If you manage storage projects, someone has told you recently that the inverters need to be grid-forming. You nodded, and then the price went up. This note is the explanation I give project managers. It has no equations.

An inverter is the box that turns the battery’s direct current into the alternating current the grid uses. The grid’s current swings back and forth 50 times a second. Everything connected has to swing in step.

Followers and leaders

Think of a rowing crew. Most rowers watch the person in front and match their stroke. That is a grid-following inverter. It reads the rhythm of the grid and copies it. It is cheap, reliable and completely dependent on someone else setting the pace.

For a century the pace was set by big spinning generators in coal, gas and nuclear plants. Their sheer weight kept the rhythm steady. As those plants close, there are fewer of them. On a windy night, some regions now run with very few.

A grid-forming inverter is a rower who can set the stroke. It holds its own rhythm and keeps it when something goes wrong nearby, like a fault on a line or a power station switching off without warning. Other equipment can then follow it.

A grid-following inverter needs a healthy grid. A grid-forming one helps to make it healthy

I ran my first trial in 2022 on a 40 MW site. We cut the site off from the grid on purpose and watched the inverters hold frequency by themselves. Seeing that on a real site, with the grid operator’s engineers in the room, convinced me more than any model had.

What it means for your project

• Cost. Expect the inverters to cost a few percent more, and the studies to grow by more than that.

• Time. Grid operators ask for extra modelling and extra witnessed tests. Allow two to three months.

• Capacity. The inverter keeps some headroom in reserve, so slightly less of the battery is free to trade.

• Revenue. Many operators now pay for this capability, and some will soon require it.

• People. Ask who on the team has tested it on a live site, and not only in software.

The question I would put to your engineers is simple. What happens at our site in the half second after a nearby fault, and how will we prove it before energisation? If the answer is clear, you are in good hands. If the answer is a product brochure, keep asking.