What G99 covers
G99 sets the requirements for connecting power generating facilities to the distribution networks of licensed DNOs in Great Britain. Clause 1.1 describes its purpose as covering the whole connection process, from standards of functionality through to site commissioning, so that customers, manufacturers and generators know what the DNO will require before the plant is accepted for connection.
It is deliberately technology-neutral. Clause 2.1 casts the net as wide as it will go, and Clause 1.2 says the same thing again about the kind of machine doing the conversion: what the plant is built from does not change which requirements apply. Solar, wind, gas engine, hydro, fuel cell and battery all run through the same framework.
The current edition is Issue 2, in effect from 10 March 2025.
The boundary with G98
Clause 2.1 sets the boundary by exclusion: G99 applies to everything not in the scope of G98. In practice that means anything above 16 A per phase, but there are two important additions.
G99 also picks up plant that would have been in G98 scope but is either not suitable for type testing or is designed to work in island mode. So being small is not enough on its own.
Clause 2.4 approaches it from the other direction and adds a requirement that catches people out: all power generating modules of 16 A per phase or less connecting to a DNO network shall be fully type tested, unless the DNO agrees that is impractical. The standard gives the case of plant designed specifically for one location, as is sometimes appropriate for micro hydro.
Small and bespoke is a G99 job
A one-off machine built for a particular site cannot be type tested in the G98 sense, so it lands in G99 however small it is. That is the usual route by which a modest hydro or a bespoke CHP set ends up in the heavier process.
Module, facility, power park module
G99's vocabulary trips people up more than its thresholds do, and getting it wrong changes the answer.
- A generating unit is any single piece of apparatus producing electricity. Storage devices and vehicle-to-grid electric vehicles count.
- A power park module is the non-synchronous kind — inverters, DFIG wind turbines. It can hold many units or just one.
- A power generating module is either a power park module or a synchronous one. A synchronous module can itself be several machines on a common shaft, which is how a combined cycle gas turbine plus its steam turbine counts as one module rather than two.
- A power generating facility is the generation as a whole: one or more modules, and it may connect at more than one connection point. Note it is the generating plant, not the site — the customer's own load and installation sit outside it.
The Type is assigned per module, not per facility. The standard's own worked examples make the point: a single 2.4 MW facility can contain several Type A synchronous modules alongside one Type B power park module, each carrying its own requirements. Adding your site up to one number and reading off a Type will give you the wrong answer.
For inverter-connected plant, registered capacity is based on the inverter rating. A battery sharing an inverter with PV does not add to the power park module's registered capacity; one with its own dedicated inverter does.
Type A to Type D
The classification comes from the European Requirements for Generators and is used throughout G99. Broadly:
| Type | Registered capacity | Connection voltage |
|---|---|---|
| Type A | 0.8 kW up to 1 MW | Below 110 kV |
| Type B | 1 MW up to 10 MW | Below 110 kV |
| Type C | 10 MW up to 50 MW | Below 110 kV |
| Type D | 50 MW and above | or connected at 110 kV and above |
The Type decides which technical section applies: Clause 11 for Type A, Clause 12 for Type B, and Clause 13 for Types C and D together. It also decides the compliance route, which runs Clause 16 through Clause 19 on the same A, B, C, D split.
The gap between Type A and Type B is the one that matters commercially. Type A is largely satisfied by type-tested equipment and a form. Type B adds simulation studies, reactive capability and fast fault current injection. Fault ride through is not purely a Type B addition — Type A has its own requirement at Clause 11.3, though it applies only where the DNO has asked for a network-security contribution.
What falls outside G99
Clause 2.6 puts several categories out of scope entirely: mobile generation operated by the DNO itself, G98-compliant plant, offshore power generating modules, and offshore transmission systems that contain generation.
