G98 vs G99 Explained Without the Industry Jargon

When I first had solar and a battery installed, I thought the regulatory side of it was somebody else’s problem. There had been a form. The form had been dealt with. My system exported to the grid, the app showed nice green arrows, and I got on with the more interesting business of teaching a battery optimiser to be clever about when to charge.

It was only later, when I started seriously thinking about expanding the system — a bigger battery, more panels, folding the EV charger into the same energy strategy — that I realised the regulatory side had never been somebody else’s problem. It had been quietly deciding what I was allowed to do next, and I had signed up to those limits without understanding them. A decision made on the day of installation, in a box I never saw, was now the thing standing between me and the system I actually wanted.

So this is the article I wish someone had handed me before I started. It assumes you have never heard the terms G98 or G99, and it explains them in plain English, because they are not complicated once you strip away the jargon — and understanding them before you install is worth real money.

The one-sentence version

Before anything else, here is the whole thing in a sentence, and if you remember only this you will already be ahead of most homeowners:

G98 is “install it, then tell them.” G99 is “ask first, then install.” The line between them is about how big your system is and how much it can push back onto the grid.

Everything else in this article is detail hanging off that sentence. Let us build it up properly.

What these things actually are

G98 and G99 are not laws exactly, and they are not product standards. They are Engineering Recommendations — documents published by the Energy Networks Association, the body that represents the companies that own and run the electricity networks in Great Britain and Ireland. When you see them written formally they carry an “EREC” prefix: EREC G98 and EREC G99. In conversation everyone just says “G98” and “G99.”

Their job is to define the rules for connecting generation to the distribution network — the wires that bring electricity to your house. Anything in your home that can push energy out onto those wires counts as generation for this purpose. Solar panels obviously do. A battery does too, because it can discharge to the grid. A hybrid inverter, which manages both, is the device that actually does the exporting, and it is the thing these rules are really written about.

The reason they exist is not bureaucratic obstruction, even though it can feel that way when a dialog box interrupts your installer. The grid was designed as a one-way street: power stations at one end, houses at the other, energy flowing downhill from big to small. Domestic generation reverses that. Suddenly thousands of houses are pushing energy back up the street, and the network has to cope with that without the voltage drifting out of safe limits, without equipment being damaged, and without a fault on the grid causing your inverter to keep energising a line that a network engineer believes is dead. That last one — anti-islanding — is a genuine safety issue. An inverter that kept feeding a “dead” cable could kill someone working on it. The grid codes are, at bottom, the rules that make sure your nice green arrows do not become somebody else’s electrocution.

So G98 and G99 both answer the same question — “under what conditions may this generation connect to the network?” — and they differ only in how big the generation is, which determines how much scrutiny the network operator wants before it says yes.

The DNO: the character you did not know was in the story

To understand the difference between the two, you need to meet the party on the other end of the form. This is the Distribution Network Operator, universally abbreviated to DNO.

The DNO owns and runs the physical distribution network in your region — the substations, the transformers, the cables in the street. They are not your energy supplier. This confuses almost everyone, so it is worth being clear: your supplier (Octopus, EDF, British Gas, whoever) is who you buy electricity from and pay your bill to. Your DNO is who owns the wires. You do not choose your DNO — it is determined by where you live — and until you install generation you may never have knowingly interacted with them in your life.

When you install solar and a battery, the DNO is the party that cares. They are responsible for keeping the network stable and safe, and your new ability to export is a change to their network that they have a legitimate interest in knowing about, or approving, depending on its size. The whole G98-versus-G99 distinction is really a distinction about your relationship with the DNO: whether you are obliged merely to inform them, or obliged to ask their permission first.

                 You buy power from          You export power to
   ┌─────────────┐        ┌──────────────┐        ┌──────────────┐
   │ Your supplier│◀──────│  Your home    │───────▶│    The DNO    │
   │ (billing)    │        │ solar+battery │        │ (the wires)   │
   └─────────────┘        └──────────────┘        └──────────────┘
        the bill              generation            G98 / G99 lives here

Keep that picture in mind. G98 and G99 are the terms of your relationship with the box on the right.

