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
Do all home batteries provide backup power during a power cut?
No. Most battery systems are grid-tied by default, meaning they shut down automatically when grid power is lost — this is required by UK G99 regulations to prevent power flowing back into the grid and endangering network workers. Only batteries with a dedicated EPS (Emergency Power Supply) or backup power module can provide power during an outage. Always confirm EPS capability before purchasing.
Which home batteries support backup power in the UK?
Batteries with backup capability include: Fox ESS T-series (5kW EPS, standard), Tesla Powerwall 3 (built-in, 11.5kW output), SolarEdge Home Battery with backup interface module, and Growatt SPH hybrid inverter systems. Not all batteries support EPS — check before purchasing, especially if backup power is a priority.
How long will a home battery last during a power cut?
This depends entirely on your battery capacity and the loads you run. A 10kWh battery powering only essentials (fridge, lights, phone charging, laptop) at approximately 0.5-1kW will last 10-20 hours. If you run additional loads like an electric shower, hob, or EV charger, runtime drops significantly. During daytime outages, solar panels can recharge the battery, extending backup duration indefinitely if solar generation exceeds consumption.
How fast does a battery switch to backup mode during a power cut?
Tesla Powerwall 3 switches within 20 milliseconds — fast enough that most sensitive electronics do not notice the interruption. Fox ESS EPS also switches within 20ms. This is faster than typical UPS devices (which switch in 5-20ms) and much faster than a generator start-up (typically 30-60 seconds). Desktop computers and smart TVs may briefly flicker; servers and medical devices with strict power requirements may need additional UPS protection.
Can my heat pump run from battery backup power?
This depends on the battery output capacity and your heat pump size. A Tesla Powerwall 3 with 11.5kW continuous output can run most domestic heat pumps (4-12kW). Fox ESS T-series provides 5kW continuous output, which can run heat pumps up to approximately 5kW — a soft-start kit can help with larger units. For heat pump backup, we recommend specifying a battery with at least 6-8kW continuous output.
What is the difference between EPS and whole-home backup?
EPS (Emergency Power Supply) typically protects only the circuits connected to the battery's backup output — usually a dedicated consumer unit or sub-panel with selected circuits. Whole-home backup covers your entire electrical installation. Whole-home backup is more complex and expensive to install (requires a transfer switch or automatic changeover switch) but ensures everything in your home works during an outage.
How much does it cost to add EPS capability to a battery storage system?
EPS capability is included in the price of Tesla Powerwall 3 and Fox ESS T-series systems at no extra cost. For systems that require a separate consumer unit for backup circuits, additional electrical work costs £300-£600 depending on complexity. We assess EPS suitability during the free survey.
How common are power cuts in East Anglia?
East Anglia has higher rates of power interruptions than the UK average, primarily due to the rural nature of much of the network. Single-phase overhead lines serving rural Cambridgeshire, Norfolk, and Suffolk are more vulnerable to storm damage and animal contacts than underground cables. Some rural properties on 11kV rural spurs experience 3-5 interruptions per year, with individual outages lasting 30 minutes to several hours. Battery backup is a practical resilience measure for rural homeowners.
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