Battery storage compliance in the UK sits at the intersection of grid connection requirements, electrical installation standards, fire safety, and installer certification. Each battery installation involves at least three separate regulatory frameworks — the ENA Engineering Recommendations (G98, G99, or G100), BS 7671 electrical wiring regulations, and MCS 012 where the battery is part of an MCS-certified installation.
Getting the connection type wrong — AC-coupled versus DC-coupled versus hybrid — is the most common source of compliance errors on battery projects. This guide works through each framework in sequence.
Battery Storage Is a Generating Unit Under G98/G99
The ENA classifies AC-coupled battery storage inverters as generating units for the purposes of G98 and G99, in the same way as solar PV inverters. This means the 16A per phase threshold applies to battery inverters — and battery inverters count toward the combined capacity calculation for a premises. Many retrofit battery installations trigger G99 requirements because they push combined generation capacity above the G98 limit.
Grid Connection: G98, G99, and G100
AC-Coupled Battery Storage
An AC-coupled battery system has its own dedicated AC inverter — separate from the solar PV inverter. The battery charges from and discharges to the AC grid side. Common examples include the Tesla Powerwall (AC-coupled variant), Sonnen eco, and some configurations of the GivEnergy AIO.
Because AC-coupled batteries have an AC inverter output, they are treated as generating units and must comply with G98 or G99 based on the inverter’s AC output current per phase:
| Battery Inverter Output | Applicable Standard |
|---|---|
| 16A per phase or less (up to 3.68 kW single-phase) | G98 — notification only, 28 days before commissioning |
| Above 16A per phase (above 3.68 kW single-phase) | G99 — formal application, 45 working days for DNO response |
| Three-phase inverter up to 11.04 kW | G98 |
| Three-phase inverter above 11.04 kW | G99 |
The Combined Capacity Rule for Battery Retrofits
The single most common compliance issue in battery storage is failing to account for the combined capacity rule. When a battery inverter is added to a premises that already has an existing solar installation:
- Calculate the existing solar inverter’s AC output per phase
- Add the proposed battery inverter’s AC output per phase
- If the combined total exceeds 16A per phase (3.68 kW single-phase), G99 applies to the battery addition
Example: A property has a 3.6 kW solar PV system with a 3.6 kW single-phase inverter — G98-notified. Adding a Tesla Powerwall 2 (AC-coupled, 5 kW continuous AC output per phase) gives a combined capacity of 8.6 kW. This exceeds the G98 threshold. The battery addition requires a G99 application.
Retrofit Battery Addition — Check Before Specifying
Before specifying a battery for an existing solar installation, confirm the existing system’s G98 notification and the current inverter AC output. If the combined capacity will exceed 3.68 kW on single-phase (or 11.04 kW on three-phase), a G99 application must be submitted and DNO acceptance received before the battery can be commissioned. Specifying a smaller battery inverter — or an export-limiting configuration — can keep the project under G98 and avoid the 45-working-day G99 wait.
DC-Coupled Battery Storage and G100
A DC-coupled battery connects on the DC side of a hybrid inverter or existing PV inverter, sharing the same AC inverter output. The battery charges from the PV array DC bus and discharges through the existing inverter. Common configurations include the GivEnergy Hybrid inverter with DC battery, Solis Hybrid, and Growatt hybrid systems.
ENA Engineering Recommendation G100 applies to DC-connected storage where the battery does not increase the AC export capacity of the installation. G100 requires notification to the DNO, but the process is simpler than G99 — it does not require a formal application with a 45-working-day review period.
However, if a DC-coupled battery is installed with a new hybrid inverter that has a higher AC output than the original solar inverter, the AC output increase is subject to G98 or G99 as applicable. In that case, both G100 (for the DC battery) and G98/G99 (for the inverter upgrade) may apply.
G99 Application for Battery Storage
If the battery addition requires G99, the application must include:
- Battery system description and AC inverter specifications
- Single-line diagram showing the battery, inverter, solar PV system, generation meters, and grid connection
- Protection settings schedule for the battery inverter (under/over voltage, under/over frequency, RoCoF)
- BMS specifications and alarm/shutdown settings
- Export limitation details if applicable
For guidance on the G99 application process, see the G99 application guide. For commissioning test requirements under G99, see the G99 commissioning test guide.
BS 7671 Section 712: Electrical Installation Requirements
BS 7671:2018 (18th Edition) with Amendment 2:2022 includes Section 712 covering photovoltaic supply systems. The requirements in Section 712 apply to the complete solar and battery installation, including the battery circuits. Key requirements:
Battery Management System (BMS)
Every battery energy storage system must have a Battery Management System that:
- Monitors individual cell or module voltage and temperature
- Prevents over-charge and over-discharge of cells
- Initiates controlled shutdown before dangerous conditions are reached
- Provides alarm and fault signals to the system monitoring interface
The BMS is typically integrated into the battery unit by the manufacturer. However, the installer must verify that the BMS is functional and that its alarms are connected to the system monitoring during commissioning.
