Battery storage under Amendment 4: siting, ventilation and isolation

Until this year, a domestic battery storage system was installed to BS 7671's general rules, the manufacturer's instructions and a patchwork of guidance. Amendment 4 changes that: BS 7671:2018+A4:2026 adds a dedicated chapter, Chapter 57, for stationary secondary batteries. For homes, the detailed fire safety rules come from PAS 63100, the specification for battery energy storage systems in dwellings. For installers, the practical questions are where the battery can go, how the location is ventilated and protected, and how the system is isolated safely.

Amendment 4 was published on 15 April 2026, and the previous version of BS 7671 will be withdrawn on 15 October 2026. Until then either version can be used; from that date, new battery installations, and additions or alterations to existing ones, must be designed and installed to A4. BS 7671 is not retrospective: an existing compliant installation does not have to be upgraded simply because the standard has changed.

The short answer

BS 7671 Amendment 4 introduces Chapter 57 for stationary secondary batteries, covering system design, inverters, protective devices for two-way energy flow, battery location, ventilation and fire risk. For homes, the siting and fire rules come from PAS 63100. PAS 63100 requires batteries to be outdoors where practicable, and never in lofts, bedrooms or escape routes. Indoor locations must be ventilated to outside and fire separated, with interlinked fire detection where the battery is in an infrequently visited space. No single enclosure may hold more than 20 kWh, and total stored energy is limited to 40 kWh or 80 kWh depending on location.

What Amendment 4 changes: Chapter 57

Amendment 4 adds a new Chapter 57: Stationary secondary batteries to BS 7671, in response to the rapid growth of energy storage with and without solar PV. According to the IET's announcement of the published amendment, it sets out requirements for:

• System design and power conversion equipment.

• Bidirectional and hybrid inverters.

• The suitability of protective devices for two-way energy flow, which matters as batteries are increasingly used for vehicle-to-home and vehicle-to-grid applications.

• Safety-critical matters including battery location, ventilation and fire-risk mitigation.

Batteries are a source, not a load

The most important practical shift is in how a battery is treated in the design. A battery system, whether AC-coupled or behind a hybrid inverter, is a source of supply: it can deliver fault current, and it can feed energy in either direction. It should not be wired in as though it were another final circuit on the consumer unit. In practice that means:

• Connect it as a source. The battery or inverter connection should be arranged so that the protective devices for final circuits still see the supply from the correct side, rather than the battery being fed through, and feeding back through, a final-circuit device.

• Check every protective device in the path for two-way current. Circuit-breakers, RCBOs and RCDs between the battery and the rest of the installation must be suitable for current flowing in both directions. Many devices are marked with line and load terminals and are not suitable for reverse feed.

• Take care with RCDs. A source connected on the load side of an RCD can prevent it operating correctly. Check the RCD type and the manufacturer's instructions for both the inverter and the RCD before connecting on the load side of one.

• Consider fault current from both sources. Protective device breaking capacity, and the prospective fault current recorded on the certificate, need to account for the contribution from the battery as well as the grid.

Check the detailed requirements in Chapter 57 and the manufacturer's installation instructions for each system.

Dwellings: PAS 63100

For batteries in homes, the detailed siting and fire safety requirements are set out in PAS 63100. The IET's Code of Practice for Electrical Energy Storage Systems (3rd Edition) says PAS 63100 should be used for battery location. In other buildings, the location of batteries and fire protection measures should follow the fire safety risk assessment for the premises.

PAS 63100:2024, Electrical installations: Protection against fire of battery energy storage systems for use in dwellings, was sponsored by the Department for Energy Security and Net Zero and came into effect on 31 March 2024. It is free to download from BSI. MCS has confirmed the IET Code of Practice's position on it.

PAS 63100 does not cover every job. It excludes dwellings over 200 m² in floor area, high-risk residential buildings, systems using second-life batteries, systems above low voltage, and systems connected before the cut-out or meter. For those, a fire safety expert should be involved.

