C-016·Components / Storage and backup hardware
Grid-forming vs grid-following inverters
External-grid synchronisation, local island formation and terminology.
A grid-following inverter synchronises its output to an existing voltage and frequency reference. A grid-forming inverter controls a voltage waveform and can establish a reference for an electrical island. The distinction is about control behaviour, not whether the box happens to be connected to a battery.
Most ordinary grid-connected PV inverters are grid-following. When the public supply is lost, they disconnect and stop exporting because they no longer have a permitted grid reference and must not energise network lines.
Local backup uses grid-forming behaviour
A compatible battery or hybrid inverter may form a local supply after it has safely separated selected circuits from the public network. In the island it regulates voltage and frequency and supplies the load from battery and, where supported, PV.
This does not mean every inverter marketed as hybrid can run during an outage. It needs island-capable hardware, a permitted changeover arrangement, energy available for startup and an earthing and protection design for the island.
Consumer literature sometimes calls this inverter “grid forming”. The term is also used for utility-scale resources that support a live transmission system with specified voltage-source behaviour, system strength, frequency response or black-start capability. A domestic EPS forming its own small island should not be assumed to provide those wider-grid services.
Anti-islanding still applies
Grid-forming capability does not defeat anti-islanding. Before a home inverter forms a local island, the installation must be disconnected from the distribution network. When grid supply returns, the system must check and synchronise before reconnecting.
The grid connection and type-test route still follow the applicable G98 or G99 requirements. Product data should distinguish grid-connected registered capacity from backup output.
Power quality and protection in an island
The inverter becomes the source for the backed-up circuits, but its output is limited. Large load steps can cause voltage or frequency deviation, and nonlinear loads can affect waveform quality. Motors and transformers can demand high starting current.
Fault current may be much lower and more tightly controlled than it is on the public supply. That changes whether fuses, circuit-breakers and RCD arrangements disconnect faults as expected. Earthing and neutral treatment can also change at the transition into island mode.
These are reasons to design and test the final island, not reasons to assume inverter supplies are unsafe. The manufacturer’s declared island performance, BS 7671 design and the storage code of practice must be considered together.
Black start and solar operation
Grid forming after an outage needs an energy source. A battery can provide the reference and absorb differences between PV production and load. If the battery has reached its shutdown floor, is too cold, is faulted or cannot wake without the grid, the system may not black-start.
PV can operate in an island only where it is within the compatible control architecture. The grid-forming device must keep generation and demand balanced, curtail PV when necessary and protect the battery from overcharge. A separate grid-following solar inverter may or may not be supported on the backup side.
Parallel grid-forming systems
Two voltage-forming sources cannot be connected together merely because each works alone. Parallel operation requires compatible control, communications or defined droop behaviour and a tested manufacturer design. Generator, inverter and UPS combinations also need coordination.
At transmission scale, grid-forming control is used to help inverter-based resources contribute to system strength and stability. Those functions have detailed technical specifications and should not be inferred from a domestic backup label.
Installed operating modes
An intentional island arrangement is defined by:
- island-mode support in the exact inverter
- the circuits, phases and loads it can supply
- continuous, surge and load-step limits
- black-start capability and its battery conditions
- PV operation and curtailment while islanded
- neutral, earth and protective-device behaviour in grid-connected and island modes
- transfer interruption for the complete system
- synchronisation and reconnection behaviour
- coordination with any other generator or inverter
Related entries
Applies to
Battery
Last reviewed
22 Jul 2026