When large-scale energy storage systems catch fire, there are understandably many headlines and safety reviews that follow, due to their scale and greater risk to property and life. A January 2025 fire destroyed nearly 300 MW of the Moss Landing energy storage facility in Santa Cruz, California, closing a nearby highway and temporarily evacuating 1,200 residents. No injuries or deaths were reported, but the situation has placed more scrutiny on battery installation safety.
Local fire officials knew how to handle the emergency at Moss Landing — which was ultimately blamed on thermal runaway and a fire suppression system failure — due to standards like UL 9540 and NFPA 855. These two standards provide assurance to installers and safety personnel that batteries, when installed correctly, are safe for operation. And now, additional testing from UL assures compact neighborhoods that residential energy storage systems are installed with safety as the highest priority.
Residential focus
UL first offered the UL 9540 standard for safety of energy storage systems and equipment in 2016, and batteries receive the certification by using certified products and completing 9540A testing. The 9540A test method purposely pushes the batteries into thermal runaway to see how flames spread. The batteries can’t pass or fail the test, but their reaction in a fire helps inform fire personnel how best to handle them in emergency situations — whether that means installing specific fire suppression systems or requiring more space around the units to thwart a bigger blaze.
Credit: Swell Energy
The 9540A test looks at what happens to one energy storage system alone. In residential setups, multiple batteries are often installed beside each other or in stacked designs. Fire marshals requested more testing data for these common residential designs, and UL responded in 2024 with the 9540B testing method for systems 20 kWh or smaller.
“9540A will test one energy storage system: What happens in a fire condition? What happens if there are faults inside the energy storage system?” said Michael Slowinske, director of principal engineering with UL Solutions. “9540B looks at if that system is installed on a wall, under an eave, under a ceiling [or] next to other energy storage systems.”
With residential energy storage systems commonly mounted in garages or basements, there were concerns about combustible items nearby, such as gas-operated lawn equipment, natural gas furnaces, paint cans or aerosols — things not associated with utility-scale battery projects. UL 9540B looks at how a battery fire may spread to nearby items, including additional energy storage systems.
“For wall-mounted units, we initiate a fire in one, which is called the initiating unit. Then there are other target units mounted next to or above or below. We see what happens in the target units,” Slowinske said. “If [batteries are] intended to be mounted under an eave, we would simulate that by building a wall with an eave, which helps capture heat.”
Whereas 9540A testing is required to get the full UL 9540 listing, 9540B is voluntary. Certain jurisdictions, especially in markets with a high residential battery deployment rate, may want to see 9540B test results to better train emergency personnel on what to do in case of an accident.
“The fire service wants to know when they’re responding to a call what they’re running into. They want more information and detail on the size of the fire and how the fire can spread,” Slowinske said. “All of that information is part of a 9540A and 9540B test report. It informs the installer, the building owner and the fire service how these behave in a fire situation, and that helps them be ready for their response.”
While still relatively new, some battery manufacturers are already voluntarily completing the 9540B testing, Slowinske said. Many jurisdictions in California, one of the country’s largest residential battery markets, have required large-scale fire testing on residential energy storage systems, and 9540B was designed to support the request. Manufacturers completing the testing can more quickly sell their products in this coveted market.
The National Fire Protection Association is currently gathering information on a proposed new standard for battery safety, due to the technology’s increasing use in an array of applications, including home energy backup.
“Currently, codes and standards exist which address aspects of battery safety, but there is no single, comprehensive code that harmonizes all pertinent codes and standards as the bigger picture,” NFPA stated. “Creating a new standard specifically addressing fire, electrical, life safety and property protection issues related to batteries will allow focused attention on the unique challenges posed by batteries.”
More safety codes may be coming to the residential battery space, but at least fire safety is being taken more seriously now with UL 9540B testing.
