G99 application for commercial battery storage

A battery is treated as a generator when it connects to the distribution network. That is the single fact that shapes the whole process: your project goes through the same engineering recommendation as a solar farm or a combined heat and power unit, and the network operator has to agree before the battery runs in parallel with its network. For a business buyer, the useful questions are which route the battery is on, what the operator will ask for, and where the time goes. This guide answers each from the Energy Networks Association's Distributed Generation Connection Guide (March 2025), which every GB network operator works to.

Is your battery G98 or G99? A quick checker

The dividing line in the ENA guide is current per phase: a Type A system above 16 A per phase is G99. Enter the combined power of the battery inverter or inverters at the premises, add any solar or other generation that can run with them, and pick the supply. The checker is a rule of thumb for the first conversation, not a determination: the operator decides from your application.

Supply Three-phase (400 V) Single-phase (230 V) Combined generation and storage at the premises (kW) Check

Enter a figure to see the likely route.

Assumes 230 V per phase: 16 A is about 3.68 kW single-phase and about 11 kW three-phase. Several units at one premises are added together. Islanding or export-limited designs need a conversation with the operator first.

G99 Types A to D

Under G99, a generating module is classed by registered capacity and the voltage it connects at. Storage counts as generation, so the same bands apply to a battery. These are the GB thresholds in the ENA guide:

Most behind-the-meter business batteries, from tens of kilowatts to just under a megawatt, are Type A. Standalone grid-scale and large site batteries from 1 MW upward are Type B or above. Two points catch buyers out. First, adding solar behind the same meter adds to the total connected generation, so a battery that would have been G98 on its own can become G99 once it is combined with PV. Second, a design that can run in island mode (backup during an outage) is not eligible for the fast-track process and goes through the standard G99 route; the ENA guide also notes that G99 applies to island-capable generation.

The application, stage by stage

Type A: below 1 MW

1. Choose the equipment first. The operator can only check compliance if it knows what is being installed. Equipment that is already type-tested and listed on the ENA Type Test Register as compliant carries its own evidence; equipment marked as awaiting assessment needs extra evidence at the applicant's expense and adds time before energisation. The application should quote the manufacturer's reference number from the register.
2. Apply before anything is installed. For systems up to 17 kW single-phase or 50 kW three-phase there is a simplified application form (Form A1-1 in G99's Annex A.1). Larger Type A systems use the ENA standard application form, the same form that operators use for bigger generators; it asks for the site, any existing generation, the storage type and rating, import and export capacities and the operating modes. Submit it to your own operator, found as described on our who is my DNO page.
3. The operator assesses and issues a Connection Offer. It states the equipment and works needed to connect, any extension or reinforcement of the network, whether the connection is constrained, the technical and commercial terms, and often whether the operator will want to witness commissioning. You get a defined window to accept, typically 30 to 90 days, and a lapsed offer is not guaranteed to be reissued on the same terms.
4. Install, commission, notify. After installation, the commissioning tests are carried out and the Installation Document reaches the operator within 28 days of commissioning, or on the day if the operator witnesses. The commissioning guide sets out what is tested and which documents to ask for.

Types B to D: 1 MW and above

1. Early conversation and information. The ENA guide recommends meeting the operator early, asking which work is contestable (open to an independent connections provider) and which only the operator can do, and optionally requesting an indicative design and budget estimate first.
2. Formal application and studies. Parts 1 to 4 of the ENA standard application form go in with the initial submission. The operator then carries out network studies, such as load flow, contingency, fault level and transient stability work. Studies can reach beyond your connection point: an 11 kV connection can affect the 33 kV network.
3. Connection Offer and agreements. You sign a connection agreement and, if you use an independent connections provider, an adoption agreement. The operator can offer a curtailable connection, which allows an earlier connection with restricted export until reinforcement is finished.
4. Draft PGMD, tests, Final Operational Notification. Submit a draft Power Generating Module Document at least 28 days before first synchronisation, give the operator details of the test scope at least 28 days before commissioning, and complete the tests. For Types B and C the operator issues a Final Operational Notification once you have shown compliance; the Energisation and Interim notifications apply to Type D.

How long the operator has, and where the time goes

The ENA guide sets out the regulated clocks for a formal quotation. They cover the operator's answer, not your whole programme.

A budget estimate, which is not open for acceptance and rests on a desktop study, is due in 10 working days below 1 MVA and 20 working days at 1 MVA or more, if the operator is asked to do all the work. The real programme is longer than any of these clocks, because the rest of the time is yours and the network's: finalising the equipment, answering the operator's questions, waiting for studies and reinforcement, then the acceptance window. Our guide to how long a G99 application takes sets out what sits outside the clocks, and the programme guide puts the DNO milestone on the critical path of a whole installation for that reason.

What a G99 application costs

The ENA guide does not publish one national fee, because there is none. The costs that can land on a battery project are these:

• Connection Offer expenses (assessment and design fees): since April 2018 operators may charge for network modelling, connection design and site surveys whether or not you accept the offer, and they apply the charges differently, so look at your operator's published statement.
• Connection works: the offer prices the equipment and any extension or reinforcement the operator has to carry out. Contestable work can be bid by an independent connections provider.
• Use-of-system charges: generators connected at low or high voltage pay under the Common Distribution Charging Methodology, and for generation the charge can be negative, that is, a credit.
• Metering: half-hourly metering is required if the export is to be metered, except for small-scale third-party generation under 30 kW, which can choose a non-half-hourly meter.

Where the cost of a connection comes from is part of the installed cost of the whole battery. See the cost guide for how connection fits into the installed price, and G100 export limitation for one way a constrained site can avoid a reinforcement bill.

New rules that apply to batteries commissioned now

Storage was brought into line with other generation in the G98 and G99 revisions that came into effect in September 2022: every electricity storage device commissioned after that date has to be fully compliant with G98 or G99 as appropriate. The ENA guide adds that storage installed after January 2026 must switch from import to export mode when grid frequency falls below a defined threshold; the clauses are 9.4.3 and 9.4.4 in G98 and 11.2.3.3, 12.2.3.3 and 13.2.5.2 in G99. Ask any equipment supplier to confirm in writing that the model is compliant with the current issue, not the one in force when the datasheet was printed.

A pre-application checklist

• Which operator, and which licence area? (Find your DNO.)
• Total generation and storage behind the meter, in kW and per phase, including solar.
• Model numbers and the ENA Type Test Register status of each inverter and battery system.
• Import and export capacities wanted, and whether export will be limited (G100).
• Whether the battery must keep working in an outage (island mode).
• Half-hourly meter data and a single-line diagram of the existing supply.
• Who submits the application and signs the compliance forms: you, or your installer. Agree it in writing before you appoint anyone; our installer checklist covers the question.

Related guides

• G98 vs G99 for a commercial battery: the same threshold, with worked examples.
• What is a G99 application? A plain-English glossary of PGMD, FON, ICP, contestable work and the other terms on your quote.
• How long a G99 application takes: the clocks, and what is outside them.
• Battery storage commissioning: tests, forms and the handover pack.
• Planning permission for battery storage, which runs in parallel with the connection offer.
• Containerised BESS installation and fire safety.