Answer
A grid-following inverter controls its output current and needs a grid voltage to follow. A grid-forming inverter sets its own voltage and frequency. It can run an island and work on a weak grid. Grid-forming control does not by itself solve the fault current that grid protection needs. This follows NERC (2021) and AEMO (2025).
Last updated: November 2026
If you design or sell solar with batteries, ask two things. Must the system keep working when the grid fails? Does it connect at a weak point on the network? If either answer is yes, you need to understand grid-forming control. This guide gives the differences, the standards and what is still open. Each fact has a named source.
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Grid-forming vs grid-following: what is the difference?
NERC is the North American grid reliability body. It says the two terms have no universal definition. Its 2021 white paper defines grid-forming (GFM) control as keeping an internal voltage phasor nearly constant for a very short time. A voltage phasor is the size and angle of the voltage wave. That lets the inverter react at once when the grid changes. Grid-following (GFL) control controls current instead. It needs a phase-locked loop (PLL). A PLL is a circuit that tracks the grid’s voltage angle.
NERC compares them like this (NERC white paper, Table 1.1):
| Question | Grid-forming (GFM) | Grid-following (GFL) |
|---|---|---|
| What does it control? | Terminal voltage size and angle, or frequency | Current size and angle |
| Does it need a PLL? | May not use one | Needs a PLL or similar |
| Can it run alone? | May be designed to run standalone | Depends on the grid or other resources |
| Fast energy in the first moments of a frequency event | Inherently provides it | Can give fast frequency response after a short delay to measure and react |
| Very weak grid | Stable at very low grid strength, for example short-circuit ratio (SCR) below 1 | Stable only above some minimum strength, for example SCR above 1 |
| Black start (restart a dead grid) | Can be the first source, if designed for it | Cannot be the first source |
| Whole grid on inverters | Can run a grid at 100% inverter-based power, if some are GFM | Cannot run a grid at 100% GFL |
SCR is a measure of grid strength. A higher number means a stronger grid.
The Australian Energy Market Operator (AEMO) uses a similar definition. A GFM inverter keeps a constant internal voltage phasor over a short time. The inverter itself sets its size and frequency. AEMO describes a GFM inverter as a voltage source behind an impedance (AEMO Voluntary Specification, 2023).
How do grid-forming inverters get their behavior?
NERC lists three common control methods:
- Droop control. The inverter’s voltage frequency moves in proportion to the active power it delivers. Reactive power and voltage work the same way.
- Virtual synchronous machine (VSM). The software copies how a spinning generator reacts to a sudden load or fault.
- Virtual oscillator control. The inverter acts like a non-linear oscillator that locks to others.
Does grid-forming solve fault current?
No. NERC says GFM technology does not solve the fault current issue. The inverter’s own current and voltage limits still decide how much current it can give in a fault. NERC adds that an inverter can be built for higher fault current, but most are not.
AEMO reaches a similar view. In its 2025 plan, it calls protection-quality fault current “a key limitation” of grid-forming inverters. Their fault current differs from that of a synchronous machine. That creates doubts about protection working reliably. AEMO’s specification lists “current capacity above continuous rating” as an additional capability. It says that capability may need hardware upgrades. One example is a larger over-current rating than a grid-following design.
So do not assume a GFM inverter gives a set multiple of rated current in a fault. Ask the maker for the fault current curve of the exact model.
Key Takeaway
Grid-forming control changes how an inverter behaves in the first moments of a grid event. It does not raise the inverter’s current limit. Per NERC and AEMO, protection design still needs the fault current curve for the exact model.
What can grid-forming inverters do that grid-following ones cannot?
According to NERC and AEMO, GFM inverters can:
- run a network with no synchronous generator on it
- keep working on a very weak grid
- add fast power in the first moments of a frequency event
- restart a dead grid (black start), if designed and equipped for it
- help nearby GFL inverters stay stable
AEMO’s specification names “core” capabilities. They include voltage source behavior, frequency-domain response, inertial response, surviving the loss of the last synchronous connection, and weak-grid operation. It says core capabilities should be achievable with no or minimal change to plant hardware. Black start and extra current capacity are “additional” capabilities.
Which one do you need for a battery hybrid system?
Choose grid-forming if the system must keep supplying loads after the grid fails. Also choose it if the system connects to a weak part of the grid. NERC notes that GFM operation in off-grid and islanded systems has been common for decades. Choose grid-following when the plant sits on a strong grid and another source sets the voltage.
Grid rules also matter. Under IEEE 1547-2018, a grid-tied energy source in North America must detect an unplanned island. It must trip within 2 seconds. That time is adjustable up to 5 seconds, says a 2018 IEEE 1547 workshop presentation by members of the 1547 working group. A planned (intentional) island is different. It is built to disconnect from the grid and run on local sources.
For a home or small business battery, use these questions with the maker:
- Does the inverter keep supplying the backed-up loads when the grid fails?
- How long is the transfer time?
- Which loads can it start, such as pumps and compressors?
- Does the data sheet state the control mode in backup?
Pro Tip
Put those four questions in writing and send them to the maker before you quote a backup system. Keep the answers with the project file. A hybrid inverter data sheet that lists backup output but not the control mode is an incomplete answer.
