If you quote a battery size, export limit or installation date before giving the Distribution Network Operator (DNO) a chance to check the project, you may end up having to change the design, price or installation date later on.
The best approach is to submit a G99 so the DNO can check how what you propose to install could affect the local electricity network.
In this guide, find out when you need to apply, which route and form to use, what to submit and what can happen after the DNO reviews it.
When does a commercial solar or battery project need a G99 application?
You normally need a G99 application if all the inverters at the site can supply more than 16 amps per phase when you add their ratings together. This is about 3.68kW per phase. Include the new inverter and any solar or battery inverters already connected at the site.
Your Distribution Network Operator (DNO) runs the local electricity network and handles new connections in its area. You deal with the DNO, not the electricity supplier or the Energy Networks Association (ENA).
G98 is the simpler route for smaller installations. It may apply if the total stays within the 16-amp limit, the equipment has passed the required tests and the site has a low-voltage connection. You can usually connect the equipment first and tell the DNO within 28 days.
Use G99 if the total is higher or the project doesn’t meet another G98 rule. In most cases, you must apply to the DNO and wait for permission before connecting the new equipment.
Does battery storage need a G99 application?
Yes. A battery can need G99 even if the client plans to use all the electricity from it on site.
For example, a warehouse might install a 100kW battery to supply its machinery when electricity prices are high. You could use a zero-export setting or G100 export limitation scheme to stop electricity leaving the site.
Even so, the inverter can supply far more power than G98 allows, so the DNO must check the proposed connection and export controls before you connect it. This is because the battery is still connected to the local network and could send electricity into it if the controls stop working.
This is why you need to check the inverter’s power in kilowatts (kW), not just the battery’s storage capacity in kilowatt-hours (kWh).
The kW figure shows how much power the inverter can supply at one time. The kWh figure shows how much electricity the battery can store.
Which G99 application route applies to my installation?
Start by checking the new and existing equipment against the rules for each G99 Fast Track category. You can use Fast Track if the equipment meets every rule for one of them.
If you can’t use Fast Track, add together the power ratings of every new and existing inverter at the site. You can normally use Form A1-1 if the total is 50kW or less on a three-phase supply, or 17kW or less on a single-phase supply.
If the total is higher, use the Standard Application Form and provide the extra technical information the DNO requests.
Can we use G99 Fast Track?
G99 Fast Track is a shorter process for connecting smaller solar or battery installations at an existing site.
You can only use Fast Track if the manufacturer has tested every new and existing inverter model and shown that it follows the G98 or G99 rules. This is called fully type-tested equipment.
The installation must also fit into one of these three categories:
| Category | Equipment limits | G100 export limit | What you do |
|---|---|---|---|
| SGI-1 | Each inverter and the combined total must be no more than 16 amps per phase. | Not needed | Connect first and tell the DNO afterwards |
| SGI-2 | Each inverter must be no more than 16 amps per phase. The combined total can be up to 32 amps per phase. | 16 amps per phase | Apply and receive permission before connecting |
| SGI-3 | Each inverter must be no more than 32 amps per phase. The combined total can be up to 60 amps per phase. | 32 amps per phase | Apply and receive permission before connecting |
You cannot use Fast Track simply by setting a zero-export limit. You must still meet the other rules, including the limit on how much power each inverter can supply.
Check every part of the installation against the chosen category before you order the equipment or promise your client a connection date. National Grid lists the current limits and forms in its G99 Fast Track guidance.
Which G99 forms do you need?
Once you know which route you’re using, the table below shows which forms you need to send. Most routes require an application before connection and another form after you’ve installed and tested the equipment.
| Route | Before connection | After installation |
|---|---|---|
| Fast Track SGI-1 | No application | Form A3-3 |
| Fast Track SGI-2 or SGI-3 | Form A1-2 | Form A3-2 |
| Up to 50kW on a three-phase supply or 17kW on a single-phase supply, outside Fast Track | Form A1-1 | Form A3-1 |
| Larger projects | Standard Application Form and the relevant technical schedules | The forms requested by the DNO |
The form you send afterwards tells the DNO what you installed and how you tested it. It also confirms whether the finished installation matches the design the DNO approved.