There is also a legacy boundary. Clause 2.1 directs three groups to the older EREC G59 instead of G99:
- Plant already connected to the DNO network before 27 April 2019
- Projects whose main plant was already under contract before 17 May 2018, where the DNO was told about that contract by 17 November 2018
- Anyone holding a relevant derogation from the Authority
That exemption is not permanent. G99 applies to any module substantially modified on or after 27 April 2019, with Clause 20.3 and Annex A.6 setting out what counts as substantial. Replacing plant on an old site can pull the whole connection into the current standard.
One further case stays in G99 but under a different clause: where generation can only ever act as an alternative supply and never runs in parallel with the network, Clause 2.7 points to the switched-alternative arrangements in Clause 7.4 rather than the main body.
Battery storage and the two dates
Storage is fully inside G99 and has been treated progressively:
- Storage commissioned before 1 September 2022 is exempt from a small number of requirements, listed in Annex A.4 (Clause 2.2).
- Storage commissioned on or after 1 September 2022 complies with G99 in full.
- Requirements covering storage in an importing mode of operation apply to storage first connected on or after 1 March 2026 (Clause 2.5). That date has now passed, so any new storage connection is inside them.
Above G99: Grid Code and medium power stations
Two thresholds sit above the Type classification and are easy to miss because they are geographic.
Clause 2.9 notes that a facility reaching the Grid Code's "large" threshold must also comply with the relevant parts of the Grid Code. Those thresholds differ across GB: 10 MW in the north of Scotland, 30 MW in the south of Scotland, and 100 MW in England and Wales. The same megawatt figure can therefore mean very different obligations depending on where you are connecting.
Clause 2.10 adds that a registered capacity of 50 MW or more in England and Wales brings the embedded medium power station requirements of Clauses 6.4.4 and 13.8.
The studies a DNO will ask for
This is where G99 connects to the rest of this site, and it is explicit in the standard's own structure. Clause 9 covers network connection design and operation, and two of its subsections are studies in their own right:
- Clause 9.3 — step voltage change and rapid voltage change. The subject of EREC P28: how far the voltage moves when your plant connects, disconnects or changes output, and how often.
- Clause 9.4 — power quality, which is not one subject but three, and G99 hands each to a different standard: Clause 9.4.6 adds DC injection.
Clause 9 also covers general design criteria (9.1), system stability (9.5), island mode (9.6) and fault contributions and switchgear ratings (9.7). Clause 6.3 covers the system analysis the DNO performs for connection design across all four Types.
In practice, a G99 application means four assessments
The generation requirements in G99 itself, a voltage fluctuation assessment against P28, a harmonic assessment against G5, and a voltage imbalance check against P29. Four separate standards with four separate methods, and the DNO will expect all of them.
P29 is the one people forget, because unbalance only bites on single-phase generation connected to a three-phase system — which describes a great deal of small solar.
Application through to operational notification
Clause 6 covers the connection application and the information you must provide. Clause 7 covers connection arrangements, and starts with the operating mode:
- OM1 is the default, and it is what almost everything actually is: running in parallel with the network for the long term, with no islanding capability. Under Clause 7.1.2 the whole of G99 applies to it, except the three sections covering the other modes.
- OM2 — long term parallel operation with islanding capability (Clause 7.2)
- OM3 — infrequent short-term parallel operation (Clause 7.3)
- OM4 — switched-alternative operation, never actually in parallel (Clause 7.4)
If none of OM2 to OM4 describes your plant, you are in OM1 and the full standard applies. That is the usual answer for a solar farm, a grid-tied battery or a CHP set exporting continuously.
Compliance and commissioning then run per Type through Clauses 16 to 19, each covering the connection process, witnessing and commissioning, and operational notification. Type D carries the most, including interim, final and limited operational notification and a process for derogations.
Clause 20 sets out the obligations that continue after energisation: periodic testing, reporting operational incidents that affect compliance, and notifying changes to the facility.
Where this page stops
This is an orientation, not the standard. It paraphrases and cites; it does not reproduce G99's text, tables, figures or forms. G99 Issue 2 runs to several hundred pages and most of it is annexes and application forms. For anything you are going to submit, work from your own copy of ENA EREC G99 Issue 2 (2025), which the Energy Networks Association publishes free.