Why UL 9540B exists
Test methods based on UL 9540A did not fully meet the requirements some North American authorities set for home batteries. The 2022 California Fire Code, for example, asks for large-scale fire data for residential energy storage. UL 9540B, published in 2024, fills that gap for residential systems of 20 kWh or less. It supplements UL 9540 and UL 9540A rather than replacing them.
What is tested
Cell test. Defines the method used to force thermal runaway and confirms that it occurs. Temperature and gas composition are measured, and the lower flammability limit of the vent gas is determined. A UL 9540A cell test can be used instead.
Fire propagation test. The full residential unit is mounted as installed, against a wall with a ceiling or eave above and a neighbouring unit alongside. Thermal runaway is triggered and the vent gas ignited. The test checks that:
- the neighbouring unit is not driven into thermal runaway by radiant heat;
- wall, ceiling or eave surface temperatures stay below the limit (150 °C);
- flames do not extend from the test wall onto the ceiling or eave.
The results support the manufacturer's installation instructions on spacing between units and on the location and total energy allowed under NFPA 855.
UL 9540A vs UL 9540B
| UL 9540A | UL 9540B | |
|---|---|---|
| Scope | Residential, commercial and utility systems | Residential systems ≤ 20 kWh |
| Levels | Cell, module, unit, installation | Cell, fire propagation |
| Ignition | Not forced | Vent gas deliberately ignited |
| Purpose | Does thermal runaway propagate? Quantify heat, smoke and gas | Effect of a developed fire on wall, eaves and a neighbouring unit |
Frequently asked questions
Does UL 9540B replace UL 9540A for home batteries?
No. Residential systems are still tested to UL 9540A for UL 9540 listing and NFPA 855. UL 9540B gives the AHJ additional large-scale fire data where local codes ask for it, such as the 2022 California Fire Code.
Do I need a separate UL 9540B cell test?
Usually not. A UL 9540A cell test covers the UL 9540B cell test requirements; the UL 9540B cell test only needs the lower flammability limit, not Pmax or burning velocity.
What is the main difference from UL 9540A?
UL 9540A measures whether thermal runaway propagates and does not force ignition. UL 9540B requires ignition of the thermal runaway gas, lets the fire develop and measures its effect on the surroundings.