A heat battery can turn surplus solar electricity into stored hot water or space heat. That may increase self-use, but “free solar” is the wrong comparison when exported electricity earns a payment.
The short answer
Use surplus solar for heat when the household needs that heat later and its value exceeds the export payment or another use for the electricity. A diverter, battery controls and the heating system must agree on when charging is allowed.
Follow one kWh
For each unit of surplus electricity, compare three routes: export it, store it as electricity, or store it as heat. Consider conversion losses, storage losses, equipment cost and whether the energy will actually be used.
Summer and winter are different
Solar output is highest when space-heating demand is lowest. Domestic hot water provides a year-round load, but an oversized store can still finish charging early on a bright day. In winter, there may be little genuine surplus.
Controls are the project
Ask how the system detects export, limits charging and prioritises other loads. A simple system that reliably heats water can outperform a clever arrangement that repeatedly imports peak-rate electricity.
Compare with the cylinder you already own
If an immersion diverter can use your existing cylinder, price that path before replacing the store. Space constraints, cylinder condition and desired flow rate may still justify a different format.
Keep the export tariff in the calculation
The opportunity cost of diverted solar is the export income given up. Tariff terms change, so use the live rate from your own agreement and test more than one season.
Your next step
Look at a year of import and export data. Estimate how often surplus coincides with an uncharged heat store. That overlap—not annual solar generation on its own—is the useful number.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026