Battery location and ventilation

Where a domestic storage battery can be installed, including fire separation, ventilation, impact and environmental limits.

The location of a home battery is a fire-safety and electrical-design decision. Cable convenience comes after safe escape, fire separation, fresh-air ventilation, equipment limits and protection from impact.

PAS 63100:2024 is the UK specification for fire safety in small-scale battery energy storage systems in dwellings within its scope. BS 7671:2018+A4:2026 adds Chapter 57 for stationary secondary batteries and directs dwelling installations to PAS 63100. An MCS installation also has to meet the applicable version of MIS 3012.

Start outdoors where practicable

PAS 63100 says storage batteries should be installed outdoors where practicable. For this purpose, “outdoors” can include a non-habitable outbuilding that is detached or separated from the dwelling by a main wall with at least REI 120 fire performance.

This does not mean any garden wall is suitable. The battery, enclosure and cabling must be approved for the actual temperature, moisture, dust, sunlight, condensation and flood exposure. The mounting structure has to carry the load, and the unit must remain accessible for isolation, inspection and replacement.

An external installation must not compromise the wall’s fire performance. Penetrations need appropriate fire stopping, cavities need suitable barriers around services, and combustible material inside the wall must not be exposed by the work.

PAS 63100 prohibits an outdoor battery within 1 m of the edge of an escape route, door, window or ventilation port. It also prohibits a battery within 2 m of stored flammable materials and fuel-storage tanks or cylinders. These are fixed requirements of the named specification, not general estimates.

Locations PAS 63100 prohibits

Storage batteries must not be installed in:

rooms intended for sleeping
landings, stairs, corridors or other routes used as a means of escape
the corridors, shafts, stairs or lobbies of protected escape routes
firefighting lobbies, shafts or stairs
cupboards, enclosures or spaces opening into sleeping rooms
voids, roof spaces or lofts
cellars or basements without access to the outside
the outdoor and flammable-material zones described above

An under-stairs cupboard is not named as a separate category, but it will normally form part of or threaten the escape route. The layout has to be tested against the actual PAS wording rather than a simplified internet list.

The prohibition is for storage batteries. A power-conversion unit such as an inverter is covered separately, and PAS 63100 allows PCE in some infrequently visited locations subject to product, fire-detection and impact requirements. “The inverter can go there” does not mean the battery can.

Indoor batteries need ventilation and separation

Where an indoor location is used, PAS 63100 requires all three of the following:

the location is not prohibited
it has fresh-air ventilation to outdoors
it has the required fire-resisting separation

The walls, floor and ceiling of an indoor battery location must provide at least REI 30 separation from the prohibited indoor locations. Service penetrations should preserve that performance.

All indoor battery locations need fresh-air ventilation to outdoors, not only lead-acid systems. The ventilation must not undermine the fire-resisting enclosure. The edge of its outside port has to be at least 1 m from the edge of doors, windows and ventilation ports serving other locations.

The airflow calculation, inlet and outlet positions and need for natural or mechanical ventilation depend on the battery technology, manufacturer data and room. A cupboard volume or grille size copied from another installation is not a design. Mechanical ventilation also needs a safe response to power or fan failure.

Do not wrap, box in or heat an outdoor battery unless the manufacturer permits the complete arrangement. An improvised enclosure can restrict cooling, collect gases, trap moisture or invalidate the product’s environmental rating.

Fire detection

If batteries are in an infrequently visited part of the dwelling, such as a store cupboard, PAS 63100 requires a smoke or multi-sensor fire detector in that location, interlinked with the dwelling’s fire-alarm system and conforming to BS 5839-6.

The PAS notes that a dwelling with BESS should have at least a Grade D2, Category LD2 system. The fire-alarm designer still has to apply the current rules for the UK nation, building and occupants and provide coverage appropriate to the battery hazard. A standalone alarm that cannot be heard in bedrooms is not the intended outcome.

The BMS and remote app do not replace building fire detection. A product alert depends on sensors, power and communications that may fail during the event.

Garages and vehicle impact

A detached garage can fall within the PAS meaning of outdoors. An attached garage is assessed by its fire separation, ventilation and relationship to the dwelling.

PAS 63100 sets a stored-energy limit of 80 kWh for batteries outdoors, in a detached garage or outbuilding, or in an attached garage or outbuilding with at least REI 60 fire separation. The limit is 40 kWh in other cases. These are totals for all battery units at the dwelling, not per cabinet.

Batteries in garages or beside vehicle movement areas need at least IK10 impact protection or equivalent local protection, such as a suitably designed barrier or enclosure. The barrier must protect the equipment without blocking access, ventilation or escape.

Do not assume that mounting a battery just above bumper height removes the risk. Vehicle type, reversing path, stored equipment and the strength of the wall all matter.

Temperature and product limits

The installation has to remain within the battery manufacturer’s permitted charging, discharging and storage conditions. These ranges differ materially between products and can be narrower for charging than discharging.

Cold weather may reduce charge power or stop charging; high temperatures can accelerate ageing or cause protective shutdown. Quote the product’s declared range and any derating in the design rather than publishing a generic “ideal battery temperature”.

The location should also avoid direct heat sources, combustible storage, persistent damp, corrosive atmospheres and places likely to be flooded. Adequate clearances are the manufacturer’s values for the installed orientation, not a universal gap around every battery.

Survey and handover evidence

A useful location assessment records:

why an outdoor location was or was not practicable
the PAS 63100 clause route for the chosen location
distances to openings, escape routes and stored fuels
wall, floor and ceiling construction and required fire performance
ventilation route and calculation
fire-detection category and interconnection
equipment environmental and temperature ratings
structural loading, fixings and impact protection
access to isolators and space for maintenance or replacement
total stored energy across the dwelling

Photographs and a plan should go in the handover pack with the PAS verification checklist, electrical certificate, product instructions and emergency information.

Existing installations

Standards are not automatically retrospective. A battery installed to an earlier valid standard is not proved unsafe merely because its location would not be selected for new work today.

It is still sensible to have a competent installer assess an older loft, escape-route or unventilated installation against current risks, product temperatures, alarms, structure and access. Do not move or enclose it yourself. Relocation changes high-current wiring, isolation, fire stopping, warranty conditions and possibly the network and MCS records.