Siting: where a home battery can and cannot go

The starting point in PAS 63100 is simple: where practicable, storage batteries shall be installed outdoors. A detached outbuilding not intended for habitation, or one separated from the house by a wall with at least REI 120 fire performance, counts as outdoors. Only where outdoors is not practicable may the battery go indoors, and then only in a permitted location with the required ventilation and fire separation.

Locations where batteries must not be installed

Locations that can work

• External wall or ground-mounted outdoors, at least 1 m from doors, windows, ventilation ports and escape routes. The installation must not compromise the fire performance of the wall: service penetrations must be fire stopped, and cavity barriers provided where cables pass through a cavity wall. External equipment may need planning consent in some cases.

• Detached garage or outbuilding. Treated as outdoors, and allows the higher stored energy limit.

• Attached or integral garage. Permitted with the fire separation required for indoor locations; with REI 60 separation it also qualifies for the higher stored energy limit.

• Indoor plant or utility space that is not excluded above, with ventilation to outdoors, REI 30 fire separation from excluded areas and fire detection where required.

Where a garage also holds vehicles, protection against impact applies: batteries need IK10 protection, and inverters and other power conversion equipment IK08, or equivalent barriers. The same applies outdoors next to driveways or on walls adjoining a public highway. For supplies to garages and outbuildings, see How to Wire a Garage Consumer Unit.

How much storage is allowed

Ventilation

PAS 63100 requires all indoor locations containing storage batteries to have fresh-air ventilation to outdoors. Three conditions apply:

• The ventilation must not compromise the fire resistance of the battery location.

• The edge of the outdoor ventilation port must be at least 1 m from doors, windows and ventilation ports for other locations.

• Detailed ventilation guidance is in the BS EN IEC 62485 series: Part 2 for lead-acid and nickel-based batteries, and Part 5 for lithium-ion.

Lithium-ion batteries do not normally give off gas in operation in the way vented lead-acid batteries do, but a cell in thermal runaway can release flammable and toxic gases. Ventilation to outside is there for that failure case, as well as for heat. PAS 63100 also notes that air-tightness requirements for new dwellings may make habitable parts of a home unsuitable for batteries, which is one more reason to go outside.

Manufacturers' clearances and ambient temperature limits still apply on top of PAS 63100, and a battery that runs hot through poor siting will also lose capacity and life.

Fire separation and fire detection

• Fire separation: any indoor location containing batteries must be separated from the excluded locations by walls, ceilings and floors with a fire performance of at least REI 30 (30 minutes). All services passing through the enclosure must be fire stopped to maintain the separation, in line with Regulation 527.2 of BS 7671.

• Fire detection at the battery: where batteries are in an infrequently visited location, such as a store cupboard, a smoke alarm or multi-sensor fire alarm must be fitted there and interlinked with the dwelling's fire alarm system, to BS 5839-6. The same applies to inverters and other power conversion equipment in lofts, store cupboards and similar spaces.

• Whole-dwelling fire alarm: PAS 63100 recommends that premises with a battery system have a fire detection and alarm system of at least Grade D2, Category LD2. On existing homes this may mean upgrading the smoke alarms when the battery goes in.

Note that inverters are not excluded from lofts in the way batteries are; they need interlinked detection instead. Batteries themselves must be in non-combustible enclosures that include the overcurrent protection, with access to the interior only by tool.

Isolation and safe working

A battery is always live. Switching off the AC supply, or isolating the inverter from the grid, does not make the battery or the DC cables between battery and inverter dead. Isolation has to be designed with every source in mind: the grid, the battery, and any PV or other generation.

What to provide

• Isolation of every source. Provide means of isolating the battery DC circuit, the inverter's AC connection and any PV input, following the manufacturer's instructions and BS 7671's requirements for isolation and switching. The installation must be arranged so that the battery cannot back-feed into circuits or the network when it should not.

• Bidirectional protection. Protective devices on the battery circuit must be suitable for current in both directions, which Chapter 57 specifically addresses.

