Adoption of UL9540 DC ESS
The Third Edition of UL 9540, published in June 2023, introduced long-needed clarity for Battery Energy Storage Systems (BESS). Adoption across North America has progressed at varying speeds as Authorities Having Jurisdiction (AHJs) work through interpretation, alignment with existing enforcement practices, and integration into local approval processes. In Canada, the standard was incorporated into the 2024 Canadian Electrical Code (CEC, Part I), formally recognizing both AC- and DC-coupled ESS architectures. However, as with many newly adopted codes, consistent application at the local level has taken time, and differing interpretations among AHJs across America have contributed to extended review cycles and uncertainty for installers into 2025/26.
What Changed in UL9540 Third Edition?
The most important update was simple but impactful: UL9540 now distinguishes between AC ESS and DC ESS.
- AC ESS: A battery paired with an integrated inverter, certified as one unit.
- DC ESS: A standalone battery system certified to UL9540, designed to pair with a compatible, separately listed inverter.
Both system types are now evaluated to the same safety requirements, including UL9540A fire testing, system-level protections, and performance validation. The primary difference is how the Power Conversion Equipment (PCE) is documented: AC ESS includes the inverter within its certification, DC ESS verifies inverter/battery compatibility through manufacturer documentation. All the safety tests
This shift enables modern DC battery systems to be certified as UL9540 DC ESS, which offers flexibility in pairing with UL 1741 or CSA 107.1-listed inverters, rather than requiring fixed, pre-engineered packages.
The following table compares AC ESS with DC ESS.
| Item | AC ESS | DC ESS |
|---|---|---|
| PCE Integration | Included as part of the ESS certification and evaluated to UL 1741 or UL 62109-1 | Evaluated separately under UL 1741 or UL 62109-1 as standalone or multi-mode power conversion equipment |
| Certification Scope | ESS includes the inverter and battery as one integrated product | ESS functions as a standalone DC system compatible with multiple PCE |
| Compatibility Documentation | Fixed configuration, each pair included a specific inverter and battery | Managed by the manufacturer per UL 9540 Clause 46.14 |
| Safety Evaluation | UL 9540A fire testing and system-level protection | UL 9540A fire testing and system-level protection (identical evaluation as AC ESS) |
UL9540 in the Electrical Code
The UL9540 DC ESS fits into the National and Canadian Electrical Code (CEC) as published in the UL9540 Third Edition, as shown below.
| Component | Required Certification | Role |
|---|---|---|
| 1. Battery System | UL 9540 (Listed as “DC ESS”) | UL 9540 certifies the battery pack, BMS, and protection as a safe standalone system. |
| 2. Inverter | UL 1741 or CSA C22.2 No. 107.1 | The UL or CSA listing certifies the inverter’s safety and grid interaction capabilities. |
| 3. Interoperability | Manufacturer Manual / Closed-Loop Integration | The battery’s UL 9540 manual lists the specific inverter brand/model as compatible, or defines the integration settings. |
Why Many AHJs Aren’t Aligned
The challenge is largely interpretive, not technical. The disconnect between the national governing bodies and many parts of North America may be due to a misalignment in the mindset of both inspectors and system designers.
- First and Second Edition:
ESS = battery + inverter. Tested and certified as a single unit - Third Edition:
DC ESS = standalone battery system + separately certified inverter. Battery and inverter are tested and certified separately
Of all the components in a solar energy system, only the battery and inverter required being tested as a single unit. None of the other components was restricted in this way. Whether it be solar panels, junction boxes, DC disconnects, AC distribution panels, and so on, each component serves a particular purpose and is tested to ensure it operates as defined, in a safe and controlled manner. However, the UL listing for these components is not determined by their pairing with another component. That would be restrictive. Compliance is not, and should not, be contingent on connecting to a specific device, which was the case for batteries and inverters in the first and second editions of UL9540.
UL 9540 Third Edition addresses this issue by decoupling the inverter and battery. UL 9540 ensures safe operation by testing the safety and functional operation of the DC ESS components, including the BMS and control logic, thermal protection, electrical safety, and system integration. The boundaries of the DC ESS are firmly defined, allowing installers and AHJs to concentrate on other aspects of the installation, such as the installation environment, component spacing, and connection methods.
This shift in what to inspect in a UL 9540 Third Edition energy storage system requires training. The lack of understanding of the new regulations has resulted in varying timelines for adoption across North America, leading to inconsistent enforcement and frequent project delays. We look forward to everyone involved gaining a better understanding of the new regulations in 2026 and to increased adoption of best practices.
Verification Checklist for AHJs
To approve a DC ESS installation under UL9540 Third Edition, AHJs can follow a simple checklist:
- Verify UL9540 listing for the battery system.
- Review UL9540A test data for spacing and fire safety requirements.
- Confirm inverter certification (UL1741 or CSA 107.1).
- Check manufacturer compatibility documentation (per Clause 46.14 in the UL9540 standard).
- Ensure installation follows the battery manufacturer’s instructions.
This ensures that the battery and inverter are individually compliant and safe as a combined system.
Supporting Documentation from Discover Energy Systems
Discover Energy Systems provides integration guides and LYNK II Gateway documentation listing validated inverter models. The HELIOS ESS manual also lists inverter brands that are supported through its native communication protocol settings. These resources help AHJs verify compatibility and provide instructions that enable proper communication between the ESS and the inverter.
Conclusion
UL 9540 Third Edition represents a significant step forward in modernizing energy storage safety for both integrated AC systems and modular DC systems. The Third Edition offers increased safety and flexibility while maintaining protection requirements for all types of energy storage systems.
The full benefit of the new standard will be realized when AHJs fully understand and apply the new framework. As inspectors transition from fixed, pre-engineered systems to modular DC ESS paired with certified inverters, consistent training and documentation become essential. With guidance and manufacturer-supported compatibility tools, jurisdictions across North America can confidently approve safe, flexible, and future-ready energy storage installations, unlocking the full value of the UL9540 Third Edition.