“Grid Code G” most commonly refers to the G98 and G99 engineering recommendations published by the Energy Networks Association (ENA) in the UK. These are the technical standards that any generator, whether a rooftop solar array or a large wind farm, must meet before connecting to the public electricity distribution network. The term can also refer to Section G of the National Grid’s broader Grid Code, which covers general legal provisions for parties connected to the transmission system. For most people searching this term, though, G98 and G99 are what matter.
G98 and G99: The Two Standards
The split between G98 and G99 comes down to the size of your generating equipment. G98 applies to smaller installations producing up to 16 amps per phase, which works out to roughly 3.68 kW on a single-phase connection. This covers the vast majority of residential solar panel systems and small battery storage setups. If your system exceeds 16 amps per phase, or produces more than 50 kW on a three-phase connection, you fall under G99 instead.
Both standards replaced their predecessors (G83 and G59) on 27 April 2019. The current version of G99 is Issue 2, published in March 2025, and is available free of charge from the ENA. Northern Ireland has its own variant, G99/NI, with a separate amendment timeline.
What These Standards Actually Require
At their core, G98 and G99 exist to protect the electricity grid from instability. When thousands of generators feed power into a network designed to carry electricity in one direction, things can go wrong: voltage spikes, frequency drift, and uncontrolled “islanding” where a section of grid stays energized after it should have shut down. The standards set rules to prevent all of this.
The UK grid operates at 50 Hz, and the national transmission system is controlled within 49.5 to 50.5 Hz under normal conditions. In exceptional circumstances, frequency can swing as high as 52 Hz or drop to 47 Hz. Connected generation equipment must handle continuous operation between 47.5 and 52 Hz, and survive at least 20 seconds if frequency dips into the 47 to 47.5 Hz range. Voltage at the 400 kV transmission level is normally maintained within 5% of its nominal value, with a hard limit of 10% that cannot persist for more than 15 minutes.
For the installer or system owner, this translates into practical requirements: your inverter (the device converting solar DC power to grid AC) must be type-tested and certified to disconnect safely if grid conditions move outside these ranges. It must also stop exporting power during a grid outage to protect workers repairing lines.
The Application Process
How much paperwork you face depends entirely on the size of your system. G99 categorizes generators into types based on capacity, and each type has a progressively more involved application route.
- Small systems under 50 kW (three-phase) or 17 kW (single-phase): If your equipment is fully type-tested and listed on the ENA Type Test Verification Report Register, you submit form A1-1 to your Distribution Network Operator (DNO). For equipment that isn’t type-tested, additional compliance verification forms are needed, with separate forms for synchronous generators and inverter-connected systems like solar panels.
- Type A (50 kW to under 1 MW): Requires a standard application form to the DNO, plus compliance evidence if the equipment isn’t type-tested.
- Type B (1 MW to under 10 MW): Standard application form, plus a Power Generating Module Document (PGMD) and commissioning compliance checks.
- Type C (10 MW to under 50 MW): Same as Type B, with additional installation documentation.
- Type D (50 MW and above, or connected above 110 kV): The most rigorous process, requiring formal energisation planning with separate interim and final operational notifications.
The smallest systems also have “fast track” routes that skip the full application entirely. Fast Track SGI 1 requires no application at all, just a notification form, provided the equipment is type-tested. Fast Track SGI 2 and SGI 3 require form A1-2 but are still significantly simpler than the full process. For all applications, parts 1 through 3 of the relevant forms should be completed at the application stage. Parts 4 and 5 can follow later but must be submitted before the connection is physically made.
What This Means for Solar Panel Owners
If you’re installing a typical residential solar system in the UK (around 3 to 4 kW), your installer handles the G98 process. They notify the DNO using standardized forms, confirm the inverter is type-tested, and the system can usually be connected without waiting for formal approval. The process is designed to be lightweight for small systems precisely because millions of homes now have rooftop solar.
Larger commercial installations, community energy projects, and battery storage systems above the 16 amps per phase threshold enter G99 territory. Here, the DNO needs to assess whether the local network can handle the additional generation. This can involve network studies, potential upgrade costs, and longer timelines. The DNO may impose conditions on how and when the system exports power.
Section G of the National Grid Code
Separate from G98 and G99, Section G of the National Grid Code covers the general legal provisions governing all parties connected to the national electricity transmission system. This applies to large power stations, transmission operators, and major industrial users rather than to residential or small commercial generators.
Section G establishes the legal framework for disputes, data handling, and contractual obligations. Disputes that cannot be resolved between parties can be referred to the energy regulator (Ofgem) under the Code’s formal dispute resolution process. Any legal proceedings arising from the Code fall under the jurisdiction of the courts of England and Wales or Scotland.
The section also includes data protection obligations. All parties must comply with data protection legislation, and no party can be forced to disclose documents under the Code that they could not be compelled to produce in civil court proceedings. Where disclosing information requires consent from the person it relates to, each party must make reasonable efforts to obtain that consent.
Consequences of Non-Compliance
In the UK, a DNO can refuse to connect generation equipment that doesn’t meet G98 or G99 requirements, or disconnect equipment that was connected without proper notification. For existing connections, non-compliance discovered during inspection can result in the DNO requiring modifications at the owner’s expense or, in serious cases, disconnection from the network.
For context on how seriously grid code violations are treated internationally: in North America, the reliability regulator (NERC) can levy penalties up to $1,000,000 per day per violation of grid reliability standards. Non-monetary sanctions include placing violators on a reliability watch list and restricting their operations. Any penalty must, at minimum, strip the violator of whatever economic benefit they gained from the non-compliant behavior. The UK system is less punitive for small generators but the principle is the same: grid stability is treated as non-negotiable.