The technical schedules provide more information about equipment such as inverters, batteries, transformers and network protection. If an inverter isn’t fully type tested, the DNO may ask for extra evidence showing that it follows the G99 rules.
The ENA publishes the standard G98 and G99 forms. Your DNO may ask you to complete the same forms through its own website, so check how it currently accepts applications before you begin.
What do I need to submit with a G99 application?
When you submit your G99 application, the DNO wants to know who the application is for, what is already connected at the site and the equipment you want to add.
It also needs to see how you’ll wire the new equipment into the site’s existing electrical system, how much power it can supply and how you will keep the site within its agreed connection limits.
Gather the following information before you start the application.
The site and client
- Site and contact details: The site address, the name and contact details of the person or company applying and the client’s written permission if you’re applying for them.
- The site’s MPAN: The Meter Point Administration Number identifies the site’s electricity connection. You can normally find it on the electricity bill.
- A site plan: A drawing showing the site boundary, its main buildings, the proposed equipment and where the site connects to the local network.
The equipment and electrical design
- Existing and new equipment: The make, model, number and power rating of the solar panels, batteries, inverters and other generators. Include equipment already connected at the site.
- Power and storage figures: The power of each inverter in kilowatts (kW), the amount each battery can store in kilowatt-hours (kWh) and the combined power of all new and existing inverters.
- A single-line diagram: A drawing showing how the solar panels, batteries, inverters, meters, switches and safety equipment will connect to the site’s electricity supply.
- Type-test information: The certificate or ENA reference number for each inverter. This shows that the manufacturer has tested the model and that it follows the relevant connection rules.
The connection, safety and timing
- Import and export limits: How much power the site can currently take from or send to the local network, along with the new limits you’re requesting.
- Protection and earthing: The settings that will disconnect the equipment during a fault and details of how the installation will carry fault electricity safely into the ground.
- G100 export controls: Details of any G100 scheme and how it will keep the electricity sent to the local network within the agreed limit.
- Expected dates: When you plan to install and test the equipment. Treat these as expected dates until the DNO has reviewed the application.
For a larger or more complicated installation, the DNO may ask for more information, potentially including:
- Transformer details
- Calculations showing how much current could flow during a fault
- A study showing whether the protection equipment will disconnect correctly
Check that all the equipment names and ratings match across the form, drawings and certificates before you send the application. If they don’t, the DNO may ask you to correct them before it starts its network study, which can delay how fast they can give you a decision.
What happens after I submit a G99 application?
The DNO first checks that the application form, drawings and certificates contain all the information it needs. If anything is missing or doesn’t match up, such as equipment names or ratings, the DNO asks you to correct it before one of its engineers starts the network study.
The engineer checks whether the local network can safely accept the proposed connection. The main concerns are whether power from the site could push the local voltage too high or make cables and transformers carry more power than they were designed for.
The engineer also checks what would happen during an electrical fault. They calculate how much current could flow and whether the protection equipment would disconnect the generating equipment from the local network quickly enough.
When making these checks, the engineer counts both the existing and new generating equipment and the maximum power the site could export.
After completing these checks, the DNO may:
- approve the connection as proposed
- allow less power to be exported or require a G100 export limitation scheme
- require a different inverter, protection settings or connection design
- require work on local cables, transformers or substations, which the client may have to pay for
If the DNO agrees that the connection can go ahead, it sends a written connection offer. The offer tells you how much generating power can be connected, how much the site may export, what must change and how much the client must pay for any network work.
The client decides whether to accept the offer. If they do, you make any required changes and install the approved equipment. You then test and commission the installation to see that the equipment works correctly and uses the settings the DNO approved.
Some DNOs may want an engineer to watch these tests. This is called witness testing. You then send the test results and final forms to the DNO. They must match the approved design or include any later changes the DNO has accepted.
Can I start installation before G99 approval?
Yes, but you take a risk if you order or install equipment before the DNO approves the connection. You can complete surveys, prepare the design and carry out other work that doesn’t connect the generating equipment to the local network.
Unless you’re using the SGI-1 notification-only route, don’t connect or commission the equipment until you receive written permission from the DNO.
The DNO might allow the site to send less electricity to the grid than planned. It could also require a smaller inverter, different protection settings or work on local cables and transformers.
