The Difference Between UL 9540 vs. UL 9540a: Demystifying Solar Battery Certifications

By Nick Holden, Senior Regulatory Engineer, Discovery Energy Systems 



  • UL 9540 is a safety standard for certification of Energy Storage Systems (ESS’s)
  • UL 9540a is a test method for gathering data and  assessing an ESS’s ability to withstand a thermal runaway event, but doesn’t offer a pass or fail verdict
  • Manufacturers use UL 9540a test results along with other compliance efforts to obtain UL 9540 certification
  • The UL 9540a test method is a comprehensive, four-step procedure evaluating an ESS starting with individual cells, battery modules, to the complete energy storage system
  • Consumers should choose energy storage systems with UL 9540 certification for peace of mind and safety
  • Be cautious of confusing terminology regarding certifications like "UL 9540a certified"
  • Proper certification like UL 9540 for ESS' is required for installation approval in certain regions, such as major urban areas of California
  • Prioritize finding a qualified professional who understands the importance of UL 9540 certification when choosing a solar installer
  • For an example of a UL 9540-certified energy storage system, please see our AES Rackmount Slimline Enclosure here: 

Harnessing the power of the sun is a fantastic way for homeowners to save money, become more energy-independent, and help our planet. However, when it comes to storing solar energy, safety is paramount. Certifications have been established to ensure the safety of consumers, but two terms often cause confusion: UL 9540 and UL 9540a.

What is UL 9540?

While UL 9540 and UL 9540a sound similar, they play very different roles. UL 9540 is a safety standard and is considered the industry benchmark to ensure energy storage systems work safely. UL 9540 is approved by the American National Standards Institute (ANSI) and the Standards Council of Canada (SCC). To meet the standard, energy storage systems must comply with strict requirements for construction methods, system safety, and system performance and perform up to specified levels on a series of tests, including UL 9540a.

What is UL 9540a? 

UL 9540a, on the other hand, is a test procedure. It assesses an energy storage system’s response to thermal runaway, a potentially dangerous situation where a battery enters an uncontrolled, self-heating state. The test gathers data on how the battery responds but doesn’t offer a pass or fail verdict. Think of UL 9540a as a single hurdle within the bigger race of achieving 9540 certification. Manufacturers use the 9540a test results along with other compliance efforts to obtain certification from an authorized third-party agency for UL 9540.

What are the steps of the UL 9540a test procedure?

The UL 9540a test is a comprehensive, four-step procedure that starts with testing the individual components and ends with testing the product as a whole. The first step, the cell level test, evaluates an individual battery cell. It’s forced into thermal runaway in a pressure vessel, and the gasses produced are gathered and analyzed. If the cell test triggers thermal runaway, the process moves to the module test.


This test assesses how the module handles a runaway cell. The goal of the module test is to evaluate how the battery responds when cell-to-cell propagation occurs. This is done as an iterative process starting with forcing thermal runaway in a single cell, and monitoring whether it propagates to adjacent cells. If yes, the flammable gases are collected and analyzed. If cell-to-cell propagation does not occur, the test is repeated but starts with forcing two cells into thermal runaway and monitoring for propagation. Then if needed, three cells, etc.


The third step, the unit-level test, evaluates the complete energy storage unit as installed per the manufacturer's installation instructions. Using the same method determined in the Module level test, a runaway event is simulated in one of the modules, and the test observes the temperature rise in the cabinet, whether any fire escapes the enclosure to adjacent units, overall safety performance, and how a thermal runaway event in one ESS will affect adjacently installed ESS's.


Finally, if the unit-level test reveals any safety hazards, such as escaping flames, the installation-level test is conducted. It evaluates potential risk mitigation strategies, such as creating specific installation instructions and limitations of installation locations.


What certification should homeowners and installers look for when assessing home solar batteries?

Consumers can achieve peace of mind by choosing an energy storage system that is certified to the UL 9540 standard. This rigorous process, conducted by an independent third-party agency, ensures home batteries meet strict safety standards, lowering the risk of fires and other potential hazards. Certification to UL 9540 standards involves simulating real-life scenarios during testing, providing an extra layer of confidence in the safety of a chosen solar battery system.


Unfortunately, some manufacturers use confusing terminology regarding certifications. This can make it difficult to know if a product is truly safe. Consumers should be cautious of manufacturers claiming their products are “UL 9540a certified.” While the test is part of the UL 9540 certification process, simply undergoing the test doesn’t guarantee that the product is safe to use. Similarly, claims of “designed to UL 9540a or UL9540” or “tested to UL 9540 or UL 9540a'' offer little assurance. These could indicate testing was performed on a preliminary component but not the whole product, or even that the test results were unsatisfactory. Look for wording like “UL 9540 listed” or “certified” to indicate the ESS has undergone independent third-party evaluation and certification to ensure it meets all requirements of the standard.


It’s a good idea to visit manufacturers’ websites and look for their energy storage systems’ test results and certifications. However, it’s important to be wary of any incomplete reports that only show the first page or two. Below are examples of a UL 9540 certificate and the first and last page of a UL 9540a test report.


Figure 1. Discover Energy Systems’ AES Rackmount Slimline Enclosure’s UL 9540 certificate.


Figure 2 & 3. The first and last page (pg. 1 & 38)  of Discover Energy Systems’ AES Rackmount Slimline Enclosure’s UL 9540a test report

Why do you see confusing language around UL 9540 and UL 9540a in the solar battery market? 

New solar battery technology is growing incredibly fast, and the regulations are changing just as quickly. Manufacturers may find it challenging to keep up with the various regulations in this rapidly evolving landscape. Since its introduction in 2016, the UL 9540 standard has undergone three revisions. The technical and legal language used in the standards can also be difficult to understand. For overseas manufacturers relying on translation tools, a few misplaced words can lead to significant misunderstandings.


At the end of the day, the installation likely won't be approved in certain places, such as California, without ESS certification to UL 9540. Proper certification ensures a safe installation, saving consumers time and money and preventing potential delays.


Why UL 9540 is important

When choosing a solar installer, prioritize finding a qualified professional who understands the importance of selecting products that are certified to UL 9540 standards. This ensures a safe and inspector-approved installation, saving you time and money in the long run. Their expertise can ultimately mean the difference between a smooth installation and costly delays.


While solar power offers a path toward energy independence and a greener future, safety remains a top priority, especially when it comes to battery storage. Don't be misled by confusing terminology. UL 9540 certification is the gold standard, ensuring a battery system meets strict safety protocols through rigorous testing. This translates to peace of mind for consumers, knowing their home is equipped with a safe and reliable energy storage solution. It’s important to remember that certification to UL 9540 standards is also mandatory for installation approval in many areas. By prioritizing safety and choosing a UL 9540-certified system and a knowledgeable installer, consumers can confidently embrace the future of solar power.

For an example of a UL 9540-certified energy storage system, please see our AES Rackmount Slimline Enclosure here: 

- By Nick Holden, Senior Regulatory Engineer, Discovery Energy Systems


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