G99 and DNO applications for commercial solar

Why the grid connection is a survey topic

A solar array connects to your building's electrical installation, and through it to the local distribution network. The network operator needs to know that your system will not overload cables or transformers, push voltages out of limits or cause protection problems for other customers. For anything above a small size, that means applying and waiting for approval before you connect.

The grid position can change the whole project. It can limit how much you export, add a cost for network work or affect the timeline. That is why the feasibility study looks at it early rather than leaving it to the end.

G98 or G99

In Great Britain the rules for connecting generation to the distribution network are set out in Engineering Recommendations published by the Energy Networks Association.

Battery storage counts as generation for these purposes, so a battery added alongside solar is included in the assessment. Our page on battery storage for business covers this.

Rules and thresholds can be revised, so check the current versions on the Energy Networks Association website or with your DNO.

What the DNO asks for

A G99 application is a technical submission. The installer normally prepares it on your behalf. The DNO typically wants:

1. The site address, MPAN and details of the existing connection and supply capacity.
2. The proposed generating capacity in kW and the inverter make, model and type test information.
3. The maximum export you are asking for, and the maximum import.
4. A single line diagram showing how the system connects, including protection and isolation.
5. A site plan showing the point of connection.
6. Details of any battery storage or other generation on site.
7. Details of any export limitation scheme, if one is proposed.

Incomplete applications are a common cause of delay, so it pays to get the supply information right at survey stage.

Timescales

Timescales vary by DNO, by region and by the size and complexity of the system, and they are not guaranteed. A small application on a strong part of the network can be answered relatively quickly. A larger system, or one on a constrained part of the network, can take considerably longer, especially if the DNO needs to study the network in detail. Connection reform across Great Britain has also been changing how queues and offers are managed. Build the application into your programme from the start and ask the installer what they are currently seeing with your DNO.

The connection offer

If the DNO can connect the system, it issues a connection offer. This typically sets out:

• The capacity you can export, and any conditions attached
• Any works the DNO needs to carry out, and their cost
• The timescale for those works
• How long the offer stays open for acceptance, and the payment terms

Read the offer carefully. If it includes significant costs or a long lead time, there may be alternatives, such as a smaller export capacity or export limitation.

Export limitation under G100

Where the network cannot accept the full output, an export limitation scheme can let you install a larger system and use most of the energy on site while capping what flows back to the grid. These schemes are covered by Engineering Recommendation G100, which sets out how they must be designed and tested. In practice, a meter or current sensor at the incoming supply tells the inverters to reduce output if export approaches the agreed limit.

Export limitation works best where your daytime demand is high. If your site uses most of its solar output anyway, a low or zero export limit costs you little. If demand drops sharply at weekends, some generation will be curtailed. Your half-hourly data shows how much.

Reinforcement costs

If the local network needs upgrading to take your export, the DNO may ask you to contribute. Ofgem reforms that took effect in April 2023 changed how these costs are shared, reducing how much of the wider network cost connecting customers pay, but contributions can still apply to generation. The connection offer itemises what you are being asked to pay for. If the cost is high, compare it with an export-limited design before deciding. The commercial solar cost page explains how grid costs fit into the overall budget.

How the survey checks your supply

MPAN and supply capacity

Your MPAN (Meter Point Administration Number) is on your electricity bill. The core part identifies your connection point, and its first two digits identify the distribution area, which tells the installer which DNO to apply to. On half-hourly contracts your bill also shows an agreed supply capacity in kVA. The survey checks that capacity, your actual peak demand and the rating of the incoming supply.

The transformer and main switchboard

Smaller sites are usually supplied at low voltage from a DNO substation that may serve other customers. Larger sites may have their own transformer, sometimes owned by the DNO and sometimes privately. The survey records the transformer arrangement where relevant, the main switchboard rating, spare ways, the earthing arrangement and the cable route from the roof to the point of connection. Long cable runs and a full switchboard both add cost, so they are worth knowing before the quote.

Which DNO covers your site

You can confirm yours using the Energy Networks Association postcode tool. Our regions pages cover local points.

Northern Ireland

In Northern Ireland, generation connections go through NIE Networks, which publishes its own application process and technical requirements. The principles are similar, but the forms, thresholds and timescales are NIE Networks' own, so check its current guidance. The Smart Export Guarantee does not apply in Northern Ireland. See solar in Northern Ireland.

How the free survey covers the application

We arrange a free survey with one MCS certified installer matched to the site. The installer's surveyor checks your MPAN, supply capacity, switchboard and connection point, and advises whether G99 applies, whether export limitation is likely to be needed, and what the DNO will want to see. If you go ahead, the installer usually prepares and submits the application. Your details go to that one installer only, and there is no obligation to proceed. See what the survey includes.