Grid-forming inverters: how solar and storage learned to hold the grid

Here is a fact about the electricity grid that almost nobody outside the control room thinks about: its stability has always been, in large part, a free side effect. A coal, gas, nuclear, or hydro plant spins an enormous metal rotor in lockstep with the grid's 60-hertz rhythm. That spinning mass stores kinetic energy, and when something on the grid trips — a plant drops offline, a line faults — all those rotors collectively resist the change, slowing the frequency's fall by sheer physical momentum. This is inertia, and for a hundred years it came bundled, for free, with every thermal power plant.

Solar panels and batteries have no spinning mass. They connect to the grid through inverters — power electronics that convert direct current to alternating current. As the spinning generators retire and inverter-based resources take their place, the grid is quietly losing the inertia that has always kept it stable. The question that now occupies grid engineers is deceptively simple: if the machines that used to hold the grid steady are leaving, what holds it steady instead? The answer, increasingly, is a new generation of inverters that don't just follow the grid — they form it.

01Following vs. forming the grid

Almost all the solar and storage on the grid today uses grid-following inverters. The name is precise: they detect the grid's existing voltage and frequency, then inject current in sync with it. They are, in effect, passengers — they need a stable grid signal to lock onto, and they ride whatever the grid gives them. When the grid is healthy and dominated by spinning generators setting the rhythm, grid-following inverters work beautifully. But they can't create the rhythm; they can only match it. In a grid with little spinning generation left, a roomful of grid-following inverters all waiting for someone else to set the beat is a real problem.

A grid-forming inverter is built on the opposite principle. It behaves like a voltage source — it establishes and holds its own voltage and frequency reference, independent of what the rest of the grid is doing. Instead of following the beat, it sets one. It provides synthetic inertia (sometimes called virtual inertia), responding to disturbances in milliseconds — faster, in fact, than a physical rotor can — and it can ride through faults while continuing to hold the grid up. Crucially, a grid-forming inverter can operate with no external grid reference at all, which is what makes it capable of the hardest job on the grid: starting from nothing.

A grid-following inverter asks the grid, "what's the beat?" A grid-forming inverter is the beat. That difference is the whole transition in a sentence.

02The inertia problem

Why does this matter now and not ten years ago? Because the share of the grid served by spinning generators is falling fast, and inertia falls with it. On a grid still full of thermal plants, a few inverter-based resources can free-ride on the inertia those plants provide. But as coal and gas retire and solar, wind, and batteries become the majority, there is less and less spinning mass to cushion disturbances. Frequency can then move faster and further after a fault — and if it moves too fast, protective systems trip, cascading into the kind of wide-area outage every grid operator fears.

The grids furthest along the transition hit this wall first, which is why the operators of high-renewables systems — Australia, Hawaii, Texas, and others — have been the earliest and most aggressive adopters of grid-forming technology. They reached the point where the old free inertia was no longer enough, and they needed inverters that could manufacture stability on demand. The lesson generalizes: every grid heading toward high renewable penetration will face the same inflection, and grid-forming capability is the engineered answer to it.

03Why batteries are the natural partner

Here's a subtlety that's easy to miss. A grid-forming inverter can command power instantly, but it can only deliver power it actually has. Synthetic inertia means injecting a burst of energy in the first instants of a disturbance — and that energy has to come from somewhere physical behind the inverter. A solar array at night has nothing to give. A battery, on the other hand, is a large, instantly available reservoir of energy, perfectly matched to the inverter's instant response.

That's why batteries are the natural partner for grid-forming. The combination — a grid-forming inverter backed by storage — can do what was previously the exclusive domain of large thermal plants: provide inertia, hold frequency and voltage, ride through faults, and even black-start a collapsed grid by establishing the first stable island of power and building outward. A grid-forming battery system is, functionally, a synchronous generator's capabilities delivered through silicon instead of steel — and faster.

Grid-forming is the brain; the battery is the muscle. Neither stabilizes the grid alone — together they replace the spinning generator that used to do both.

04From novelty to requirement

Grid-forming has crossed the line from research demonstration to interconnection expectation in a remarkably short time, and the standards have moved with it. IEEE 2800-2022 set the first comprehensive interconnection and performance requirements for inverter-based resources at transmission voltages (69 kV and above), defining a common technical "language" for how these resources must support the bulk grid — from fault ride-through to reactive power. Alongside it, the U.S. Department of Energy published specifications for grid-forming inverters, and the UNIFI Consortium has developed detailed grid-forming performance specs that vendors and utilities increasingly build to.

Grid-following vs. grid-forming inverters

Capability	Grid-following	Grid-forming
Needs an existing grid signal	Yes — locks onto it	No — sets its own
Provides synthetic inertia	No	Yes (milliseconds)
Holds frequency & voltage	Follows	Establishes
Black-start capable	No	Yes (with storage)
Behaves like	A current source	A voltage source

The market signal is now unmistakable. Developers submitting interconnection requests are increasingly being asked whether their equipment can operate in grid-forming mode — or switch into it during contingency events. What was a differentiator two years ago is becoming a baseline expectation. The trajectory points one way: on a grid built increasingly of inverters, the ability to form the grid rather than merely follow it is becoming table stakes.

05What it means for a hybrid plant

For a solar-plus-storage campus, grid-forming capability changes what the plant is from the grid's point of view. A conventional solar farm is a source of energy that the grid has to work to accommodate. A grid-forming, storage-backed hybrid plant is a source of energy and stability — it can hold up its corner of the grid, provide the fast frequency response that keeps the whole system steady, and, in the limit, help restart the grid after an outage. It stops being a guest the grid has to manage and becomes part of the grid's backbone.

That's a meaningful shift in value. As inertia becomes scarce, the services grid-forming resources provide — stability, fault ride-through, black-start — become more valuable, and grids are increasingly willing to require and reward them. A hybrid plant designed with grid-forming storage isn't just selling megawatt-hours; it's offering the reliability backbone that a renewables-heavy grid needs to function at all. That's exactly the kind of resource we think the grid of the next decade is built around.