The short answer
Grid charging can reduce energy costs when the delivered energy replaces more expensive imports by enough to cover losses and other costs. Assess the whole schedule, not just the cheapest unit price.
Calculate the delivered-energy cost
With an illustrative charging rate of 8p/kWh and round-trip efficiency of 90%, the charging cost per delivered kWh is about 8 ÷ 0.90 = 8.89p.
If it replaces a 28p import, the gross difference is about 19.11p per delivered kWh before wear, standby use and other costs. These example rates are not a supplier offer.
Check the demand it will actually replace
Do not credit the battery with avoiding an expensive unit that the household would not otherwise have imported. If the house is empty during a peak window, the useful opportunity may be smaller.
Keep the before-and-after tariff identical when testing the battery schedule. Evaluate a tariff switch as a separate change.
Leave room for the next charging opportunity
Filling the battery overnight can reduce the capacity available for daytime solar. Compare the value of stored cheap electricity with the solar energy that may then be exported.
A practical assessment uses the household load profile, the storage limit and the expected generation together. It should retain unfavourable outcomes rather than displaying savings by assumption.
Understand the calculator’s boundary
SolarIQ’s methodology describes the supported fixed-rate windows and scheduling rules. It does not connect to a live supplier tariff, operate the physical battery or calculate battery-to-grid trading revenue.
For a real installation, confirm that the manufacturer supports the schedule and reserve settings you want to use.
Before your assessment
Bring these questions with you.
- Rate and length of the cheap window.
- Loss-adjusted cost of delivered energy.
- Demand during the periods being avoided.
- Room for solar and any required backup reserve.