UL 9540A is the internationally recognised test method for evaluating thermal runaway propagation in energy storage systems. The standard defines four hierarchical test levels.
| Level | Test subject | Objective |
|---|---|---|
| Level 1 | Single cell | Thermal runaway characterisation: gas emissions, energy release |
| Level 2 | Module | Cell-to-cell propagation within a module |
| Level 3 | Unit/Rack | Propagation between modules and racks |
| Level 4 | Installation | System behaviour incl. ventilation, detection, suppression systems |
UL 9540A test results are a prerequisite for permitting and insuring BESS projects. Missing or incomplete test results are a deal-breaker during technical due diligence. PV-BESS-Assessor reviews and evaluates UL 9540A reports as part of pre-investment assessments.
UL 9540A is not legally required in Germany but is expected by insurers, investors, and authorities as proof of propagation safety. In combination with VDE-AR-E 2510-50, it represents the current state of the art for BESS safety assessments.
UL 9540A: Thermal Propagation Evaluation for 40 MWh BESS
Real anonymised investigation case with measurement data, timeline, and economic evaluation.
Last updated: 2026-06-16