Forgotten Heat Battery

The Forgotten Heat Battery

Why Britain's energy storage strategy should start with heat, not electricity

Heat is the energy problem we keep ignoring

Britain's energy storage debate is almost entirely about electricity: grid batteries, interconnectors and pumped hydro. Yet heat, not electricity, is where most of our energy goes.

The most detailed government breakdown found that heat accounted for 78% of all non-transport energy use, close to half of UK final energy. Transport took 42% of final energy in 2024, and most of what remains still goes on warming homes, offices and industrial processes.

If heat is the biggest demand, heat should be the biggest store. It also happens to be by far the cheapest form of energy to store. This article argues that the "heat battery" deserves to be at the centre of UK energy strategy, not on its fringes.

The tank we threw away

For most of the 20th century, a British home had a heat battery and nobody called it that. It was the insulated hot water cylinder in the airing cupboard, heated by the boiler or an Economy 7 immersion and drawn on through the day.

The switch to combination boilers quietly dismantled that stored capacity. Around 17 million UK homes now have a combi boiler and no hot water cylinder. Combis were sold on convenience and space, and they delivered both. But they turned hot water from a stored resource into an instantaneous demand.

That shift created an on-demand problem at national scale:

• Peaks instead of buffers. Every tap turned on fires a burner at full output. Millions of morning showers become a synchronised spike in gas demand, rather than a slow overnight top-up of a tank.
• No flexibility. A cylinder could be heated whenever energy was cheapest or most plentiful. A combi can only heat at the moment of use.
• A barrier to heat pumps. A heat pump heats water slowly and needs storage. Homes that ripped out the cylinder and repurposed the cupboard now face a space problem when they try to decarbonise.

Oversized by design

There is a further, hidden cost. To heat mains water instantly for a shower, a combi needs a large burner, typically 24–35 kW. A typical home's actual heating need is a fraction of that. In practice, peak hot water demand, not the building, sets the boiler's size. The result is a boiler that is oversized for the job it does most of the time.

A UCL study modelled a common 28 kW combi in a typical EPC band C home whose design heat load was just 3.3 kW. When a boiler cannot turn down far enough to match a small heating load, it repeatedly fires and shuts off. The study found that this short-cycling, at typical levels of oversizing, cuts efficiency by 6–9%. In heavily oversized cases the simulated boilers cycled more than 50 times a day, matching what is measured in real homes.

So the combi is penalised twice. It turns hot water into peak demand, and it locks millions of homes into a boiler several times larger than their heating needs, running less efficiently all winter. A stored-heat system breaks that link: the heat source can be sized for the building, while the store handles the bursts of hot water demand. In effect, the UK moved its hot water storage upstream into the gas network. The gas grid became the nation's heat battery, and it is a fossil one.

Heat versus electricity: cost and duration

Storing heat costs a fraction of storing electricity per kWh, and it lasts far longer. The catch is that heat is cheap to store but costly to turn back into power.

The Vojens figure is worth dwelling on. That pit store holds 12,180 MWh of heat and cost €5.01 million, so its capital cost per kWh is hundreds of times below a lithium battery's.

Three points follow:

• When the end use is heat, store heat. Thermal stores return 90% or more of their energy as heat. Turning heat back into electricity typically recovers only 30–40%.
• Duration is where heat wins outright. Lithium is economic for a few hours. Heat stores run for days, weeks or, in the Danish case, from summer to winter.
• Losses rise with time. Heat leaks, so seasonal stores lose more than daily ones. Danish pits lose between 6% and 24% of throughput a year, which scale and good insulation keep in check.

Heat batteries are already working, at every scale

This is not speculative technology. Heat batteries are operating today from the airing cupboard to the factory floor.

In the home. Edinburgh's Sunamp makes compact phase-change heat batteries that replace a hot water cylinder in a fraction of the space. They solve exactly the problem the combi created: putting storage back into homes with no cylinder cupboard. Sunamp has partnered with tepeo, whose Zero Emission Boiler stores off-peak electricity as heat for space heating. Together they offer a combi-like system with no gas at all. Units start at around £1,600–3,300, and Sunamp reports that pairing with solar PV lets a household use up to 80% of its PV output as heat.

In the town. In Pornainen, Finland, Polar Night Energy's sand battery is a 13-metre silo of crushed soapstone storing 100 MWh. It is now the main heat source for the town's district heating network, covering close to a week of winter demand or a month in summer. It has been charged at prices 70–80% below the market average, does not degrade, and has an estimated life of 25–30 years. A second plant at Vääksy, twice the size, is under construction.

Across the season. Denmark has built several large water pit stores that hold summer solar heat for winter. They are, as one Danish engineer describes them, essentially a hole in the ground with a liner, water and a floating insulated lid. Simple, but they have brought seasonal storage costs to around €24–38 per cubic metre.

In industry. In California, Rondo Energy's 100 MWh brick-based heat battery charges from on-site solar, stores heat above 1,000°C, and supplies continuous steam. It claims over 97% efficiency and needs only six hours of the cheapest electricity each day to supply heat around the clock.

