Economy 7 gives a lower electricity rate during a defined off-peak period and a higher rate at other times. A heat battery may use that window to charge, but the tariff only helps if enough demand can move into the cheap hours.
The short answer
Check four things together: the cheap rate, the day rate, the exact off-peak hours and the battery's required charge time. Ofgem notes that the seven hours vary by supplier and location; do not assume midnight to 7am applies to your meter.
Read the whole tariff
Compare the annual bill, not only the attractive night rate. Daytime appliances remain on the higher rate, and standing charges can differ. Use your actual consumption split when comparing a single-rate deal.
Will the store finish charging?
Divide the energy required by the charging power to estimate the minimum charging time. A 10 kWh charge at 2 kW needs at least five hours before losses and control behaviour. The installer should also confirm that the supply, circuit and controls support the load.
Meter and switching arrangement
Economy 7 needs a meter capable of recording peak and off-peak use separately. Confirm whether the heating circuit is permanently live and controlled by a timer, or switched by the meter. This changes how the battery's controls should be configured.
A cheap window can still be too early
Hot-water use concentrated late in the evening may sit far from the previous night's charge. Storage capacity and standing loss matter more when the gap is long.
Smart time-of-use tariffs
Some smart tariffs offer shorter or varying cheap periods. They can suit controllable storage, but the system must respond reliably and should have a safe fallback when prices or connectivity change.
Your next step
Ask your supplier to confirm the off-peak times for your meter in writing. Give those times and both unit rates to the installer, then ask for a charge schedule that fits them.
Sources
Primary and independent references checked by our editorial team.
- Economy 7 tariff guidance — Ofgem Checked 10 Nov 2026