Notification versus permission: the actual difference

Now we can state the difference properly, because it is genuinely the heart of the whole thing.

G98 is a notification process. If your installation is small enough to fall under G98, you are allowed to install it and connect it, and then tell the DNO — typically your installer submits the notification on your behalf, usually within a defined window after commissioning (28 days is the figure that gets quoted). You do not wait for permission. The DNO’s answer is assumed to be yes, because the equipment is fully type-tested to standards that guarantee it behaves safely on the network, and it is small enough that the network can absorb it without individual assessment. This is sometimes described as “connect and notify” or “fit and inform.” The dialog box in my first article in this series — the one my EV charger installer dismissed — was the system confirming its G98 compliance.

G99 is an application process. If your installation is larger than the G98 threshold, you must apply to the DNO before you connect, and wait for their approval. They assess your specific case: what you want to install, how much you want to export, and what the network in your area can actually accommodate. They may approve it as requested. They may approve it with conditions — most commonly a cap on how much you are allowed to export. They may, in constrained areas, require network reinforcement before they will say yes, which can carry a cost and a delay. G99 is “ask first,” and the asking is a real assessment, not a rubber stamp.

The practical difference in your life is enormous. A G98 installation can be booked, fitted, and running in the time it takes to do the physical work. A G99 installation has a DNO application in the critical path, and that application takes time — potentially weeks, occasionally longer if the network is constrained — during which nothing can be commissioned. If nobody has told you which category you are in, you can get an unpleasant surprise about timelines, or worse, a system that gets installed and then cannot legally be switched on at full capacity until an application that should have gone in weeks earlier is finally approved.

Where the line sits

So where is the threshold? This is where I have to be careful, because the exact numbers are the sort of thing that gets revised, and I would rather you understand the shape of the rule than memorise a figure that might change — always confirm the current thresholds with your installer or DNO. But the shape has been stable and it is what you need.

The dividing line is defined in terms of the generation’s rating per phase, and the number that matters is 16 amps per phase. Below that, on a fully type-tested system, you are in G98 territory. Above it, you are in G99 territory.

For a normal single-phase home — which is most UK homes — 16 amps at 230 volts works out to roughly 3.68 kW. That number, 3.68 kW, is the one you will hear again and again, and now you know where it comes from: it is the single-phase expression of the 16-amps-per-phase line.

For a three-phase supply, the same 16-amps-per-phase rule applies to each of the three phases, so the total is roughly three times as much — on the order of 11 kW before you cross into G99. This is one of the reasons a three-phase supply is quietly valuable for anyone with ambitions, and I will come back to it.

Here is the comparison laid out, because a table is worth a hundred words of hedging:

A crucial subtlety that trips people up: the threshold is about the generation and export capacity, not simply the number of panels on your roof. A modern hybrid inverter’s rating, and specifically how much it can push to the grid, is what gets measured against the line. This is exactly why the export configuration matters so much, and it leads directly to the concept that ties the whole thing together.

Export limitation: how to have a big system and stay small on paper

Here is the clever bit, and the bit that most changed how I understood my own installation.

Suppose you want a system that is genuinely larger than the G98 line — a decent-sized inverter and a big battery, because you want the ability to charge fast from cheap overnight electricity and run a lot of the house from stored energy. On paper that pushes you over 3.68 kW and into G99, with all the application and assessment that implies.

But what if you do not actually need to export at that full rate? What if the size is about your own use — self-consumption, fast charging, running the house — rather than about dumping large amounts back onto the grid? This is where export limitation comes in.

