A standard grid-connected BESS disconnects during a grid outage — safety rules prevent back-feeding a dead network. Backup power for critical circuits requires an islanding configuration, which is an optional addition to the standard programme.
A commercial property owner installing battery storage for income generation often assumes the battery will also supply the building if the grid fails. In a standard grid-connected configuration this assumption is incorrect, and it is worth understanding why before deciding whether a backup power capability is needed.
When the grid fails, the distribution network operator's protection system detects the loss of voltage and frequency on the local network. Any generating or storage asset connected to the network must disconnect automatically. This is mandated by G99 in the UK and equivalent Swedish DSO technical standards.
The reason: if the battery continued to supply power to the building's circuits after the grid failed, current could flow back from the building's circuits into the distribution network — creating a live network that appears dead to the DNO's repair crew. This is a serious safety risk. The automatic disconnection is not a design limitation; it is a deliberate and mandated safety mechanism.
After disconnection, the battery retains its stored charge but cannot supply the building until the grid is restored and the battery's connection protocols confirm grid voltage and frequency are within normal parameters.
An islanding configuration — sometimes called backup power or island mode — allows the battery to supply a defined set of building circuits during a grid outage. It requires:
A transfer switch — a switching device that isolates the critical circuit panel from the rest of the building's distribution when the grid fails. This prevents the battery from back-feeding into the wider distribution network while still supplying the critical circuits.
An islanding-capable inverter — not all inverters support island mode. An inverter with islanding capability detects the grid failure, initiates the transfer switch, and transitions to island operation within milliseconds.
A defined critical load list — the circuits to be supplied during an outage are specified in advance. They must be within the battery's output capacity (in kW) and the battery must hold enough charge to supply them for the desired backup duration (typically two to eight hours).
These requirements mean islanding must be designed into the installation from the outset. It is not a switch that can be enabled on an existing standard installation — the transfer switch, inverter specification, and circuit separation must be in the original installation design.
A battery configured for islanding must maintain a minimum state of charge — the energy reserve needed to supply critical circuits for the desired backup duration. This reserved charge is not available for balancing market participation or energy arbitrage during normal grid-connected operation.
The revenue impact depends on the size of the reserve relative to the battery's total capacity. A 430 kWh battery reserving 100 kWh for backup has 330 kWh available for market participation. The site assessment quantifies this trade-off — the revenue lost to the backup reserve versus the value of the backup capability.
Islanding is an appropriate addition to the standard programme where the property has critical loads whose failure during a grid outage carries significant cost — refrigeration systems, server rooms, emergency lighting, fire suppression pump rooms, or production equipment that is costly to restart. The backup value is measured against the cost of the downtime it prevents, not against energy income foregone.
For properties where grid resilience is a primary requirement rather than a secondary consideration, HNordic's site assessment specifies the islanding configuration as a primary design constraint — the system is sized to provide backup capacity first and income optimisation second.
See also: Does the property owner manage the battery system? · Does installing energy infrastructure affect building insurance? · Full FAQ