Where electrical storage still belongs

None of this makes electrical batteries redundant. They should be used for the jobs they do best, which are small-scale and short-duration ones.

Lithium-ion has become remarkably cheap for short durations. BloombergNEF's benchmark cost for a four-hour battery fell 27% in 2025 to $78 per MWh. Batteries respond in under a second, which makes them ideal for frequency response, smoothing local solar output, and shaving daily peaks.

For longer gaps in renewable output, durable chemistries are emerging. Iron-air and iron-salt batteries, developed by companies such as Form Energy and Germany's VoltStorage, target 12–100 hours of storage. They avoid lithium's fire risk and degradation, though they give up efficiency to do it.

The mistake is asking batteries to do heat's job. Buffering a winter heating peak with lithium means paying electrical storage prices for a task a tank of water or a silo of sand performs for a tiny fraction of the cost.

The efficiency we forgot: fabric first

For decades, UK policy has asked which fuel heats our homes, not how much heat they waste. The combi boiler and the “all-electric” agenda share the same blind spot: both start with the heat source, not the building.

The “fabric first” principle reverses that order. Insulate, draught-proof and glaze first, then size the heating to the much smaller load that remains. Every kWh not lost through walls and lofts is a kWh that never has to be generated, transmitted or stored.

Britain abandoned that principle just as it was working. Between 2008 and 2012, around 700,000 homes a year had cavity wall insulation fitted under supplier obligation schemes. When the Green Deal replaced them in 2013, installations collapsed: in April 2013 just 1,138 cavity walls were insulated, against 49,650 a year earlier, a fall of 97%. Loft insulation fell even more sharply.

Fabric and storage work together:

A smaller load needs a smaller heat source. A well-insulated home can be heated by a correctly sized boiler or heat pump, ending the oversizing problem described above. Stored heat lasts longer. In a home that loses heat slowly, a heat battery or cylinder charged overnight covers far more of the day. The building becomes a store. Insulated thermal mass holds warmth for hours, so heating can be shifted to cheap or surplus periods without loss of comfort.

Heat pumps need it. Heat pumps deliver about 3 kWh of heat per kWh of electricity, but only reach that efficiency running at low, steady temperatures in a well-insulated building.

The same logic applies across the system. A conventional gas power station turns roughly half its fuel into electricity and discards the rest as heat. CHP captures that heat and puts most of the fuel to use.

Direct electric heating from such a grid is the least efficient route of all. Electrification is not the problem, but electrification without fabric improvement and without storage repeats the combi’s mistake at national scale.

Fabric first should therefore come before every measure in the strategy that follows.

 A heat-first strategy

The constraint in the UK is not storage technology. It is the absence of the infrastructure that makes cheap heat storage usable. A large thermal store needs a heat network or an industrial heat user to draw from it, and Britain has relatively few of either.

This is where local energy systems come into their own. Islington's Bunhill Heat Network already combines CHP with waste heat recovered from the London Underground. Add a large thermal store, and the same network can:

• Absorb surplus wind and solar as heat when power is cheap or would otherwise be curtailed.
• Let CHP run when power prices are high, banking its heat in the store instead of dumping it or throttling back.
• Flatten the morning and evening peaks that combi boilers created, drawing on stored heat rather than firing burners on demand.
• Back up electrical storage, leaving batteries to handle fast, short-duration balancing.

A heat-first policy would mean:

1. Treating heat storage as storage in energy policy and market rules, alongside batteries.
2. Supporting local heat networks with thermal stores built in from the start.
3. Encouraging hot water storage back into homes, through compact heat batteries where cylinders will not fit.
4. Rewarding flexibility, so households and networks are paid for charging their stores when the grid has surplus.
5. Pairing local CHP, local renewables and thermal storage as one system, instead of funding them in separate silos.

Conclusion: put the tank back

We once had millions of small heat batteries in British homes, and we removed them for convenience. Getting that buffering back, at household, town and industrial scale, is the cheapest storage win available to the UK.

Electrical storage has a place: small, fast and short-duration. But with heat taking nearly half of our final energy, heat should drive the storage strategy. The technology is proven, the costs are low, and the lifespans run to decades. What is missing is the policy will to treat the humble heat battery as the serious infrastructure it is.

Sources

• DESNZ – Estimates of heat use in the UK
• DESNZ – Energy Consumption in the UK 2025
• Heating & Plumbing Monthly – combi boiler homes without cylinders
• BloombergNEF – Battery storage costs hit record lows
• Dominion Electric – Commercial battery storage costs
• PatSnap – Long-duration energy storage 2026
• Solarthermalworld – Molten salt storage vs lithium-ion
• IEA SHC – Danish pit heat storage data
• pv magazine – Pornainen sand battery
• TechCrunch – Sand battery economics
• IO+ – The sand battery one year on
• Energy-Storage.news – Rondo 100 MWh heat battery
• RenewEconomy – Rondo heat battery operations
• Sunamp – Heat batteries for heat pump installs
• Sunamp and tepeo partnership
• Plumbing Superstore – Sunamp prices
• Energy-Storage.news – Sunamp PV heat batteries