Do not assume a hybrid inverter is grid-forming. A hybrid inverter joins PV and battery in one unit. Whether it forms a voltage in backup is a separate point on the data sheet. See also our guides to island mode with grid-forming inverters, backup power and battery design and AC-coupled versus DC-coupled batteries. Related terms: hybrid inverter and anti-islanding.
Which standards and specifications apply?
- IEEE 1547-2018 sets the 2-second island trip rule above for distributed energy resources in North America.
- AEMO Voluntary Specification for Grid-forming Inverters (May 2023) defines the core and additional capabilities. AEMO added a simulation test framework in January 2024. It says the specification is voluntary and does not replace existing National Electricity Rules.
- UNIFI Specifications Version 3 (released 30 January 2026) come from a US consortium. The National Laboratory of the Rockies (the former NREL), the University of Texas at Austin and EPRI lead it. The specifications group GFM devices into four tiers. Tier 3 is a battery system with full GFM capability and islanding. Tier 4 adds black start. UNIFI notes that AEMO, ERCOT, MISO and others use or propose versions of NERC’s grid-forming battery specification. Source: UNIFI Version 3 on OSTI.
- MISO (a US grid operator) published a draft white paper in July 2024. It proposed grid-forming requirements for stand-alone battery systems, limited to inverter software changes. Whether MISO adopted them later is not covered here. Read the latest MISO documents before you rely on this.
How big is the grid-forming market?
This guide uses AEMO data because it is public and primary. The NEM is Australia’s main electricity market.
- In November 2025, AEMO counted 10 operational GFM assets in the NEM. All were on battery systems. It counted 94 more in the connections pipeline (AEMO 2025 Transition Plan for System Security).
- In October 2026, AEMO’s Quarterly Energy Dynamics Q3 2026 reported on the battery pipeline. Of 44.7 GW of battery capacity, 38.5 GW is grid-forming and 6.0 GW is grid-following. It says grid-forming capacity in the pipeline has doubled since September 2025, from 19.2 GW to 38.5 GW.
- Grid-forming38.5 GW
- Grid-following6 GW
Source: AEMO Quarterly Energy Dynamics Q3 2026, checked November 2026.
The pipeline is projects that have applied, not projects that are built. No global market size or United States microgrid figure is given here, because no primary source was found.
Sizing hybrid systems for backup?
Model PV yield and battery savings on a real site before you decide what the inverter must do in an outage.
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Does a grid-forming inverter cost more?
No price premium is published in a primary source, so none is given here. Two sourced points help. AEMO says core capabilities should need no or minimal hardware change. MISO’s 2024 draft limited its proposed requirements to software changes. Extra current capacity or a bigger energy buffer can need hardware, per AEMO. Ask for a written price for the exact model and the exact control mode.
What about manufacturers?
This guide does not list which makers ship grid-forming products. A maker’s claim should come from its own data sheet for the exact model and firmware. When you compare products, ask for three data sheet lines. They are backup or island operation, the transfer time, and the fault current. For where design tools fit in the workflow, see solar design software, or book a demo to walk through a PV-plus-battery project.
Frequently Asked Questions
What is the difference between a grid-forming and grid-following inverter?
A grid-following inverter controls its output current. It uses a phase-locked loop to track the grid voltage. A grid-forming inverter sets its own voltage size and frequency. If it is designed for it, it can run a network with no other generator. NERC says there is no universal definition, so check how a maker uses the terms.
Do I need a grid-forming inverter for a home battery system?
Only if you want the battery to keep supplying the home when the grid fails. Ask the maker three things. Does the inverter keep supplying the backed-up loads in an outage? How long does the transfer take? Which loads can it start? A system with no backup function can run grid-following.
Is a hybrid inverter the same as a grid-forming inverter?
No. A hybrid inverter joins PV and battery in one unit. Whether it forms its own voltage in backup is a separate point. Check the data sheet for backup or island operation and the control mode.
Does grid-forming control solve fault current?
No. NERC says GFM technology does not solve the fault current issue, because the inverter’s own current limits still apply. AEMO says protection-quality fault current remains a key limitation of grid-forming inverters. Ask the maker for the fault current curve of the exact model.
What standards and specifications cover grid-forming inverters?
IEEE 1547-2018 covers distributed energy resources in North America, including a 2-second trip for unplanned islands. AEMO published a Voluntary Specification for Grid-forming Inverters in May 2023 and a test framework in January 2024. The UNIFI consortium released Version 3 of its specifications on 30 January 2026. MISO published a draft proposal in 2024.
How much grid-forming battery capacity is planned in Australia?
AEMO’s Quarterly Energy Dynamics Q3 2026 covers the battery pipeline in the National Electricity Market. Of 44.7 GW, 38.5 GW is grid-forming and 6.0 GW is grid-following. Pipeline capacity is projects that applied, not projects that are built.
Will grid-forming inverters replace grid-following inverters?
NERC says a grid cannot run at 100% grid-following power, so some grid-forming resources are needed. It also says that in most cases existing grid-following inverters may not need to be changed to grid-forming.
Where this fits
This article is part of SurgePV's Solar Technology hub, which works through the topic from first principles to the decisions a project team actually has to make.
