UL 9540 third edition: what energy storage makers need to know
The third edition of UL 9540 splits AC and DC energy storage systems, ties capacity limits to UL 9540A fire tests and adds requirements for explosion control, liquid cooling and remote software updates.
- 01UL Standards & Engagement published the third edition of UL 9540 in June 2023.
- 02It introduces separate AC and DC energy storage system concepts.
- 03Capacity and separation limits now align with UL 9540A results and NFPA 855.
- 04New topics: explosion control, liquid-cooling leaks, noise and remote software updates.
- 05Lithium-ion systems of 500 kWh and more need external warning communication.
Why it matters
UL 9540 is the North American safety standard for energy storage systems (ESS). Installation codes such as NFPA 855 and the US fire and electrical codes refer to it, so a UL 9540 listing is effectively the entry ticket for commercial and utility storage in the US and Canada.
The third edition was published by UL Standards & Engagement in June 2023 and has since been revised as a bi-national ANSI/CAN standard.
The main changes
| Topic | What changed |
|---|---|
| System types | Separate concepts for AC ESS (with the inverter inside the listing) and DC ESS |
| Fire testing | Capacity and spacing limits linked to UL 9540A large-scale fire test results, aligned with NFPA 855 |
| Explosion | Explosion control requirements aligned with NFPA 855 |
| Large lithium-ion systems | External warning communication for systems of 500 kWh and above |
| Liquid cooling | Leak detection and requirements for coolant and other hazardous fluids |
| Other technologies | New criteria for thermal storage, revised flywheel and mechanical ESS testing |
| Software | Requirements for remote software updates |
| Environment | Noise-level criteria |
Lead-acid and nickel-cadmium batteries can use alternative evaluation methods aligned with UL 1973.
AC or DC: decide early
Under the third edition, an AC ESS includes the power conversion system in the listing, while a DC ESS is listed without it and is later combined with a separately listed inverter. The choice affects which tests run on the complete system, which documents the certifier reviews and how much retesting a later inverter change triggers.
How UL 9540 fits with related standards
- UL 1973: batteries for stationary use.
- UL 1741: inverters and power conversion systems, including grid-support functions (SB).
- UL 9540A: the fire test method; its results feed into the installation limits in UL 9540 and NFPA 855.
- IEEE 1547 / 1547.1: interconnection requirements and their test procedures.
What to plan for
- Confirm the AC or DC system architecture before the certification project starts.
- Run UL 9540A at cell and module level early; the data shapes spacing and capacity limits.
- Document cooling, gas detection, explosion control and software update processes.
- Align PCS testing to UL 1741 and IEEE 1547.1 with the system test plan.
Our partner laboratory network tests energy storage converters up to 2.5 MW for grid behaviour, safety and EMC, which supports the PCS part of a UL 9540 project.
FAQ
Is a UL 9540A report the same as a UL 9540 listing?
No. UL 9540A is a fire test method that produces data. UL 9540 is the safety standard under which a system is listed.
Does an AC ESS need a separate UL 1741 certificate for the inverter?
The PCS is evaluated as part of the AC ESS listing, and the evaluation draws on UL 1741 requirements. Check the exact route with your certification body.
Does the 500 kWh threshold apply to all chemistries?
The external warning communication requirement described in the third edition applies to lithium-ion systems of 500 kWh and above.