If the equipment you’ve already ordered or installed no longer meets these conditions, you may have to replace it or redo part of the installation. This will cost more and push back the completion date.
How long does a G99 application take?
A qualifying Fast Track application may take around 10 working days. For example, National Grid aims to respond to G99 Fast Track applications within this time.
A standard G99 application usually takes several weeks. It can take longer if the DNO needs more detailed studies or must plan work on local cables, transformers or substations.
The DNO cannot begin its full assessment until it has all the information it needs. If a document is missing or the inverter details don’t match the drawings, it will ask you to correct the application first.
Check the response time published by the DNO before giving your client an expected date. Allow separate time for ordering the equipment, installing it and completing any work required on the local network.
How much does a G99 application cost?
There is no fixed national fee for a G99 application. The client’s cost depends on what you charge to prepare the application and whether the DNO charges for its engineers’ time.
Before the DNO gives its decision, the client may need to pay for:
- your time to prepare the drawings, complete the forms and deal with questions from the DNO
- the DNO’s assessment and design work, if it charges for this
- any extra technical studies requested by the DNO
Other costs may arise after the DNO has checked the proposed connection. These can include:
- a G100 controller or changes to the inverter and protection settings
- witness testing by a DNO engineer
- work on local cables, transformers or substations
These are not all part of the application fee. The DNO should list its charges in the connection offer. Show your client the application costs, equipment changes and network work separately so they can see what they are paying for.
What should the client proposal say about G99?
Your proposal needs to tell the client that the DNO must approve the connection before you connect the equipment. It should also show which parts of your quote may change after the DNO checks their application.
Include the following:
- The application: Who will prepare and submit it, what this work costs and whether that cost is included in your quote.
- The proposed design: The inverter size and export limit you have used to prepare the quote.
- Possible changes: Explain that the DNO may allow less export or require a different inverter, protection settings or connection design.
- Extra costs: State whether your quote includes DNO fees, witness testing and any work on local cables, transformers or substations.
- Ordering the equipment: Explain whether you will wait for DNO approval before placing the order and what happens if equipment has already been ordered.
- Changing the quote: Explain how you and the client will agree any changes to the design, price or installation date.
Don’t guarantee the export limit or installation date before the DNO replies. Describe them as assumptions. If the DNO changes either one, the client can then see why the design, price or installation date may also need to change.
Plan commercial solar and battery projects with GridVolt
GridVolt works directly with commercial solar and battery installers and EPCs across Great Britain and Ireland. Our free project simulator helps installers compare solar and battery designs using the client's electricity use, tariff, solar production and connection limits.
For a Great Britain project, the installer can use the G99 application to establish what the site may connect and export. The installer can then enter the agreed limits into the simulator instead of building the proposal around an export assumption that the DNO may not accept.
GridVolt's Energy Manager can also connect to compatible third-party batteries and site equipment after installation. It uses the site's actual tariff and operating limits to decide when a battery should charge or discharge and when suitable equipment could use electricity at a cheaper time.
Speak to GridVolt if you want help modelling the commercial case for a solar or battery project before presenting it to the client.
Frequently asked questions
Does G99 apply in Northern Ireland?
Yes, but Northern Ireland uses its own version, called G99/NI. You apply to NIE Networks using the form that matches the type and size of the installation.
Don’t use the G99 forms for England, Scotland and Wales. NIE Networks publishes the current forms and requirements for G99/NI installations.
Does the Republic of Ireland use G99?
No. For a site in the Republic of Ireland, you apply to ESB Networks instead.
ESB Networks uses different forms depending on how much power the equipment can supply and whether the site will export electricity. If the client doesn’t plan to export, use its process for parallel non-exporting generators.
What should I do if the inverter
Tell the DNO before you connect and commission the replacement inverter. The DNO approved the make, model, power rating and protection details in your original application.
Send the DNO the new inverter details, its type-test information and an updated single-line diagram. The DNO will tell you whether its original approval still applies or whether you need to change the application.
What should an installer do if the inverter changes after G99 approval?
Tell the DNO before commissioning a different inverter. The DNO assessed the make, model, rating and protection information in the approved application, so it may ask for updated documents or a revised application before the replacement equipment connects.