UK Grid codes and complianceGuide

UK Grid Codes and Compliance for Commercial Solar

An informational guide to UK grid codes and compliance, explaining G98 and G99 regulations, DNO application processes, and technical requirements.

The UK Grid Code is the technical rulebook for connecting to and using the National Electricity Transmission System (NETS), administered by NESO and enforced at distribution level through Engineering Recommendations G98 and G99. It exists so generators, suppliers, and network operators connect under the same technical rules: system stability, security of supply, safe operation, and a transparent connection process. Commercial solar almost always connects in parallel with the public distribution network, so G98 and G99 are the rules that decide how you apply, what you must prove, and when the DNO will allow the system to operate. Get the code right first. Everything else in the connection process follows from that decision.

Tags:UK Grid codesComplianceG98G99Commercial Solar

G98 vs G99: Which Grid Code Applies to Your Commercial Solar Project?

G98 applies when generating equipment stays at or below 16 A per phase. G99 applies when current exceeds 16 A per phase. That 16 A figure is a current limit on each live conductor of the supply, not a round kW marketing size, and it is the single decision that sets your application route. Do not convert a brochure kW figure into a code choice until you have worked current per phase for the actual voltage and number of phases.

On a single-phase 230 V supply, 16 A is about 3.7 kW (16 A × 230 V = 3,680 W). On a three-phase 400 V supply, 16 A per phase is about 11 kW (16 A × 400 V × √3 ≈ 11.1 kW). Most commercial arrays sit above that level, so they fall under G99 rather than the simpler G98 notification route. A few-kilowatt system on a small premises may still qualify for G98. A typical commercial rooftop does not.

Work the current from your own kW rating before you assume G98. A 20 kW three-phase system at 400 V draws 20,000 W ÷ (400 V × √3) ≈ 28.9 A per phase. That exceeds 16 A, so the project requires G99. Use the same method for your design: divide watts by (line voltage × √3) on three-phase, or by 230 V on single-phase, then compare the result with 16 A. If the result is close to the limit, ask the DNO or a qualified installer rather than defaulting to the simpler pathway.

Attribute	G98	G99
Threshold	Up to 16 A per phase	Above 16 A per phase
Typical system size	Small arrays, often up to about 3.7 kW single-phase or about 11 kW three-phase	Most commercial rooftop and ground-mount solar
Application process	Notification to the DNO, not a full connection application	Formal application, assessment, connection offer, then commissioning sign-off
Requirements	Compliant small-scale generation with defined interface protection	Full generating-unit data, protection settings, and DNO agreement before parallel operation
Complexity	Lower	Higher, and the default path for commercial sites

Treat G99 as the working assumption for commercial solar unless your calculated current per phase is genuinely at or below 16 A. Engineering Recommendation G100 may also apply when multiple generators sit on one site, because the DNO then has to manage combined export and control. Once the threshold decision is settled, use the G99 and G100 application process as the next level of detail.

The G99 Application Process: Step-by-Step to Grid Connection

The G99 application process is a nine-step journey from choosing the right code through to final DNO approval after commissioning. The DNO is the point of contact at every stage. G98 uses a simpler notification process rather than this full application sequence. Self-managing a G98 notification is often feasible. For G99, professional support from the installer or a grid consultant is the lower-risk option, because incomplete data, wrong protection settings, or an assumed export figure can stall the offer. Confirm in writing who owns the application: you, the EPC, or the installer. Do not assume it is included.

1. Determine the applicable code

Calculate current per phase from system kW, voltage, and number of phases. Confirm whether G100 is also in play if more than one generator will share the site.

2. Prepare technical documentation

Assemble system ratings, single-line diagrams, inverter and protection specifications, proposed settings, export intention, and site address with MPAN or equivalent supply details.

3. Submit the application to the DNO

Send the G99 forms and supporting files to the DNO that owns the local network. Incomplete packs stall the clock.

4. DNO assessment

The DNO checks network capacity, fault level, protection coordination, and whether reinforcement or export limits are needed.

