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Safety Testing for Residential Energy Storage Systems (ESS)

UL 9540B test protocol addresses a more robust ignition scenario and enhanced acceptance criteria to evaluate large scale fire propagation characteristics of residential energy storage systems (ESS).

A battery energy storage system in a garage

Since the beginning of energy storage system adoption, safety has remained a key pillar in the evolution of systems. We have seen the technology around residential ESS evolve and adapt to accommodate applications throughout various environments and installations. As manufacturers, installers, local authorities and homeowners embrace the energy transition and incorporate ESS into their own homes, we see the need to address additional risk and considerations around safety for residential use.

The addition of UL 9540B the Outline of Investigation for Large-Scale Fire Test for Residential Battery Energy Storage Systems will provide large-scale fire testing for ESS used in residential applications. The driving forces behind the development of UL 9540B come from the growing desire for evaluation of the fire propagation ability of an electrochemical ESS and its thermal impacts to its surroundings—in addition to feedback from local authorities having jurisdiction (AHJ).  

Leading the way in battery safety technology

As a leader in battery safety technology, we offer:

  • Full fire testing facilities capable of executing test methodologies that support demonstrating compliance with UL 9540B.
  • Experienced engineering and research teams that can help develop insights using in-depth knowledge of battery chemistries and fire suppression.
  • Demonstrable track record of success serving organizations and entities such as fire departments, as well as electrical and building inspectors.

Frequently asked questions

Why was UL 9540B created?

Manufacturers relying on UL 9540A test reports for their residential ESS received feedback from authorities having jurisdiction expressing the need for large-scale fire tests. The AHJs determined that UL 9540A test series by itself does not adequately fulfill the requirements of the 2022 California Fire Code.

UL 9540B was created with insights from fire marshals and fire chiefs, as well as other fire service representatives—coupled with safety science research and extensive UL 9540A testing experience—to develop a test method that addressed the concerns brought forth by the various jurisdictions and fire service.

Will UL 9540A—the Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems—be going away?

UL 9540A is an established test method that has been referenced in NFPA 855 since its inception and continues to be referenced. It also is referenced in UL 9540, Energy Storage Systems and Equipment to support the safety certification of energy storage systems that have separation distances less than and capacity requirements greater than the values specified in NFPA 855 for installations.

UL 9540A is not going away because it plays a critical role in the safe deployment of energy storage systems. We recognize that some jurisdictions will enforce requirements beyond UL 9540A to comply with local codes for installation of residential ESS. UL 9540B was developed to fill that gap for those jurisdictions.

What are the differences between UL 9540A and UL 9540B?

UL 9540B specifically addresses residential energy storage systems that are 20 kilowatt hours or less. It does not address commercial or industrial energy storage systems. Industrial energy storage systems are still addressed in UL 9540A.

When compared with UL 9540A, UL 9540B removes the module level test. Rather than conducting three tests (cell, module, unit) under UL 9540A, under UL 9540B only two tests are required -- – cell test and fire propagation test. An added benefit is that residential energy storage systems that have previously undergone the cell level test under UL 9540A can often use that test data for the UL 9540B cell test.

A key difference between the UL 9540A and UL 9540B is that UL 9540B includes the purposeful ignition of vented gases during a thermal runaway propagation event.

The output of UL 9540A is three test reports that evaluate the ability of an electrochemical ESS to experience and manage thermal runaway propagation due to cell failure(s) leading to propagation.

The output of UL 9540B is a single test report that evaluates the fire propagation of an electrochemical ESS and its thermal impact to the surrounding exposures. The large-scale fire test report can be used to assess whether the residential battery energy storage systems can be installed as indicated in the manufacturer’s installation instructions or if they must be installed in accordance with NFPA 855 and the International Residential Code (IRC) separation requirements for residential energy storage systems.

Will the methods of UL 9450B be used in UL 9450A?

UL 9540A has a different objective and purpose compared with UL 9540B and UL 9540B is not intended to replace the tests of UL 9540A. The scope of UL 9540A is to evaluate thermal runaway propagation behavior within an ESS. The scope of UL 9540B is to evaluate fire propagation behavior if  the vented gases from a battery inside the residential ESS are ignited.

How does UL 9540B help me get my products accepted into the US or other markets?

UL 9540 certification and UL 9540A testing are required in UL 9540 and NFPA 855 to get products accepted into the U.S. and other markets. However, some jurisdictions in North America do not consider UL 9540A to be representative of a large-scale fire test as required by their local fire code—the 2022 edition of the California Fire Code, for example. In those cases, the Code Authority requires an additional large-scale fire test for residential energy storage systems, and this is where UL 9540B fits in. UL 9540B was developed to address the concerns of code authorities related to fire propagation risks in a residential ESS experience due to a thermal runaway propagation event.

Where is UL 9540B required – how does it help my product gain acceptance by AHJs?

UL 9540B is intended to address the needs for code authorities who request a large-scale fire test to be done on residential energy storage systems in accordance with their local fire codes. The test results of UL 9540B will support—or not support—the manufacturer's installation instructions regarding separation between individual residential energy storage system units.

Will the testing be conducted at a UL Solutions lab location?

UL 9540B testing will be done at our global facilities that presently conduct UL 9540A testing, with anticipated future capabilities to conduct this testing at our brand-new North America Advanced Battery Laboratory in Auburn Hills, Michigan.

Is UL 9540B intended to become a Standard?

Yes, there are plans to make UL 9540B a consensus-based standard like UL 9540A. It was released as an outline to address the immediate needs of the Code Authorities/fire service who do not accept UL 9540A testing as a large-scale fire test for residential energy storage systems, as noted in local codes, such as the California Fire Code (CFC 2022).

Can we still conduct residential tests under UL 9540A?


If UL 9540A testing has already been completed on a product, can any data be used for UL 9540B?

The UL 9540A cell level test report can be used for the UL 9540B cell test. However, the fire propagation test in UL 9540B must still be completed because it is a different test methodology.

If the UL 9540A cell level test has been completed, can it be used for the cell test in UL 9540B?

Even though the test methods are very similar between UL 9540A and UL 9540B, UL 9540A requires additional test data (maximum pressure and burning velocity) that is not required for UL 9540B. Therefore, a UL 9540B cell test cannot be used to replace a UL 9540A cell level test.

Does a residential energy storage system have to be tested under UL 9540A and UL 9540B?

No. In order to achieve a UL 9540 certification or listing, a residential energy storage system must meet the unit level performance criteria of UL 9540A when the spacing between individual battery energy storage systems is less than 3 ft (0.9 m) in accordance with the installation codes (NFPA 855, IRC).

We recognize that some jurisdictions may request manufacturers to provide test data for a large-scale fire test in accordance with their local code requirements, such as the California Fire Code (2022). However, there is no published test method for a large-scale fire test. UL 9540B was developed to provide a consistent test method for a large-scale fire test that can be utilized to meet the requirements for those jurisdictions.


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