IEC 62933-1 Terminology
Defines the vocabulary of electrical energy storage — system boundary, ratings, states — used consistently across the series.
IEC 62933
Electrical energy storage (EES) systems
IEC 62933 is the international standard series for grid-connected electrical energy storage systems: terminology, unit parameters and test methods, environmental issues and — most critically — safety, including the prevention and containment of battery thermal runaway. It works alongside UL 9540A and NFPA 855.
Document structure
IEC 62933-2-1 Unit parameters and test methods
How to specify and test an EES unit: power and energy ratings, round-trip efficiency, response time, the parameters that make systems comparable.
IEC 62933-4-1 Environmental issues
Environmental requirements and the assessment of impacts over the storage system lifecycle.
IEC 62933-5-1 / 5-2 Safety requirements
Safety of EES systems: -5-1 general safety considerations, -5-2 safety for grid-integrated EES using electrochemical (battery) storage — hazard identification, thermal runaway prevention and mitigation.
Key concepts
- EES system boundary
- An energy storage system is more than cells: it includes the battery management system (BMS), power conversion, thermal management, controls and enclosure. IEC 62933 defines the boundary so that ratings and safety claims refer to the same thing.
- BMS vs EMS
- The Battery Management System protects each cell (voltage, current, temperature, balancing); the Energy Management System decides what the asset does (frequency regulation, peak shaving, arbitrage). Safety lives mainly in the BMS; value mainly in the EMS.
- Thermal runaway & propagation
- A failing cell can self-heat, vent flammable gas and ignite — and drag neighbouring cells with it (propagation). Preventing initiation and containing propagation is the central safety problem of battery storage and the focus of IEC 62933-5-2.
- Round-trip efficiency
- Energy out divided by energy in over a charge-discharge cycle. Li-ion systems reach roughly 85-90% at the AC terminals; the losses (conversion, auxiliaries, thermal management) directly affect the business case.
- Power vs energy rating (C-rate)
- A system is defined by power (MW) and energy (MWh); their ratio is the C-rate. A high-power, short-duration system suits frequency regulation; a high-energy, long-duration one suits peak shifting. The duty drives the cell choice and the thermal design.
- UL 9540A (companion test)
- A fire-safety test method that measures cell-to-cell and unit-to-unit thermal runaway propagation. Its results feed the separation distances, ventilation and suppression required by installation codes — referenced widely even outside North America.
- NFPA 855 (companion install standard)
- The installation standard for stationary storage: setback distances, fire detection and suppression, ventilation and explosion control. It uses UL 9540A test data to size the protections — the practical counterpart to the IEC 62933 safety case.
Notes & guidance
Making storage comparable — and safe
A battery storage system is the piece that lets a grid run on variable renewables: it absorbs surplus, delivers at peak and stabilises frequency in milliseconds. IEC 62933 is the series that makes such systems describable (terminology, parameters), comparable (test methods, efficiency) and above all safe — because the same energy density that makes storage useful makes a failing cell dangerous.
What the series covers
The parts split the problem: -1 fixes the terminology and the system boundary (a storage system is cells plus BMS, conversion, thermal management, controls); -2-1 defines unit parameters and test methods — power and energy ratings, round-trip efficiency, response time — so two systems can be compared honestly; -4-1 addresses environmental issues across the lifecycle.
The safety problem: thermal runaway
The heart of storage safety is thermal runaway: a failing cell self-heats, vents flammable gas and can ignite, then drag its neighbours with it. IEC 62933-5-2 structures the safety case for grid-integrated electrochemical storage — hazard identification, prevention of initiation, and containment of propagation.
It does not stand alone. The UL 9540A test method measures how runaway propagates cell-to-cell and unit-to-unit, and NFPA 855 uses that data to set separation distances, ventilation, detection and suppression. Incidents like the 2019 McMicken explosion in Arizona — where vented gas exploded as firefighters opened the door — are exactly what this stack of standards now exists to prevent.
Grid integration
A storage system that acts on the grid must speak its protocols and be cyber-secure. In practice IEC 62933 sits alongside IEC 61850 for substation communications, IEC 62443 for control-system cybersecurity, IEC 61000 for electromagnetic compatibility, and ISO 55000 for managing the asset across its life.
Applicable industries
- Grid-scale battery storage (BESS)
- Solar-plus-storage and wind-plus-storage hybrids
- Commercial and industrial behind-the-meter storage
- Utilities, IPPs and system integrators