Over-Current Protection
Battery circuits require dedicated over-current protection devices (fuses or MCBs) appropriately rated for the maximum charge and discharge current of the battery. The protection device must be:
- Rated for DC service (AC MCBs are not suitable for DC battery circuits)
- Located at the battery connection point, not only at the inverter
- Accessible without opening the battery enclosure
DC-Rated Protection Devices Only
Standard AC MCBs are not rated for DC circuits. Battery circuits operate on DC at voltages of 48V to 800V depending on system design. Using AC-rated protection devices on DC battery circuits is a serious safety fault — the arc quenching characteristics of AC MCBs are not suitable for DC fault currents. Always specify DC-rated fuses or MCBs for battery circuit protection, with a voltage rating appropriate for the battery system voltage.
DC Isolation
The battery system must have a means of isolation on the DC side that:
- Can be operated without opening the battery enclosure
- Is lockable in the open (isolated) position
- Is clearly labelled with the battery circuit voltage
- Is accessible to the DNO and emergency services
Many battery manufacturers integrate the DC isolator into the battery enclosure. Where this is the case, verify that the isolator handle is externally accessible without opening the main battery cover.
Wiring and Connections
DC wiring on battery circuits must:
- Be rated for the maximum temperature the wiring may reach in service
- Be protected against short-circuit faults by appropriate circuit protection
- Not use standard twin-and-earth cable (designed for AC use only)
- Use appropriately rated DC solar cable (typically tinned copper to BS EN 50618) or manufacturer-specified cable
Fire Safety: Lithium-Ion Batteries
The majority of residential and small commercial battery storage systems installed in the UK use lithium-ion chemistry (lithium iron phosphate, LFP, being increasingly common for its improved thermal stability). Lithium-ion batteries carry a thermal runaway risk — a condition where a failing cell generates heat, which triggers adjacent cells to fail in a chain reaction that can result in fire or explosion.
BS EN IEC 62619:2022 (Safety requirements for secondary lithium cells and batteries for stationary applications) is the applicable standard for lithium-ion batteries in stationary storage applications. Battery products should be certified to this standard.
Installation Safety Requirements for Lithium-Ion Batteries
| Requirement | Specification |
|---|---|
| Clearance to combustible materials | Minimum as per manufacturer’s installation manual — typically 300mm to combustible surfaces |
| Ventilation | As per manufacturer specification — many sealed lithium-ion batteries require no ventilation, but the installation area must not accumulate off-gassing products |
| Proximity to ignition sources | Do not install adjacent to boilers, gas meters, electrical distribution boards, or other ignition sources |
| Enclosure | Garage, utility room, or purpose-built battery enclosure — not under stairs or in living spaces without specific fire containment measures |
| Fire detection | Smoke alarm or heat detector in the same room as the battery installation |
| Emergency signage | DNO and fire service require permanent signage at the meter position indicating battery storage on the premises |
LFP Chemistry Offers Better Thermal Stability
Lithium iron phosphate (LFP) batteries have a higher thermal runaway threshold than NMC (nickel manganese cobalt) chemistries used in some earlier systems. Many residential battery products installed in the UK since 2022 — including the Tesla Powerwall 3, GivEnergy batteries, and Pylontech units — use LFP chemistry. Check the battery product datasheet for chemistry type and thermal runaway onset temperature before installation.
Lead-Acid Battery Storage
Older battery storage installations may use sealed valve-regulated lead-acid (VRLA) or flooded lead-acid batteries. These chemistries have different compliance requirements:
- Ventilation is mandatory — lead-acid batteries off-gas hydrogen during charging, which is explosive at concentrations above 4% in air. The installation space must have adequate ventilation to prevent hydrogen accumulation.
- Flooded lead-acid batteries require ventilation to the outside air at both high and low levels in the installation room.
- VRLA (AGM/gel) batteries produce minimal off-gassing under normal conditions, but still require some ventilation as a precaution.
- Lead-acid batteries must be in a dedicated room or enclosure not accessible to children.
MCS 012: Battery Storage Certification
The Microgeneration Certification Scheme’s installation standard MCS 012 has been updated to cover battery energy storage systems installed alongside solar PV. For an MCS certificate to be issued for a solar and battery installation (required for Smart Export Guarantee eligibility), both the solar PV and the battery storage must be installed to MCS 012 standards.
What MCS 012 Requires for Battery Installations
- The battery product must be listed in the MCS product directory
- The installer company must hold MCS certification for battery storage installations
- The battery must be installed per the manufacturer’s installation manual and the MCS 012 requirements
- Commissioning must be completed and recorded using the MCS commissioning checklist for battery storage
- The MCS certificate issued must include both the PV system and the battery system details
Retrofit Battery Additions to Existing MCS Systems
When a battery is added to an existing MCS-certified solar installation:
- The battery addition must be carried out by an MCS-certified installer
- A revised or supplementary MCS certificate should be issued covering the battery addition
- The SEG supplier should be notified of the battery addition
Failing to certify a retrofit battery addition under MCS can affect the property’s SEG eligibility and the installation warranty under the MCS Guarantee.
Building Regulations: Battery Rooms and Ventilation
Battery storage installations in dwellings are subject to Part P (Electrical Safety) and, where structural or ventilation modifications are made, potentially Part F (Ventilation) and Part B (Fire Safety) of the Building Regulations.
