Domestic heat storage falls into three broad families: phase-change materials, heated solid materials and hot water. All hold thermal energy, but they release it differently and are not interchangeable in a heating design.
The short answer
A phase-change battery stores much of its energy as a material melts and solidifies. A solid-core battery raises the temperature of a dense material such as ceramic. A cylinder or thermal store holds heat in water. Start by asking whether the unit must supply taps, radiators or both.
| Storage type | What changes during charging | Questions to ask |
|---|---|---|
| Phase-change | Material changes between solid and liquid | Transition temperature, usable kWh, output rate |
| Solid-core | Solid material becomes hotter | Core temperature, airflow or water connection, standing loss |
| Water | Stored water becomes hotter | Volume, temperature range, pressure and heat exchanger |
Why labels overlap
“Thermal battery”, “heat battery” and “thermal store” are sometimes used as broad commercial terms. Two products carrying the same label may serve completely different loads. One may replace a hot-water cylinder; another may replace a boiler and feed a wet heating system.
Direct and indirect charging
Direct electric charging uses a heating element. Indirect charging accepts heat from another appliance, such as a heat pump. Some products offer both. The connection method changes efficiency, electrical demand and controls.
Compact does not mean equivalent
Energy density helps reduce the storage vessel, but the installed system still needs valves, pipework, controls and access. Compare useful output and total installed space rather than the box alone.
Your next step
Write “taps”, “heating” or “both” at the top of your shortlist. Remove any product whose technical documents do not explicitly support that job.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026