A common misconception among people enquiring about home battery systems for the first time is that having a battery automatically means the house will continue to have power during a grid outage.
It doesn’t.
A standard grid-connected battery system will normally stop supplying power when the grid fails, even if the battery is fully charged. To provide backup power, the installation needs additional equipment that allows the house to be safely disconnected from the electricity network.
What makes a battery system capable of backup?
A backup-capable system normally includes a gateway, or equivalent changeover equipment.
When a grid outage occurs, the gateway automatically disconnects the property from the electricity network. This prevents electricity from the battery feeding back onto the grid, where engineers may be working.
Once the house is safely isolated, the battery can supply it independently.
There may be a brief interruption — typically only a second or two — while this changeover takes place.
What can the battery power?
There are two things to consider: how much power the system can provide at any one time and how much energy is stored in the battery.
Lighting, refrigeration, internet equipment and televisions generally use relatively modest amounts of power. Electric showers, ovens, kettles and EV chargers can place much greater demands on the system.
The backup system therefore needs to be capable of supplying the loads you expect to use during an outage.
How long will the battery last?
This depends mainly on how much electricity the house is using.
For example, at an average demand of around 0.5 kW:
- 2 kWh of usable battery capacity would provide roughly 4 hours of power.
- 10 kWh would theoretically provide around 20 hours.
At an average demand of 2 kW, the same 10 kWh would last nearer 5 hours.
Actual figures will vary slightly because of usable battery capacity, reserve settings and system losses.
During a longer outage, avoiding unnecessary or high-power appliances can therefore make the battery last considerably longer.