An export limitation scheme is a control function, built into the inverter or added alongside it, that actively caps how much power the system pushes to the grid, regardless of how big the system itself is. It measures the flow at your connection point and throttles export so it never exceeds a set limit — often, deliberately, set to the magic 3.68 kW. The relevant engineering recommendation here is EREC G100, which is the standard that governs how these export-limitation schemes must work to be trusted by the DNO. If you hear “G100” mentioned, that is what it is: the rulebook for capping export reliably enough that the network operator will believe your cap.

The result is that you can have a system that is physically capable of far more than 3.68 kW, but which is guaranteed never to export above the limit. Depending on the specifics and your DNO, this can keep you within a simpler connection process, or it can be the condition under which a G99 application is approved — “yes, you may install this larger system, provided export is limited to X.” Either way, the principle is the same: export limitation decouples how big your system is from how much it is allowed to push onto the grid.

This matters enormously if your goal is self-consumption and smart-tariff arbitrage rather than selling power back. My own battery optimiser mostly cares about importing cheaply and running the house from the battery, not about maximising export — for that kind of system, a sensible export limit costs you almost nothing in practice while keeping the connection simpler. But it is a decision, and it should be a conscious one, because the export limit you accept on installation day is a ceiling on part of your system’s future behaviour.

A couple of worked examples

Abstractions are slippery, so let me ground this in the kind of decisions real households actually face.

Example one: the modest starter system. A single-phase house puts up a 3.6 kW array with a 3.6 kW hybrid inverter and a small battery. The inverter’s export sits at or below 3.68 kW. This is comfortably a G98 job: install it, notify the DNO within the window, done. No application, no waiting. Straightforward, and for a lot of households genuinely the right size. The catch, which nobody may mention, is that you have now built a system with essentially no headroom above the G98 line — which becomes the whole subject of the next article when you decide, two years later, that you want more.

Example two: the ambitious self-consumer. A single-phase house wants a 5 kW inverter and a large battery, because they are on a half-hourly tariff and want to charge hard in cheap windows and run the house from storage. That inverter is over the G98 line. Two routes open up. One: apply under G99 and let the DNO assess it, possibly approving full export, possibly approving with a cap. Two: install the larger system but configure a G100-compliant export limit at 3.68 kW, so the system is big for the household’s own use but never exports beyond the line. Which route is right depends on whether they actually want to export at high power — and if they mostly want self-consumption, the export-limited route can be much simpler. The important thing is that this is a fork in the road with real consequences, and it should be chosen deliberately, not defaulted into.

Example three: the three-phase household. A house with a three-phase supply has roughly three times the G98 headroom — around 11 kW — before G99 applies. This is why anyone with three phases, or the option to get them, has quietly more room to grow. A system that would force a single-phase home into a G99 application might sit comfortably under G98 on three phases. If you have three phases and did not know it mattered, it matters.

The misconceptions worth killing

Because I held some of these myself, here are the ones I most want to correct.

“The form is just a formality my installer handles.” Sometimes it is — for a small G98 job the notification genuinely is routine. But treating all of it as a formality is how you end up over the line without an approved G99 application, or accepting an export limit you never understood. The form is a formality only when you are firmly in G98 territory and someone has actually confirmed that. Otherwise it is a decision wearing the costume of a formality.

“G99 means I can’t have a big system.” Wrong, and the opposite of the truth. G99 is the route to a big system. It is the process by which you get approval for something above the standard threshold. It is more work and it takes longer, but it is the door, not the wall.

“Export limit is about being green / being nice to the grid.” The export limit is a technical and regulatory constraint that also happens to shape your economics. It is not a moral setting. Understanding it as “the maximum rate at which my system can sell or spill to the grid” is far more useful than thinking of it as some kind of environmental courtesy.

“Battery capacity is what the DNO cares about.” Not directly. The DNO cares about the rate at which you can export — power, measured against that 16-amps-per-phase line — not how many kilowatt-hours you can store. You can have a very large battery behind a modest, export-limited inverter and remain small in the eyes of the grid code, because what matters is the size of the tap, not the size of the tank.

