Battery Storage and Power Cuts: What You Need to Know
Not all home batteries provide backup power during an outage. We explain which batteries support backup, how long they last, and why rural East Anglia homeowners should think carefully about this when choosing a system.
Most batteries do not automatically provide backup power
UK electrical regulations (G99) require grid-tied inverters to shut down when mains power is lost. Unless your battery system specifically includes an EPS (Emergency Power Supply) feature, it will shut down during a power cut — even if it is fully charged. This surprises many homeowners.
Rural East Anglia has one of the higher rates of power interruption of any region in England. Single-phase overhead lines across the Fens, Breckland, and coastal areas are exposed to winter storms, and individual spurs can lose power for hours when a pole or transformer is damaged. For homes that depend on electrically-powered heating (heat pumps) or medical equipment, power resilience is a genuine practical concern.
A home battery with EPS (Emergency Power Supply) capability transforms your system from a financial asset into a resilience asset. During a grid outage, the battery automatically disconnects from the grid (for safety) and switches to supply your home from its stored energy — all within 20 milliseconds.
How Battery Backup Works
1. Grid outage detected
The battery inverter detects loss of grid voltage within milliseconds.
2. Grid isolation
The system opens the grid connection relay — required by G99 to prevent dangerous backfeed to the grid.
3. Island mode activation
The battery switches to 'island mode', generating its own 230V AC supply from stored DC energy.
4. Protected circuits powered
Selected circuits (or the whole home, depending on installation type) receive power from the battery.
5. Solar recharging (if daytime)
During daytime outages, solar panels continue generating and recharge the battery via the hybrid inverter.
6. Grid restoration
When grid power returns, the battery safely synchronises and reconnects — no action required by the homeowner.
Battery Backup Power Comparison
| Battery | Capacity | Backup Output | Included? |
|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW continuous | Included |
| Fox ESS T-Series | 5.1-10.2 kWh | 5 kW (EPS) | Included |
| Fox ESS EVO | 5-10 kWh | 5 kW (EPS) | Included |
| SolarEdge Home Battery | 9.7 kWh | 5 kW (with module) | Add-on |
| Pylontech Force-H2 | 7.1-14.2 kWh | 5 kW (EPS) | Included |
How Long Will Your Battery Last in a Power Cut?
| Scenario | Load | 9.5 kWh battery | 13.5 kWh battery |
|---|---|---|---|
| Essentials only | 0.5 kW (fridge, lights, phone) | ~19 hrs | ~27 hrs |
| Light household use | 1.0 kW (+ TV, laptop) | ~9.5 hrs | ~13.5 hrs |
| Normal household use | 2.0 kW (+ cooking) | ~4.75 hrs | ~6.75 hrs |
| Heat pump running | 3-4 kW (depending on size) | ~2.4 hrs | ~3.4 hrs |
| Heat pump + essentials | 4-5 kW combined | ~1.9 hrs | ~2.7 hrs |
Assumes 100% depth of discharge (practical maximum 80-90%). Daytime solar recharging can extend these periods significantly.
Our Recommendation for East Anglia Homes
If you are on a rural single-phase supply in Cambridgeshire, Norfolk, or Suffolk and experience regular power interruptions, we strongly recommend specifying a battery with built-in EPS when planning your solar and battery system. The additional cost is minimal compared to the peace of mind of knowing your home stays powered during outages.
For homes with a heat pump as the primary heat source, the Tesla Powerwall 3's 11.5kW output is the most practical option — it has the power capacity to run the heat pump, maintaining home warmth during extended outages.
Frequently Asked Questions
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