A grid-following inverter measures the voltage waveform the grid already provides and injects current in step with it. A grid-forming inverter creates its own voltage waveform and holds it steady, behaving more like a traditional spinning generator. Almost every rooftop solar inverter is grid-following. Grid-forming control is what allows a home battery to run a house during a blackout, and at utility scale it is becoming an important tool for keeping the power system stable as more electricity arrives through inverters rather than spinning machines.

How spinning generators held the grid together

For most of the last century, electricity came from large synchronous generators in coal, gas and hydro plants, all rotating in lockstep at a speed tied to the grid frequency of 50 hertz. Those machines gave the grid several stabilising properties without anyone having to design them in:

• Inertia: their rotating mass resists sudden changes in speed, so when a large generator trips, frequency falls gradually rather than instantly, buying time for controls to respond.
• System strength: they hold voltage firm and supply large currents during short circuits, which keeps the waveform stable and helps protection equipment detect and clear faults.
• A reference signal: the voltage waveform they create is what everything else synchronises to.

Grid-following inverters

Solar panels and batteries work with direct current, so they connect to the grid through inverters and are described as inverter-based resources. The basic DC-to-AC conversion is covered in how solar power works. A grid-following inverter uses a control loop, commonly a phase-locked loop, to measure the grid's voltage angle and frequency, then switches its electronics to deliver current precisely aligned with that waveform.

This approach is efficient, well proven and ideal for exporting as much solar as possible. Its limitation is dependence: it needs a strong, stable voltage to lock on to. On weak parts of the network, or when few synchronous machines are running, large numbers of grid-following devices can interact with each other and with disturbances in ways that make stable control harder.

Grid-following does not mean passive. Inverters connected in Australia must comply with AS/NZS 4777.2, which sets requirements for responses such as reducing output when local voltage rises too high (volt-watt), absorbing or supplying reactive power to support voltage (volt-var), and reducing output when frequency climbs. They must also disconnect when the grid is lost, a protection known as anti-islanding.

Grid-forming inverters

A grid-forming inverter controls its output as a voltage source. Rather than asking what the grid is doing, it sets a voltage and frequency and lets current flow to whatever is connected. When conditions change, it responds immediately and smoothly, much as a synchronous machine slows slightly under a sudden load.

A grid-forming inverter needs energy behind it to be useful. Batteries are the natural partner because they can supply or absorb power instantly in either direction. Solar on its own cannot provide extra power when a cloud passes unless it is deliberately run below its available output.

System strength on a high-renewables grid

As more electricity comes through inverters and fewer synchronous machines are online, particularly around the middle of sunny days, the grid can lose some of the inertia and system strength it once received for free. Several approaches are used to replace it:

• Synchronous condensers: spinning machines that produce no energy but provide inertia and fault current.
• Grid-forming batteries: large batteries whose inverters deliver fast, inertia-like responses and help hold voltage steady.
• Improved grid-following behaviour: tighter connection settings and ride-through requirements so inverters stay connected through brief disturbances.
• Network planning: strengthening weak areas and setting system strength requirements for new connections.

Grid-forming inverters have limits too. Unlike a heavy generator, power electronics cannot deliver many times their rated current during a fault without damage, so protection schemes may need rethinking. It is an active area of engineering, and rules and standards are still evolving; policy developments are tracked in industry news and policy updates.

What it means for home backup

The same principle works at the scale of a single house. When the grid fails, every grid-connected inverter must stop exporting within a short, defined time. A backup-capable hybrid or battery inverter then isolates the backup circuits from the grid and switches into grid-forming mode, creating a stable 230-volt, 50-hertz supply for those circuits. The mechanics are described in how a hybrid inverter works. Several everyday behaviours follow from this:

1. Solar only keeps running in a blackout if it is connected on the backup side of a grid-forming inverter. A separate grid-following solar inverter elsewhere on the switchboard stays off.
2. AC-coupled solar needs coordination. When a battery inverter forms a backup network with an existing solar inverter attached, it can raise the frequency slightly to signal that solar inverter to reduce output once the battery is full, using the frequency response built into compliant inverters. The two wiring approaches are compared in AC-coupled vs DC-coupled batteries.
3. Power limits matter. A home inverter forming a backup supply can deliver only its rated power, and large motor starts such as air-conditioners or pumps can briefly exceed it and trip the supply.
4. Changeover time varies. Some systems switch almost seamlessly, while others have a short interruption that can reset clocks and sensitive electronics.

Next steps

If blackout backup is part of your plans, the inverter's grid-forming capability, rated power and the circuits it will support are the details to compare. The Blackout Protection Package (from $10,490, battery and backup installed) and the Essential-Load Backup Circuit (from $990, added to a battery installation) are listed in the energy market. Prices are indicative and confirmed after a site assessment. To work out which backup design suits your home, request a free assessment from Blue Energy Solar.

Frequently asked questions

How can I tell whether my inverter can form its own supply?

Look in the datasheet or manual for terms such as backup output, EPS, off-grid mode or island mode. An inverter with a dedicated backup port and a connected battery can generally form a local supply for the circuits wired to that port. A standard solar-only inverter without these features is grid-following and shuts down in a blackout. Your installer can confirm how your system is wired and configured.

Could home batteries help stabilise the wider grid?

Collectively, they can contribute. Batteries enrolled in Virtual Power Plants already respond to price and frequency signals, and coordinated fleets can provide fast frequency support. While the grid is present, most home systems still operate as grid-following devices, and they must disconnect in a blackout for safety. How distributed batteries might support system strength is still being worked through in standards and market rules.

Does grid-forming control make a system less efficient?

Not in a way a household would notice. While connected to the grid, a backup-capable home inverter normally operates in grid-following mode and only forms its own supply when the grid is lost. For large grid batteries, operating in grid-forming mode can mean holding some capacity in reserve to respond to disturbances, which is a design and commercial choice rather than a loss of conversion efficiency.