A solar immersion diverter sends surplus PV electricity into an immersion heater, using the hot-water cylinder already in the home. A heat battery may store the same surplus more compactly, but requires a larger change.
The short answer
If the existing cylinder is sound, correctly sized and has a suitable immersion, price the diverter route first. Consider a heat battery when space, cylinder condition, flow performance or a wider heating redesign justifies replacement.
| Check | Diverter and cylinder | Heat battery |
|---|---|---|
| Existing store | Reused | Replaced or supplemented |
| Project scope | Controls and electrical connection | Storage, plumbing, electrical and controls |
| Space | Existing cylinder remains | May reduce storage footprint |
| Evidence | Cylinder and diverter limits | Product-specific output and charge data |
Available surplus sets the ceiling
Neither option can store solar that does not exist. Review export data by season and the store's uncharged capacity during those periods.
Temperature and control
The diverter must work with the immersion, thermostats and any other heat source. A heat battery needs its own compatible input and priority logic.
Export value
Include export income forgone. A higher export tariff can weaken the financial case for diverting, even though self-use rises.
Replacement timing
An ageing cylinder due for replacement creates a different comparison from a recently installed, well-insulated cylinder.
Your next step
Get two written scopes: controls added to the existing cylinder, and complete heat-battery replacement. Compare net cost, retained equipment and hot-water outcome.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026