BESS Grid Connection Requirements UK: A 2026 Guide

Battery energy storage capacity is growing faster than almost any other technology on the GB grid, but the constraint has shifted. It's no longer cell chemistry or inverter cost — it's the connection itself. Understanding BESS grid connection requirements UK-wide is now a prerequisite for any viable project, because a technically sound battery design means little if it can't secure a compliant, timely connection point. 

This is due to the structure of the process. Grid-scale storage sits at the point where two distinct regulatory procedures meet: the Engineering Recommendation (ER) G99 process managed by Distribution Network Operators (DNOs) and, for larger schemes, the transmission-level connection process. To build a project, a developer must engage with the National Energy System Operator (NESO). To connect, a developer must meet both the technical rulebook and the queue management process. 

G98 vs G99: Where BESS Projects Sit 

Most battery storage grid connection applications in Great Britain fall under one of two ENA engineering recommendations: 

• The G98 ("connect and notify") procedure applies to small installations with a current rating of less than 16A per phase (approximately 3.68kW for a single-phase system and 11kW for a three-phase system). The installer must connect the installation and then notify the DNO within 28 days—pre-approval is not necessary. 

• G99 applies to anything larger, including virtually all commercial and utility-scale battery storage. Under G99, written DNO approval is mandatory before the system energises. 
  

Because export capacity — not panel or cell capacity — determines the threshold, even modest commercial BESS installations typically trigger BESS G99 connection requirements. Developers should assume G99 applies unless a system is genuinely domestic scale. 

G99 Technical Requirements for Battery Storage 

G99 connection requirements for storage cover a specific set of dynamic performance criteria, including: 

• Fault Ride-Through (FRT): the system must remain connected and support the network during voltage sags rather than disconnecting, in accordance with the defined voltage-duration profiles. 

• Dynamic Frequency Response: rapid active power adjustment in response to system frequency deviations. 

• Reactive power and voltage control: maintaining power quality and supporting local voltage stability. 

• Protection settings and anti-islanding: preventing the storage system from energising an isolated part of the network during an outage. 

• Commissioning and witness testing: DNOs typically require site tests to verify the modelled performance matches reality before final connection. 

  
  

The requirements are alongside the international standards which are being referred to more and more in UK due diligence and lender technical advisor reports—specifically IEC 62933 (relating to the performance and safety of electrical energy storage systems) and the BS EN harmonised equivalents that cover power quality and electromagnetic compatibility. Projects that state compliance with both G99 and these standards generally face fewer queries during the DNO's technical review. 

BESS DNO Connection Requirements: What to Prepare 

All GB’s DNOs by region have their own application portal and assessment process, but the documentation. Typical BESS DNO connection requirements include: 

• Single Line Electrical Diagrams and Site Layout Plans 

• Inverter/PCS technical datasheets and G99 type-test certificates 

• Protection and control philosophy documents 

• Export/import capacity requested (MVA) and operating profile 

• Fault level and short-circuit contribution data 

• Planning status and land rights evidence 
  

Application fees vary significantly by DNO — commonly ranging from a few hundred pounds for straightforward assessments to several thousand for schemes requiring detailed network studies or reinforcement work. Constrained substations may require the developer to fund upstream reinforcement, which can materially change project economics, so an early "connection feasibility" enquiry before formal application is worthwhile. 

Beyond G99: NESO's Connections Reform and the Gate 2 Queue 

For large embedded or transmission-connected batteries it is necessary but not sufficient to comply with G99; since 2025 NESO has been carrying out Connections Reform (Target Model Option 4+), replacing the previous 'first come, first served' queue with a Gate 2 process which is based on readiness and strategic alignment. 

For storage developers the practical impact has been considerable since NESO's Gate 2 assessment found there to be a large excess of battery projects compared with the requirements set out in Clean Power 2030, which has led to a great number of applications either being removed or having their priority revised, whereas the 'protected' projects keep their place in the queue and their indicative connection dates. Another application period at Gate 2 is anticipated to begin in 2026, provided that the timeline continues to slip as the DNOs and Transmission Owners carry out further network studies. 

That is to say, for BESS grid integration on a large scale, the connection date and connection point can no longer be regarded as fixed when an early-stage offer is received—they have to stay provisionally agreed upon until formal Gate 2 modification offers have been made and, finally, until signed connection agreements have been reached. 