Part P applies to all fixed electrical work in the dwelling including battery circuit wiring, inverter connections, and consumer unit modifications. A Competent Person Scheme-registered installer (NICEIC, NAPIT, or similar) can self-certify the electrical work.
Part F may apply where a ventilation system is added to a battery room. Natural ventilation (e.g., an air brick or louvred vent) typically does not require a Building Regulations application, but a powered ventilation system may be notifiable.
Part B considerations arise where a battery is installed in a location that affects fire compartmentation — for example, in an integral garage separated from the dwelling by a fire-rated door and wall. Installing a battery in such a space must not compromise the fire separation.
DNO Notification: Practical Steps for Battery Retrofits
When adding battery storage to an existing solar installation, follow this sequence:
Check the Existing G98 Notification
Obtain the existing G98 notification reference number and the notified inverter AC output. This is the baseline for calculating whether the battery addition triggers G99. The homeowner should have a copy of the G98 notification and the post-commissioning confirmation. If these cannot be located, the DNO can confirm the notified capacity using the property MPAN.
Calculate Combined Capacity
Add the proposed battery inverter’s AC output per phase to the existing solar inverter’s AC output per phase. If the total exceeds 16A per phase (3.68 kW single-phase), G99 applies. If it remains at or below 16A per phase, G98 applies. For DC-coupled configurations, check whether the hybrid inverter’s AC output exceeds the original solar inverter’s rated output — if it does, the increase is subject to G98/G99.
Submit Notification or Application
For G98: submit to the DNO’s online portal at least 28 days before commissioning. For G99: submit the full application including single-line diagram, battery specifications, and protection settings. Allow 45 working days for the DNO response. Do not commission the battery until the 28 days (G98) or written acceptance (G99) is received.
Commission and Test
Commission the battery system per the manufacturer’s commissioning procedure and MCS 012 requirements. Test and record all protection relay settings on the battery inverter. Verify BMS functionality. Complete the MCS commissioning checklist. Send the G98 post-commissioning notification (or G99 post-commissioning notification) to the DNO within 28 days of energisation.
Using Solar Design Software for Battery System Planning
Accurate system design is particularly important for battery storage compliance. The generation and financial tool in solar design software allows you to model the interaction between PV generation, battery storage, and grid export — confirming whether a proposed battery inverter size will keep the combined capacity within G98 limits, or whether a G99 application will be required.
This modelling step at the design stage avoids situations where a battery has been specified, ordered, and delivered before anyone has calculated that a G99 application is required — and the customer faces a 9-week wait before the battery can be turned on.
Design Battery Storage Systems with Automatic G98/G99 Threshold Checks
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Related UK Compliance Guides
For a detailed comparison of G98 and G99 requirements, see the G98 vs G99 guide. For commissioning test requirements under G99 (which apply to battery inverters over 16A per phase), see the G99 commissioning test guide. For the full UK solar compliance framework including planning and building regulations, see the UK compliance hub.
Frequently Asked Questions
Does a home battery storage system need DNO notification in the UK?
Yes. Any AC-coupled battery storage system with an AC inverter output is treated as a generating unit under ENA Engineering Recommendations G98 or G99. If the battery inverter output is 16A per phase or less (up to 3.68 kW single-phase), G98 notification applies. Above 16A per phase, G99 application is required. DC-coupled batteries connected behind an existing inverter may fall under G100. DNO notification is required before the battery is commissioned and connected to the grid.
What is G100 and when does it apply to battery storage?
G100 covers small-scale embedded generation connected on the DC side of a grid-connected inverter. It applies to DC-coupled battery storage where the battery is connected to the DC bus of an existing PV inverter or hybrid inverter, rather than having its own separate AC inverter. G100 applies where the battery connection does not increase the AC export capacity beyond existing G98/G99 limits. If adding DC-coupled storage would increase the AC output of the inverter, G98 or G99 governs the addition.
What happens to the G98/G99 threshold when a battery is added to an existing solar system?
The combined AC output of all generating equipment at the premises determines the applicable threshold. A property with a 3 kW solar inverter (G98-notified) adding a 3.68 kW AC-coupled battery inverter has a combined capacity of 6.68 kW, which exceeds the G98 limit for single-phase. This triggers a G99 application for the addition. Always calculate combined output before specifying the battery inverter size.
What does BS 7671 require for battery storage electrical installations?
BS 7671:2018 Section 712 requirements extend to battery storage connected to PV systems. Key requirements include: a battery management system with over-temperature and over-charge protection, DC-rated over-current protection on battery circuits, accessible isolation means, and appropriate ventilation. Lithium-ion batteries require thermal runaway mitigation measures and specified clearances to combustible materials.
Is MCS certification required for battery storage installations?
MCS 012 covers battery energy storage systems installed alongside solar PV. Where a battery is installed as part of an MCS-certified solar installation, or as a retrofit to an existing MCS installation, the battery installation must comply with MCS 012 for the system to retain its MCS certificate. MCS certification is required for Smart Export Guarantee eligibility — a system with an uncertified battery retrofit may lose its SEG eligibility.