5. Receive the connection offer

The offer sets the technical conditions, any works, and the commercial terms for connecting. Read export limits, protection, and customer works before you treat the design as frozen.

6. Accept the offer and pay fees

Acceptance is what turns a proposal into a live connection project. Unpaid or unsigned offers do not authorise installation in parallel with the network.

7. Install the system

Build to the accepted design. Do not treat equipment substitutions as trivial if they change ratings, protection, or type-test status.

8. Commission and test

Prove interface protection, loss-of-mains behaviour, and the agreed operating limits, then record results for the DNO.

9. Receive final approval

The DNO confirms the generating unit may operate under the connection agreement. Keep that paperwork with the O&M file.

Prepare a tight pack before step 3: kW and kVA ratings, number of phases, inverter make and model, ENA type-test evidence where held, protection settings, earthing notes, proposed export, and a clear site plan. Sequence the DNO work ahead of procurement freezes and installation dates. For form-level detail, follow the G99 and G100 application process rather than improvising from a G98 notification template.

The Regulatory Bodies: Ofgem, NESO, DNOs, and ENA Explained

Ofgem regulates Great Britain's energy market, NESO operates the national transmission system and administers the Grid Code, DNOs manage local distribution networks and process G98 and G99 applications, and the ENA publishes the Engineering Recommendations those DNOs apply. Your day-to-day contact for commercial solar is the DNO, not NESO or Ofgem. Power still flows generator to transmission to distribution to the building. NESO balances the high-voltage system in real time. G98 and G99 sit on the distribution side of that chain, which is why a rooftop solar job is a DNO process even though the Grid Code is a national document.

Body	Role	When you interact
Ofgem	Energy regulator for Great Britain. Oversees industry codes and consumer protection.	Rarely on a standard commercial solar job. Relevant if a code dispute or licence issue escalates.
NESO	National Energy System Operator. Runs the NETS and administers the Grid Code.	Directly only for transmission-scale connections. Indirectly because G98 and G99 sit under that national framework.
DNO	Distribution Network Operator. Owns and operates the local network and processes G98/G99.	Throughout application, offer, works, commissioning, and the connection agreement.
ENA	Energy Networks Association. Coordinates DNOs on technical standards and publishes G98, G99, and G100.	When you use Engineering Recommendations, the Type Test Register, or a postcode lookup to identify your DNO.

The Grid Code described here applies to Great Britain. Northern Ireland uses separate arrangements through SONI and EirGrid. DNOs also work to the Distribution Code, the distribution-level counterpart to the Grid Code. Transmission-level access for very large plant also sits under the Connection and Use of System Code (CUSC). Typical commercial solar stays on the distribution network and therefore with the DNO.

Technical Compliance: Type Testing vs Individual Assessment

Technical compliance for commercial solar is shown either by type-tested equipment on the ENA Type Test Register or by a project-specific individual assessment. The inverter is the critical item, because it is the device that converts PV DC to grid AC and implements the protection and power-quality behaviour the DNO will check. While you may be selecting from a range of commercial solar panel types for the array, the panels themselves do not set the grid interface. The inverter does.

Type testing means the product has already been tested against the relevant Engineering Recommendation. Listed inverters carry pre-approved compliance status, which cuts the volume of site-specific engineering the DNO must review. Individual assessment applies when plant is not type-tested, or when the installation falls outside the tested configuration. That route needs extra studies, protection evidence, and time, and it usually costs more. Choosing a listed inverter is therefore a commercial decision as well as a technical one: it is the main way to keep G99 on the simpler compliance path.

Choose type-tested inverters unless there is a clear engineering reason not to. Then confirm the exact model, firmware, and configuration against the ENA Type Test Register before you freeze the bill of materials. A listed family name is not enough if the specific variant or firmware is outside the registered test.

Match inverter rating, phases, and intended export to the G99 (or G98) threshold you calculated.

Prefer hardware listed on the ENA Type Test Register for the code that applies.

Keep protection settings inside the DNO’s accepted ranges for under/over voltage, under/over frequency, and loss of mains.

