What is a G99 application? A plain-English jargon buster

Connection paperwork is written in abbreviations, and a quote that says “G99 Type A, DNO to carry out non-contestable works, FON on completion” is not obviously readable. This page defines the terms in the order you will meet them. Definitions follow the Energy Networks Association’s Distributed Generation Connection Guide (March 2025). For the process itself, see the G99 application guide.

The documents

• G98 and G99. The two engineering recommendations (Engineering Recommendation, EREC) that every generator connecting to the GB distribution network follows. G98 covers small units up to 16 A per phase, connected and then notified; G99 covers everything larger and needs the operator’s agreement first. Electricity storage is treated as generation under both. See G98 vs G99.
• G100. A separate recommendation for export (and import) limitation schemes. It is how a site can be held inside an agreed capacity. See the G100 guide.
• Type A, B, C, D. The size bands within G99: Type A is 0.8 kW to under 1 MW, Type B 1 MW to under 10 MW, Type C 10 MW to under 50 MW, and Type D 50 MW or more (or a connection at 110 kV or above), for connections below 110 kV for A to C.
• Type Test Register. The ENA’s online list of equipment that has been type-tested and approved against G98 or G99. Using listed, compliant equipment cuts the evidence you have to supply.

The parties

• DNO (distribution network operator). The licensed company that owns the local electricity network; there are 14 licence areas run by six companies. It, not your supplier, approves a battery connection. See who is my DNO.
• IDNO (independent distribution network operator). A company that builds and runs a network that extends a DNO’s, typically on new business parks and housing. It has no service area of its own.
• MPAN. The Meter Point Administration Number on your bill; its first two digits (the distributor ID) identify your DNO or IDNO.
• ICP (independent connections provider). A company that can carry out the contestable part of a connection in competition with the DNO.
• NESO. The National Energy System Operator, which can become involved where a project affects the transmission system.

The process words

• Standard application form. The ENA’s form used by all DNOs. There is a simpler Type A form for systems up to 17 kW single-phase or 50 kW three-phase.
• Budget estimate (or indicative estimate). An early, desktop-study price that is not open for acceptance and is subject to change. Contrast a formal quotation, based on detailed design and open for acceptance.
• Connection Offer. The DNO’s formal offer: the works, any reinforcement, whether the connection is constrained, the commercial terms and often whether it will witness commissioning. It has an acceptance window, typically 30 to 90 days.
• Connection Offer expenses (assessment and design fees). Charges for network modelling, design and surveys that a DNO may levy whether or not you accept the offer.
• Contestable and non-contestable work. Contestable work (such as building new connection infrastructure) can be bid for by an ICP; non-contestable work (often reinforcing the existing network) only the DNO can do.
• Section 16 and SLC15. Shorthand for the two routes: Section 16 of the Electricity Act, where the DNO does all the work; and SLC15 (a licence condition), where an ICP does the contestable part and the DNO the rest. Their quotation clocks differ.
• Firm, non-firm and curtailable. Whether the connection keeps its capacity when part of the network is out, or may be restricted at times. Operators use the words differently, so make sure your offer defines them. A curtailable connection can be quicker, in exchange for restrictions until reinforcement is done.
• Use-of-system (UoS) charges. Ongoing network charges. For generation connected at low or high voltage they can be negative, that is a credit.
• Long Term Development Statement (LTDS). A document each DNO publishes about its network.
• LV, HV, EHV. Low voltage (below 1 kV, in practice 400/230 V), high voltage (1 kV to 22 kV, in practice 6.6, 11 or 20 kV) and extra high voltage (22 kV and above). The voltage of your connection sets the quotation clock.

The compliance words

• Installation Document / commissioning forms. The forms that confirm a Type A system has been installed and commissioned to G99; they reach the DNO within 28 days of commissioning, or on the day if the DNO witnesses.
• PGMD (Power Generating Module Document). The compliance file for Types B and above: a checklist of criteria and pointers to the evidence. A draft goes to the DNO at least 28 days before the first synchronisation, and a final version after commissioning.
• EON, ION, FON. The Energisation, Interim and Final Operational Notifications. For Types B and C the only one you normally receive is the Final Operational Notification; EON and ION apply to Type D (an interim notification can be agreed for B or C if testing cannot all be finished). See commissioning.
• Island mode. Running on the battery when the site is disconnected from the grid; island-capable generation must meet G99 and cannot use the domestic fast-track.

Where to go next

If a term on your quote is not here, ask the installer to define it in writing: it is a fair test of whether they will manage the DNO process well (our installer checklist has the questions). To see how long the process takes once you have applied, read how long a G99 application takes. If you would like to talk it through, send an enquiry; we reply by email, and this site is not an installer.