“Once it’s set, it’s set.” Also wrong, and this cuts both ways. You can go back to the DNO to increase an export limit or upgrade a system — but it is another application, another assessment, and potentially another cost, especially if your local network is constrained. It is not that change is impossible. It is that change made later is more expensive and slower than headroom designed in earlier, which is the theme of the whole series.

Why understanding this before you install saves money

Let me connect it back to the pound notes, because this is not an academic exercise.

Every decision in the G98/G99/G100 space sets a ceiling, and ceilings are cheap to raise before the build and expensive to raise after. If your installer sizes an inverter with no headroom above 3.68 kW because it kept you in the simple G98 process, that was probably the right call for the system you asked for — but if your real intention was to grow, you have just bought a component you will have to replace rather than one you can build on. Replacing an inverter to add capacity later can cost thousands. Choosing a slightly larger, export-limited inverter on day one might have cost very little and left the door open.

Similarly, if you know you will want to expand — a second battery, an EV charger folded into the same energy strategy, more panels — then the conversation about whether to go G99 from the start, or to design for a straightforward future application, is a conversation to have now, while the walls are open and the design is fluid, not in two years when every change is a retrofit. The DNO application you might need later is far easier to plan for than to discover you needed.

And there is a quieter saving: understanding your own export limit lets you configure your system to actually exploit your tariff. If you know your system is export-limited to 3.68 kW, you design your self-consumption and battery strategy around that reality rather than around a fantasy of unlimited export. My battery makes money precisely because I understand what it is and is not allowed to do at the grid boundary. A system whose owner does not know its own limits cannot be optimised against them.

The lessons I took from getting this wrong

I did not lose money on this, but only because my ambitions arrived late enough that I could still plan around the constraints I had unknowingly accepted. Here is what I would tell my earlier self.

The regulation is a design input, not paperwork. I filed G98/G99 mentally under “boring admin the installer does,” and that was the mistake. It is not admin. It is one of the constraints that shapes what your system can be, exactly like the size of your roof or the position of your consumer unit. Treat it as a design input from the first conversation.

Know your number. You should be able to say, out loud, what your export limit is and which process — G98 or G99 — your installation went through. If you cannot, you do not yet understand your own system, and neither, possibly, does whoever installed it. This is not hard information to have. It is just information nobody hands you unless you ask.

Headroom is the whole game. Almost every regret in this space is a headroom regret — a limit accepted, a component sized exactly to requirement, a ceiling set at today’s need with nothing above it. Understanding G98 and G99 early is really just understanding where the ceilings are early, so you can decide consciously how much room to leave above your head.

Summary

• G98 and G99 are Engineering Recommendations from the Energy Networks Association that govern how generation — solar, batteries, hybrid inverters — connects to the distribution network in Great Britain. They exist for genuine reasons: voltage stability, equipment protection, and the safety of anti-islanding.
• The party on the other end is the DNO (Distribution Network Operator), who owns the wires — not your energy supplier. G98 and G99 define the terms of your relationship with them.
• G98 is notification: small, fully type-tested systems can be installed and then reported to the DNO. G99 is permission: larger systems must be applied for and approved before connection, with a real assessment in the critical path.
• The dividing line is 16 amps per phase — roughly 3.68 kW single-phase, roughly 11 kW three-phase. The threshold is about export/generation rating, not the number of panels or the size of the battery.
• Export limitation (governed by EREC G100) decouples how big your system is from how much it can push to the grid. A large, export-limited system can stay simple on paper — valuable for self-consumption and smart-tariff strategies where you do not need high-rate export.
• Every one of these decisions sets a ceiling, and ceilings are cheap to raise before installation and expensive afterwards. Understanding G98, G99 and export limits before you build is some of the cheapest insurance available.
• Know your own number: what your export limit is, and which process your system went through. If you cannot state it, you do not yet fully own your system.

Next in the series: if I started my solar installation again, I’d design it very differently — the hindsight article, and the one where all these ceilings turn out to matter.