Treat power quality (voltage, harmonics, flicker) and frequency response as inverter functions, not optional extras.

Do not swap to a non-listed model after the offer is accepted without checking whether the compliance pathway changes.

Timeline and Cost Factors for Grid Connection

Grid connection time and cost move with the G98 versus G99 pathway, DNO workload, how complete the application is, spare network capacity, and whether upgrades are required. DNO timescales are regional and indicative, not guarantees, and connection approval is often the longest lead item on a commercial solar programme. Installation, scaffolding, and inverter lead times only matter once the network will accept the plant.

Timeline drivers

G98 notification is simpler and usually faster than a full G99 application.
Incomplete drawings, missing inverter data, or unclear export figures send the file back.
Constrained networks, high fault levels, or requested export above available headroom extend assessment.
Reinforcement, new switchgear, or witness testing add weeks that sit outside your installation programme.

Cost drivers, without fixed figures

DNO application and assessment fees.
Engineering studies where plant is not type-tested or the site is complex.
Network works if the existing connection cannot accept the generation.
Installer or consultant time to prepare the pack, answer DNO queries, and commission the system.

Start the grid application before you lock equipment procurement and before you schedule installation. An accepted connection offer should sit on the critical path next to landlord consent and structural design, not after scaffolding is booked. Build contingency for DNO questions rather than assuming the first submission is the last. If the offer later imposes an export limit or extra works, you still have room to resize inverters or revise the business case before capital is committed.

Working with Your DNO: Regional Variations and Practical Advice

The DNO that processes your application is set by the site’s location in Great Britain, not by who supplies your electricity. A Manchester rooftop and a Cornwall rooftop do not use the same operator, and local application habits, portal rules, and assessment queues differ even though G98 and G99 are national Engineering Recommendations. Treat national Engineering Recommendations as the rulebook and the local DNO as the organisation that applies them to your feeder.

Identify the DNO with a postcode lookup on the Energy Networks Association website. For projects in Manchester and the wider North West, that will be the licence-area operator for North West England, which is a different organisation from DNOs in other regions. Contact that operator only after you know the licence area. Do not copy another region’s form-filling folklore.

Early engagement

Speak to the DNO before you freeze array size, export, and inverter selection.

Written confirmation

Confirm unusual requirements in writing, including export limits and protection settings.

Single source

Keep one document set and one named contact so queries do not fork into conflicting versions.

Supply data

Record MPAN, existing supply capacity, and any on-site generation already connected.

Common mistakes are treating every DNO as identical, starting the application after equipment is already on order, assuming G98 applies because the roof looks “small”, and relying on verbal assurances instead of the written offer. Early engagement is cheaper than a redesign when the network cannot accept the export you assumed.

Why Compliance Matters: Risks and Business Impact

Operating a generator without a valid connection agreement is a breach of the Grid Code. Compliance is not paperwork after the panels go down. It is the legal and technical permission to run in parallel with the public network, and it is what protects the investment once the array is energised.

Disconnection

The DNO can require the generating unit to be switched off until the agreement and protection are in place.

Penalties and contractual exposure

Unauthorised parallel operation can trigger enforcement and commercial claims. Specific penalty sums are a matter for the relevant codes and the DNO, not a quote on a solar brochure.

Insurance

Running plant outside the agreed connection conditions can undermine cover after a fire, shock incident, or network disturbance.

Safety

Incorrect protection or an uncleared islanding condition puts people working on the network and on your site at risk.

Treat application fees, possible reinforcement, and DNO lead time as line items in the solar business case from day one. Early compliance planning is what stops a viable IRR from slipping because the array is built and then sits idle waiting for an offer, a setting change, or a witness test. Understanding the broader commercial solar demand in the UK context can also help businesses justify these regulatory costs as part of a strategic energy shift. A compliant, agreed connection also supports system longevity: the plant can stay connected, export as allowed, and remain insurable for the life of the asset rather than becoming a stranded rooftop until settings or hardware are rebuilt.

