Performance tests of a BESS in operation verify, after commissioning, that the system still delivers what was contracted: its real storage capacity, its charge and discharge power, its round-trip efficiency, its auxiliary consumption, its availability and its response to power and frequency setpoints. IEC TS 62933-2-3:2025 defines those parameters and their test methods during on-site operation, measured at the point of connection. Compared with the commissioning baseline, they show degradation and serve to enforce warranties.
Why test a BESS that already works
A battery energy storage system (BESS) loses capacity with use and with time. Its supply contract almost always carries warranties: a minimum capacity after a certain number of years or cycles, an efficiency and an availability. To know whether they are met you have to measure, and always measure the same way. The parts of the system are explained in BESS: parts, functions and standards.
The standard: IEC TS 62933-2-3:2025
IEC TS 62933-2-3:2025, Electrical energy storage (EES) systems – Part 2-3: Unit parameters and testing methods – Performance assessment test during site operation, is a technical specification of IEC TC 120, published in May 2025. According to its scope, it specifies the parameters and test methods that validate and evaluate the performance of storage systems after commissioning. It is based on IEC 62933-2-1:2017, on general parameters and methods, and on IEC 61000-4-30 for power quality measurements.
Input and output energies are measured at the system’s point of connection. The standard also covers the general requirements of the evaluation: the stakeholders, the evaluation intervals and data sampling, and the data requirements.
What is evaluated
| Group | Parameters |
|---|---|
| General | Real storage capacity, rated input and output power, round-trip efficiency, consumption of auxiliary systems |
| Energy | Available hours, equivalent operating factor |
| Efficiency | Overall efficiency, energy loss rate, round-trip efficiency over a duty cycle |
| Reliability | Planned outage factor, unplanned outage factor, availability factor |
| Grid connection | Influence of grid conditions, active power control, reactive power control, primary and secondary frequency regulation, power quality |
In addition, the standard groups tests for the typical application modes: frequency regulation, smoothing of fluctuations, voltage control with reactive power, power quality management, peak shaving or shifting, renewable firming and island-mode operation. A BESS associated with a solar farm is tested in the modes it actually uses.
The procedures, conditions and formulas are in the text of the standard and are not reproduced here.
What each parameter tells you
Real capacity. It is the energy the system delivers in a complete discharge within its operating limits, measured at the point of connection. Compared with the commissioning value, it gives the degradation. It is the parameter that warranties usually fix.
Round-trip efficiency. The energy that comes out divided by the energy that went in over a complete cycle. It says how much it costs to store and how the losses change with time.
Auxiliary consumption. Air conditioning, ventilation, control systems and fire protection. At hot sites it can weigh on overall efficiency, and an increase may point to an air-conditioning problem.
Availability. How long the system was in condition to operate, separating planned and unplanned outages. It is usually another warranty in the contract.
Grid functions. The response to active and reactive power setpoints and to frequency. Their initial verification is in commissioning of BESS; in operation it is repeated to check that the response has not changed.
Difference from commissioning
| Aspect | Commissioning | Performance in operation |
|---|---|---|
| When | Once, before commercial operation | Periodically or when in doubt |
| Purpose | Accept the system and set the baseline | Track degradation and verify warranties |
| Normative reference | IEC 62933-2-1, contract, utility | IEC TS 62933-2-3:2025, contract |
| System condition | Newly installed | In service, with its cycling history |
What the performance test is not
The performance test measures the complete system from the outside, at the point of connection. It does not replace component tests or the internal diagnosis of the batteries: voltage per cell or module, impedance or conductance and inspection are used to find where a loss is that the performance test only quantifies. The internal management of the battery management system belongs to the manufacturer. Battery test techniques are in substation battery tests: capacity and impedance.
How to prepare
- Agree on the procedure before there is a claim: which parameters are measured, where, with which instruments and at which states of charge.
- Keep the commissioning baseline with its conditions.
- Record the operation: cycles, temperatures, states of charge and events, because they explain the degradation.
- Coordinate the test window with the operator, because a complete discharge takes the system out of its normal operation.
The safe disconnection of the system for any intervention is in safe disconnection of a BESS.
What the report should contain
For a performance test to hold up against a warranty, the report must allow it to be repeated and compared:
- the measurement point and the instruments, with their accuracy and calibration;
- the initial and final states of charge, the charge and discharge power and the temperature during the test;
- the input and output energies, and how the auxiliary consumption was treated;
- the calculated parameters, with their method;
- the comparison with the commissioning baseline and with the guaranteed values;
- the events and alarms recorded during the test.
A result without the conditions under which it was obtained cannot be compared with next year’s.
We test your batteries
Our team tests the batteries of the BESS and of the substation bank: capacity or discharge, impedance or conductance, voltage per cell or per monobloc, and inspection. It also tests the transformer, switchgear and protection of the system, as part of the commissioning of BESS, in Colombia or elsewhere on request. If your contract requires a performance evaluation in operation, we help you plan it together with the manufacturer. Depending on the state of your system we recommend which test to do and with what equipment, and include it in the quotation: that way you do the tests you really need, without paying for extra tests. Write to us through contact.
Related guides
- BESS commissioning: tests and requirements
- BESS: parts, functions and standards
- Substation battery tests: capacity and impedance
- AC-coupled or DC-coupled BESS in solar farms
- Safe disconnection of a BESS: sequence and risks
- BESS cybersecurity: BMS, PCS and EMS
- Solar farm testing by stage: from FAT to O&M
References
- IEC TS 62933-2-3:2025, Electrical energy storage (EES) systems – Part 2-3: Unit parameters and testing methods – Performance assessment test during site operation. https://webstore.iec.ch/publication/77334
- IEC 62933-2-1:2017, Electrical energy storage (EES) systems – Part 2-1: Unit parameters and testing methods – General specification.
- IEC 61000-4-30, Electromagnetic compatibility (EMC) – Part 4-30: Testing and measurement techniques – Power quality measurement methods.
Frequently asked questions
What is the difference between commissioning and the performance test of a BESS?
Commissioning demonstrates, once, that the newly installed system meets the contract and sets the baseline. The performance test in operation is repeated throughout the life of the system to see how it evolves: how much energy it stores today compared with day one, how much it loses in each cycle and how long it was available. IEC 62933-2-1 covers the general parameters and methods; IEC TS 62933-2-3:2025, the evaluation with the system already in operation.
Is the capacity shown by the BMS valid as a test?
It is an estimate by the battery management system, useful for operating, but it does not replace a measurement: IEC TS 62933-2-3 evaluates real capacity from the energy measured at the point of connection during a defined discharge. When a warranty is at stake, the measurement is made with independent instruments and an agreed procedure.
Why is the measured efficiency lower than the datasheet value?
Because the datasheet usually gives the efficiency of the battery or the converter separately, at rated conditions. Measured at the point of connection, round-trip efficiency includes the transformer, the converter, the batteries and, depending on how it is defined, the consumption of auxiliary systems such as air conditioning. That is why the contract must say where and how it is measured.