Surplus solar is not automatically free. Sending it into a heat store gives up the export payment and any other use the same electricity could have served.
The short answer
Compare the value of useful heat produced with export income forgone. Then account for storage loss, the cost of controls and whether the home would otherwise buy energy later to make that heat.
Route A: export
The value is the export rate for each accepted kWh, subject to the tariff terms. Export has no storage loss inside the home.
Route B: store as heat
The value is the later heat purchase avoided, adjusted for conversion and storage. If the store is already full or the heat is not used, the diverted energy has little value.
Route C: store as electricity
A home electrical battery can serve a wider range of loads later but has its own capital cost, conversion loss and capacity limits. Compare the marginal use of surplus, not the marketing category.
Seasonal mismatch
Solar surplus is abundant in summer when space-heating demand is low. Domestic hot water offers a steadier load, yet the store may fill early on bright days.
Control priority
Decide whether the system serves household loads, an EV, electrical storage, heat and export in a fixed order or by value. Make that priority visible in controls.
Your next step
Take a month with strong solar output. Count exported kWh only during periods when the heat store would have had spare capacity and later useful demand.
Sources
Primary and independent references checked by our editorial team.
- Energy storage options explained — Energy Saving Trust Checked 10 Nov 2026