Key Terms Explained: Plain-English Glossary

A plain-English glossary defines the Grid Code terms you will meet in DNO letters, installer quotes, and connection agreements. Use it when a facilities manager or business owner needs to follow the conversation without becoming the project engineer.

Grid Code

The technical rulebook for connecting to and using the National Electricity Transmission System in Great Britain, administered by NESO.

G98

Engineering Recommendation for small generating plant up to 16 A per phase, handled mainly by DNO notification.

G99

Engineering Recommendation for generating plant above 16 A per phase. This is the usual code for commercial solar.

G100

Engineering Recommendation that can apply where generation is limited or where multiple generators share one site and combined export must be controlled.

DNO

Distribution Network Operator. The company that owns the local cables and substations and processes your G98 or G99 file.

NESO

National Energy System Operator. Runs the high-voltage transmission system and administers the Grid Code.

Ofgem

The energy regulator for Great Britain. It oversees the market and the industry codes, including how networks treat customers.

ENA

Energy Networks Association. The body that coordinates DNOs on technical standards and publishes G98, G99, and G100.

Connection agreement

The contract with the network operator that sets the terms on which your generator may connect and run.

Commissioning

On-site testing that proves the installed system matches the agreed design and protection settings before final approval.

Protection

Settings and devices that disconnect generation when voltage, frequency, or network conditions are unsafe.

Power quality

How cleanly the system holds voltage, frequency, and waveform so it does not disturb other customers.

Inverter

The unit that converts solar DC to grid AC and carries most of the compliance functions the DNO cares about.

Type testing

Factory and laboratory testing of a product against the Engineering Recommendation, used to avoid a full individual assessment.

ENA Type Test Register

The published list of equipment that has demonstrated that type-tested compliance status.

Transmission system

The extra-high-voltage national backbone (the NETS) that moves bulk power between regions.

Distribution system

The lower-voltage local network that supplies most commercial buildings and accepts most commercial solar connections.

Frequently Asked Questions

These answers cover the Grid Code, who regulates electricity in Great Britain, G98 versus G99, how to obtain a G99 connection, non-compliance consequences, and how to find your DNO.

What is the UK Grid Code?

The UK Grid Code is the technical rulebook for connecting to and using the National Electricity Transmission System (NETS) in Great Britain. NESO administers it. For commercial solar on the local network, the practical rules are Engineering Recommendations G98 and G99, applied by your DNO.

Who regulates electricity in the UK?

Ofgem regulates the energy market in Great Britain. NESO operates the national transmission system and administers the Grid Code. DNOs run the local networks and process G98 and G99 applications, while the ENA publishes those Engineering Recommendations. Northern Ireland is outside this Grid Code and uses SONI and EirGrid arrangements.

What is the difference between G98 and G99?

G98 covers generating plant up to 16 A per phase and uses a DNO notification route. G99 covers plant above 16 A per phase and requires a full application, offer, and commissioning approval. A 20 kW three-phase 400 V array is about 28.9 A per phase, so it is G99, as are most commercial solar systems.

How do I get a G99 grid connection?

To get a G99 grid connection, calculate that you are above 16 A per phase, prepare inverter and protection data, and submit the application to your DNO. After assessment you receive an offer, accept it, install to that design, commission the plant, and obtain final approval. The DNO remains the contact for the whole sequence.

What happens if I don't comply with grid codes?

Non-compliance, including parallel operation without a valid connection agreement, is a breach of the Grid Code. The DNO can require disconnection, and you can face enforcement, insurance problems, and safety risk on site and on the public network. Build compliance cost and time into the project before you buy equipment.

How do I find my DNO?

To find your DNO, use a postcode lookup on the Energy Networks Association website. The operator is set by the network licence area, so a Manchester or North West England site will not use the same DNO as a site in another region, even if the electricity supplier brand is the same.

Plan Your Next Solar Project

Put what you have read into practice. Request a free site survey and receive a detailed, itemised quotation for your commercial solar project.

Request A Free Quote