Fundamentally, a heat battery boiler almost entirely decouples the purchasing of electricity from the consumption of heat. A standard electric boiler draws power from the grid the exact moment that the thermostat is turned on, which complicates grid balancing during peak hours. A heat battery, conversely, charges up when grid electricity is at its cheapest and greenest (usually overnight or during a surge in renewable generation), stores that energy as heat, and then releases it precisely when the home demands it.

The Tepeo ZEB is designed to store enough energy to heat a family home for an entire day (40kWh) within something the size of a typical white goods appliance sitting in a kitchen or utility room. It integrates into existing water-based central heating systems. End-users control the heat battery boiler via an app. It acts just like a traditional boiler, but under the hood there is a high-temperature thermal storage unit paired with smart charging software.

The thermal core. The core of the boiler is built around a high-density, magnetite-based ceramic material, heated to temperatures pushing 800°C using off-peak electricity. The high-density core makes up the majority of the unit’s 375kg mass. The appliance uses three 3kW electric resistance element banks to charge the store in under five hours. The proprietary ceramic core is designed to withstand thermal cycling with zero degradation in performance.

Thermal management. It is critical that the external casing does not get too hot, as this could cause harm to users and will negatively impact efficiency. The enclosure is therefore designed to safely and effectively trap that thermal energy with minimal heat loss using insulation with a thermal conductivity of 0.02–0.03 watts per metre-Kelvin (W/mK) to keep the outer shell not more than warm to the touch.

The heat exchanger. The most challenging mechanical aspect of operating a heat battery is not storing the heat; it is extracting it precisely. It has to transfer 800°C stored heat into water flowing at around 60°C without flashing it to steam or stressing the home’s existing pipework. Carrying out that function involves a highly dynamic, variable-speed heat exchange mechanism. This boiler design uses air to extract heat from the core. It modulates the temperature of this air before it is passed over the heat exchanger, which then delivers the heat to the central heating system. The cooled air is then returned to the core in a continuous closed-loop cycle. A fan controls air flow over the heat exchanger to deliver the required water flow temperature (assuming sufficient charge).

The brains. Since a physical heat battery is useless if it is empty when needed, the boiler runs predictive charging algorithms, powered by machine learning, which take in the home’s historical heat loss profile, user preferences, next-day weather forecasts and dynamic time-of-use electricity tariffs. In addition, a state of charge algorithm indicates the heat battery charge level, which is a key part of optimising charging to both ensure comfort and minimise charging costs. The system automatically generates a charging schedule to satisfy the heat demand profile to ensure heat is delivered throughout the day for the lowest possible cost and carbon intensity. If desired, the user app can also be configured to give customers control over the boiler’s charging behaviour.

Among the system parameters monitored is electricity grid frequency. This allows the device to respond in real-time to fluctuations in grid frequency, supporting it by avoiding power draw at times of high demand, or charging to take advantage of excess renewable generation. It can also respond to external signals (for example from aggregators), facilitating participation in full grid services.

Heat batteries can be installed in the home, although their size and weight (375kg) might require some floor reinforcement  
Photo: Tepeo  
 

Wider implications

A widespread fleet of smart heat batteries such as this one offers considerable consumer led flexibility (CLF) capabilities. Instead of domestic heating being a grid liability (where millions of homes demand peak power at 6pm), heating could become a flexible grid asset that soaks up excess wind generation at 3am, relieves local substation constraints and helps keep the national grid balanced.

If the 20% of UK homes where the UK government believes heat pumps are not practical or possible, were heated by heat batteries, this would add around 200GWh of storage (equivalent to more than 20 Dinorwig pumped hydro plants) and 50GW of highly dispatchable flex to the electricity grid.

Heat battery boilers are a good complementary technology to heat pumps, which provide high coefficients of performance but are relatively inflexible. Running costs are also typically similar to those of heat pumps and gas boilers. Whilst the battery boiler is highly efficient, it cannot compare to the high coefficients of performance of a heat pump. However, its flexibility enables the vast majority of charging to be shifted into low-cost off-peak periods. There are many time-of-use tariffs now available to customers (such as electric vehicle tariffs) which provide five or more hours of off-peak electricity at approximately a third of peak rate. When billed in this way, the running costs of a battery boiler are similar to a heat pump with a seasonal coefficient of performance of 3.

The views and opinions expressed in this article are those of the author and do not necessarily reflect those of the Energy Institute.

• Further reading: ‘Heat pumps or hydrogen boilers? The 2026 decision that could shape Britain’s net zero future’. The choice between rolling out heat pumps or hydrogen boilers at scale will determine the size, cost and complexity of the entire upstream energy system required to meet the UK’s net zero target, write Cranfield University Energy Bridge Researcher Lin Gao and Visiting Lecturer Philip Naylor.
• ‘Beyond boilers – how multi-utility connections can unlock the UK Future Homes Standard’. The trend for domestic heating to become increasingly electrified or networked opens an opportunity for more coordinated planning and building of utility network connections, argues John Marsh, Chief Innovation Officer with multi-utility network provider GTC.