High Voltage Substation Design for Utility-Scale BESS 

For most projects with a capacity of more than 10 to 50 MW, connection is made at either 33 kV or 132 kV, which in turn calls for a dedicated high-voltage substation design. The main points to take into account are: 

• Transformer and switchgear ratings specified for agreed export/import capacity 

• Coordination of protection with the DNO/TO network, including relay settings and grading studies 

• Earthing design to manage fault currents safely 

• Space and access provision for future capacity upgrades 

• Compliance with National Grid Electricity System Operator (NESO) Grid Code technical requirements for larger Power Generating Modules and Electricity Storage Modules 

  
  

Connection Route Comparison 

Route	Typical Capacity	Approval Body	Pre-Connection Approval	Key Requirement
G98	Below 16A/phase	DNO (notify only)	No	Connect then notify within 28 days
G99 (Distribution)	Above 16A/phase to ~50MW	Local DNO	Yes	Full G99 connection requirements, witness testing
Transmission/Large Embedded	Above ~50MW	NESO + TO/DNO	Yes	Grid Code compliance, Gate 2 queue position, CUSC arrangements

A Practical Readiness Checklist 

• Early on, before you get into the details of your design, confirm your export capacity and connection voltage. 

• Request a pre-application connection feasibility study from the relevant DNO 

• Complete your G99 submission, including type-test evidence for the PCS 

• For transmission-connected or large embedded projects, monitor the Gate 2/NESO queue. 

• Allow for a budget contingency for network reinforcement costs and changes to connection dates. 

• Align substation design with both G99 and Grid Code technical envelopes from the outset 

Conclusion 

Meeting the BESS grid connection requirements across the UK currently means having a thorough understanding of two separate systems — the DNO-led G99 technical compliance and NESO's ongoing connections reform. Those projects which regard these as one continuous process, from the feasibility stage right through to commissioning, are able to avoid the delays and cost surprises which affect the rest. For engineering teams and developers who are dealing with G99 applications, substation design, or their position in the Gate 2 queue, working with experts who keep a close watch on these frameworks is what distinguishes between a connection date that remains on paper and one that is actually achieved. VSS Power provides support to utility companies, EPC contractors and developers throughout this entire process — from carrying out feasibility studies to substation design and dealing with the DNO. 

Key Takeaways 

1. Nearly all commercial and utility-scale battery storage is classified as G99, not G98 — mandatory is the obtaining of pre-connection approval from the local distribution network operator. 

2. The G99 Connection Requirements cover ride-through faults, frequency response, reactive power control, commissioning tests, etc. They are being aligned more closely with IEC 62933 and BS EN standards. 

3. BESS DNO connection requirements differ from one network operator to another in cost and procedure — early feasibility inquiries help avoid surprises. 

4. The reform to NESO's Gate 2 has changed the queue for large projects; connection dates will still be provisional until formal offers are issued. 

5. The design of high voltage substations must meet both the protection requirements set by the local distribution network operator and the technical specifications prescribed by the national Grid Code. 

   
   

FAQs 

1. What is the difference between G98 and G99 for battery storage? 

G98 applies to small systems (those under 16A per phase) which connect first and then notify the distribution network operator (DNO). G99 applies to larger systems, including most commercial battery energy storage systems (BESS), and requires written approval from the DNO before connection. 

2. How long does a BESS G99 application take in the UK? 

The time taken depends on the DNO and the complexity of the project, taking a few months in the case of simple distribution connections and much longer when network reinforcement or detailed studies are involved. 

3. Does NESO's connections reform affect distribution-connected batteries? 

Certainly. Although Gate 2 mainly alters the transmission-level queue, larger embedded distribution projects can also be influenced by protection status and reprioritisation. 

4. What standards apply to BESS grid integration in the UK? 

Developers often refer to the IEC 62933 series standards and the BS EN power quality standards, especially when carrying out due diligence for lenders and insurers, together with ENA G99. 

5. What voltage level do utility-scale BESS projects typically connect at? 

Most large-scale battery projects in the UK are connected at either 33 kV or 132 kV, necessitating the design of a dedicated high-voltage substation and the installation of protection systems that comply with the Grid Code.