• Tool-only access to DC connections. PAS 63100 requires that DC power cable connections cannot be removed without a tool, or without removing a cover secured by a tool.

• DC fuses must be accessible only with a tool, or arranged so they can only be removed after opening an isolator suitable for on-load isolation. Pull-out fuse carriers that anyone can withdraw under load do not meet this.

• Parallel strings. Where battery assemblies are connected in parallel strings in one or more enclosures, common overcurrent protection must be provided.

Notices and labels

PAS 63100 requires an instruction notice showing the presence of a battery storage system at the origin of the installation, at the meter position if it is elsewhere, and at the consumer unit or distribution board to which the inverter is connected, using the PV-and-battery or battery-only symbol given in the PAS. This is in addition to BS 7671's warning notices for installations with alternative or additional sources of supply, and the user instructions, which must include shut-down and battery-fire emergency procedures.

Working on battery systems

The recommended working practices in Annex A of PAS 63100 include safe isolation as far as practicable, separating strings before working inside enclosures to reduce arc flash risk, insulated tools, fused test leads and test equipment, and arc flash PPE suited to the declared incident energy. Manufacturers must declare the arc flash incident energy, and batteries above 8 cal/cm² are not suitable for dwellings at all.

Choosing equipment

Much of PAS 63100 is met by selecting the right product, and should be confirmed from manufacturer documentation before quoting:

• Battery enclosures to BS EN IEC 62485-1, BS EN IEC 62933-5-2 and, for lithium-ion, BS EN IEC 62485-5; cells to BS EN IEC 62281 for lithium.

• System controls to BS EN IEC 62933-5-2, with a battery management and monitoring system that gives visual and audible warnings of dangerous conditions for indoor systems.

• A declared arc flash incident energy, and enclosures meeting the related requirements.

• Inverters to BS EN 62109-2, and DC-DC converters and chargers to BS EN 62477-1 or BS EN IEC 61204-7.

• Second-life batteries are outside PAS 63100 and need a designer's declaration of safety under Regulation 511.2 of BS 7671.

Verification and certification

The battery system is inspected, tested and certified under BS 7671, with the extent and location of the system recorded on the Electrical Installation Certificate. PAS 63100 adds a requirement for the installer to provide a statement of conformity to PAS 63100, appended to that certificate. The PAS includes an example verification checklist and a suggested statement.

The battery is also generation for grid connection purposes. It must be notified or applied for under ENA EREC G98 or G99, and adding a battery to an existing solar system usually takes the site above the 16 A per phase G98 limit into G99. Surge protection on the PV and battery circuits forms part of the design: see What Is an SPD in Electrical Installations?. On older properties, an EICR is a sensible first step to confirm the existing installation can accept the new source.

Checklist for installers

For a full summary of the other Amendment 4 changes, see the IET's A4 publications page and NICEIC's Amendment 4 guidance.

Frequently asked questions

Can a home battery be installed in a loft?

No. PAS 63100, the UK fire safety specification for batteries in dwellings, lists voids, roof spaces and lofts as locations where batteries must not be installed. An inverter can go in a loft, but it then needs an interlinked smoke or multi-sensor alarm.

Does a battery storage system need ventilation?

If it is installed indoors, yes. PAS 63100 requires fresh-air ventilation to outdoors for any indoor location containing batteries, with the outdoor port at least 1 m from doors, windows and other ventilation ports, and without compromising the fire separation.

What is the maximum battery storage allowed in a house?

Under PAS 63100, no single enclosure may hold more than 20 kWh. The total in a dwelling is limited to 80 kWh where batteries are outdoors, in a detached garage or outbuilding, or in an attached garage with REI 60 fire separation, and to 40 kWh in all other cases.

Do existing battery installations have to be changed for Amendment 4?

No. BS 7671 is not retrospective, so an existing installation does not have to be upgraded. New battery installations, and additions or alterations to existing ones, are designed and installed to Amendment 4.