cupboard or utility space, you want lowest cost, you value 50+ years of proven technology, your installer is most experienced with cylinders (most are).
- Choose a heat battery if: space is critically constrained (flat, small home, no airing cupboard), you have a listed property or conservation area, you specifically want the slightly lower standby losses, or your installer specialises in heat batteries.
For 90% of UK heat pump installs in 2026, the cylinder is still the right answer. The 10% where heat battery wins is real but specific.
2030 outlook
Heat battery technology is improving fast. Sunamp's latest Thermino generation has reduced cost-per-litre by ~25% since 2022. As tech matures and volumes increase, expect the cost gap to close. By 2030, heat batteries may be the default for new builds where compactness and low standby losses matter more.