How it works
Off-peak tariffBattery charging
Conversion lossesStored energy
Usable kWhDaytime home use
Peak import displaced
The inverter/charger schedules charging and later supplies household loads. Solar is optional. A grid-connected battery still falls within DNO connection rules; “no export” settings do not automatically remove notification or approval obligations.
Four checks before payback
Worked example At 90% round-trip efficiency over 300 days, input energy is 8 ÷ 0.90 = 8.89 kWh. Daily input costs about £0.62 and displaced peak electricity £2.40, leaving £1.78 gross. This excludes standby use, degradation/cycle cost, finance, maintenance and export opportunity. Tariffs change: replace both rates and check supplier terms.
Capacity is not enough
Battery capacity tells you how much energy can be stored, while inverter power determines how quickly the battery can be charged or discharged. During a two-hour cheap-tariff window, a 5 kW charger can draw up to 10 kWh from the grid, while a 3 kW charger can draw up to 6 kWh. The amount actually stored will be slightly lower because of charging losses and battery limits.
The inverter also limits how much power the battery can supply to the home at once. For example, if household demand is 7 kW and the inverter can supply 5 kW, the battery provides up to 5 kW and the remaining 2 kW comes from the grid.
Cycle economics
One useful screening method assigns a lifetime energy-throughput cost: battery-system cost allocated to storage ÷ warranted or realistically modelled lifetime delivered kWh. Warranty terms, calendar ageing and retained capacity complicate the estimate. Do not treat every shifted kWh as free after charging.
When the case weakens
- the peak/off-peak spread is narrow or temporary;
- the cheap window cannot refill the useful capacity;
- most household use already falls off-peak;
- export or flexibility income is assumed but not contractually available;
- complete installed cost is compared with gross rather than net savings.
Choose Battery Only in the calculator.
Enter your own tariff, capacity, power and cost assumptions.
Sources
- Energy Saving Trust battery guidance covers time-of-use charging.
- ENA energy-storage connection guidance.
- Ofgem Smart Export Guarantee for export context; eligibility and tariffs require supplier checks.