G99 application: connecting a battery or generator to the grid
A G99 application is the permission your distribution network operator gives before generating or storage equipment is connected. On a commercial battery project it is normally the longest lead item, and the thing most likely to move your energisation date.
If you are adding a battery, solar array, CHP unit or standby generator that can push power onto the network, the connection is governed by Engineering Recommendation G99. Smaller installations fall under the simpler G98 process instead. This page sets out which applies, who submits the application, what it contains, and the two points — export limitation and backup operation — that most often catch commercial sites out. Where the application sits among the other rules for a battery is set out in our buying guide.
Planning a battery on a constrained site?
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Within 3 working days: a 2-page summary with your indicative system size, installed cost band, the value streams that apply to your site and an honest payback range. If storage does not stack up, it says so.
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G98 or G99: which one applies?
G98 covers small installations with an aggregate registered capacity of up to and including 16 A per phase — 3.68 kW per phase. It is the light-touch route: for type-tested equipment the installer can often connect first and notify the DNO afterwards.
G99 covers everything above that threshold. In practice that is every commercial-scale battery. The arithmetic is unforgiving: a 4 kW single-phase inverter draws 4,000 ÷ 230 = 17.4 A, already past the G98 limit, so a 250 kW commercial system is not a borderline case.
Within G99 there is a faster route for smaller type-tested units, sometimes called fast track, which uses its own capacity bands. Your installer will know which band your equipment falls into; the important point for planning is that a fast-track connection still involves the DNO, and a full application involves a network study.
Who submits it, and what you should see
The application is submitted by your installer or electrical contractor, because it needs equipment type-test evidence and protection settings they hold. You are still the connection customer. Ask to see three things: the application as submitted, the DNO's offer in full, and any conditions attached to it. Conditions are where the surprises live — an export limit, an Active Network Management arrangement that curtails you at busy times, or a requirement for works you had not budgeted for.
What goes into a G99 application
- Site and supply details — address, MPAN, existing import capacity and the voltage you connect at.
- Equipment data — make and model of inverter or power conversion system, its registered capacity in kW and kVA, and the type-test evidence showing it complies with G99.
- A single-line diagram — how the battery, inverter, protection, metering and existing installation fit together.
- Protection settings — including loss-of-mains protection, so the equipment disconnects if the network fails.
- Operating intention — whether the system exports, is export-limited, or is designed to island and carry the site in an outage.
- Any limitation scheme — the G100 export (and sometimes import) limitation arrangement, if one is used.
Export limitation: how constrained sites still connect
On a busy part of the network there may not be capacity to accept everything your equipment could export. Rather than paying for reinforcement, many commercial batteries are connected with a G100 export limitation scheme: the system is configured and protected so it cannot export beyond the agreed figure. For a behind-the-meter battery whose job is to serve the site, that is rarely a commercial problem — the savings come from displacing imported power, not from export.
The same logic applies in reverse on a growing site. A battery that discharges through the site's peaks can let you add EV chargers, plant or a shift without exceeding the import capacity you already hold, which is often faster and cheaper than asking for more. See peak shaving for how that is modelled.
Backup power has to be declared
A grid-connected battery must disconnect when the network fails; that is what loss-of-mains protection is for. If you want the battery to carry essential loads through an outage, the islanding arrangement, the transfer switching and the protection that keeps the site from backfeeding a dead network all belong in the application. Retrofitting that intention later means going back to the DNO. Our guide to battery, UPS or generator backup covers the design side.
Where G99 sits in the programme
Because the DNO's assessment can require a network study, and because any reinforcement it identifies has its own lead time, the application should be submitted as early as the design allows — ideally as soon as the system's power rating and operating intention are fixed. Fire and siting design, procurement and installation planning can all run alongside it. What cannot run alongside it is energisation.
Ask the DNO for its published timescales for your connection type at the point of applying, and treat the date you are given as the project's critical path rather than an administrative formality.
What this means for your business case
Two connection outcomes change the numbers. An export limitation usually does not, for a behind-the-meter battery. A curtailment condition, or a requirement for reinforcement works, can. That is why a feasibility summary should state the assumption it has made about the connection, rather than quietly assuming an unconstrained one. See the cost guide for what the equipment itself costs, and grants and funding for the tax treatment.
G99 application: common questions
What is a G99 application?
It is the application a distribution network operator (DNO) requires before generating or storage equipment is connected to its network. Engineering Recommendation G99 sets the technical requirements — protection settings, fault ride-through, loss-of-mains — that the equipment must meet, and the application is how the DNO checks the proposed installation against them and against the capacity of the local network.
When do I need G99 rather than G98?
G98 covers small installations of up to and including 16 A per phase, which is 3.68 kW per phase. Anything above that needs G99. In practice every commercial-scale battery, and almost every commercial solar array, is a G99 job. A 4 kW single-phase inverter already draws 17.4 A, just over the G98 limit.
Who submits the G99 application?
Your installer, or the electrical contractor doing the connection work, submits it on your behalf — they hold the equipment data and protection settings the DNO needs. You remain the connection customer, so ask to see the application and the offer before anything is signed.
Does a battery need a G99 application even if it never exports?
Usually yes. Storage is treated as generation for connection purposes because it can push power back onto the network, so it falls under G99 whatever your intention. A battery that is designed never to export is normally connected with an export limitation scheme, which the application has to describe.
How long does a G99 application take?
It depends on the size of the system, the voltage you connect at and how much headroom the local network has, and it is usually the longest lead item on a battery project. Rather than relying on a rule of thumb, ask the DNO for its published timescales for your connection type when you apply, and get the target date in writing.
What is a G100 export limitation scheme?
G100 is the engineering recommendation covering export limitation. Where the network has little spare capacity, the system is configured and protected so the site never exports more than the agreed limit, which can allow a connection that would otherwise need reinforcement. It has to be declared in the application, because the DNO is approving the limitation scheme as much as the equipment.
What if I want the battery to provide backup power?
Say so in the application. A system that can run the site with the grid disconnected (islanding) needs an islanding-capable inverter, a transfer arrangement that separates the site from the network, and protection that prevents it energising a dead network. The DNO assesses that arrangement, so it cannot be added quietly afterwards.
Sources
- Energy Networks Association, Engineering Recommendation G98 and G99 — the connection standards referenced throughout. The G98 threshold of 16 A (3.68 kW) per phase is quoted from EREC G98 and SSEN's published summary of it.
- Installed cost ranges and tax treatment are set out on their own pages, with the figures dated there.
Find out what your connection will need
Send the postcode and rough annual spend. We model the system from your consumption and flag the connection questions your installer will have to answer.
What you get, within 3 working days: a 2-page battery feasibility summary naming your indicative system size (kW and kWh), the installed cost band, which value streams actually apply to your site (peak-shaving, red-band DUoS avoidance, load shifting, solar self-consumption, grid services), and an honest payback range. If storage does not stack up for your site, the summary says so.
- We do not run a lead auction. Your details are never sold or passed to a panel of installers.
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- We will not oversize a system to inflate a quote, or model a payback we cannot defend.
Published by SEO Dons, a specialist commercial energy information network. We are independent: we are not an energy supplier and not a battery manufacturer.