UL 9540 - Battery Design

Standard for Energy Storage Systems and Equipment

UL 9540 is a system-level safety standard published by UL Solutions (UL) for stationary energy storage systems (ESS). It covers the complete integrated energy storage system: including battery, power conversion, controls, and enclosure, rather than individual components. UL 9540 is the primary listing standard required by US electrical codes (NEC, IBC) and fire codes (NFPA 855, IFC) for stationary battery energy storage, and is also adopted in Canada under CAN/UL 9540. The 3rd edition (April 2023) expanded scope, updated definitions, and aligned terminology with NFPA 70 and the International Residential Code.

Published by: UL Solutions — also designated ANSI/CAN/UL 9540

Current edition: 3rd edition, April 2023 (ANSI/CAN/UL 9540:2023) — effective September 2024 for new certifications

Replaces: UL 9540:2018 (2nd edition); UL 9540:2016 (1st edition)

Primary market: United States and Canada — required for grid-connected and off-grid stationary ESS installation

Scope

UL 9540 applies to energy storage systems for stationary installations — both residential and non-residential. The standard covers:

Electrochemical ESS: lithium-ion, lead-acid, flow batteries, and other battery chemistries
Other storage technologies: flywheel, capacitor-based, compressed air, and thermal energy storage
Installation types: indoor, outdoor, and mobile (vehicle-mounted) ESS
Operating modes: standalone/off-grid, self-supply, and grid-interactive systems
Voltage: both AC-coupled and DC-coupled ESS; systems up to and including utility-scale ground-mount

What UL 9540 does NOT cover independently: The individual battery cells or modules (covered by UL 1973 or UL 1642), the inverter or power conversion equipment (covered by UL 1741), or individual component certifications. UL 9540 requires that those components be separately listed and then tests them as an integrated system.

Key Requirements

System-Level Safety

Thermal runaway fire propagation: the 3rd edition mandates that lithium-ion ESS be tested per UL 9540A — a separate test method that evaluates whether a thermal runaway event in one cell propagates to adjacent cells, modules, or adjacent ESS units. The test hierarchy runs from cell → module → unit → installation
Fire containment: at the system boundary the ESS must either prevent thermal runaway propagation or contain it within the enclosure. Results define required separation distances and suppression requirements for the installation
Electrical safety: dielectric withstand, insulation resistance, overcurrent protection, ground fault detection, arc fault detection (where applicable)
Enclosure: environmental protection ratings (IP or NEMA), structural integrity, ventilation, and gas management for off-gas events

Marking and Instructions (3rd Edition Updates)

AC ESS and DC ESS: revised scope and new definitions for AC-coupled and DC-coupled systems; new mandatory diagrams showing system architecture
Marking: updated marking requirements aligned with NEC 2023 and IRC terminology for residential ESS
Installation instructions: requirements for documentation accompanying the ESS listing, including installation clearances, wiring diagrams, and maintenance procedures

Component Requirements

Battery subsystem: must be listed to UL 1973 (stationary/light rail batteries) or equivalent standard
Power conversion equipment: must be listed to UL 1741 (inverters/converters for DER)
Controls and BMS: functional safety assessment and integration testing required as part of system certification

Certification and Code Requirements

NEC (NFPA 70): Article 706 (Energy Storage Systems) requires ESS to be “listed” — UL 9540 listing satisfies this requirement in the United States
NFPA 855: references UL 9540 listing as the baseline requirement for ESS installation; NFPA 855 then adds location, separation, and suppression requirements based on UL 9540A fire propagation test results
IFC (International Fire Code): references UL 9540 for permitted ESS installations in commercial and industrial occupancies
IBC (International Building Code): references UL 9540 listing for building-integrated ESS
Canada: adopted as CAN/UL 9540; required by Canadian Electrical Code (CEC Part I, Rule 64-056) for ESS installations
California: CalFire, CPUC, and SDG&E/PG&E/SCE interconnection agreements all reference UL 9540 listing

Related Standards

UL 9540A — Test method for evaluating thermal runaway fire propagation in BESS (mandatory input to UL 9540 certification for Li-ion systems)
UL 1973 — Standard for batteries for stationary, vehicle auxiliary power, and light electric rail (the battery subsystem standard within a UL 9540-listed system)
UL 1741 — Standard for inverters, converters and controllers for DER (the power conversion subsystem standard)
NFPA 855 — Installation standard for stationary ESS; uses UL 9540 as baseline and UL 9540A results for installation requirements
IEC 62619 — IEC safety standard for industrial Li batteries; international equivalent role to UL 1973 within a BESS

Disclaimer: This is not a definitive guide, just an indication and it is important that you always go back to the source for legislation.