Between “the containers have arrived” and “the project is earning” sits a testing programme that most buyers underestimate. It is where a supplier’s claims stop being claims, and it is the last point at which you have real commercial leverage.
Get the test regime right and you have a defensible record of what you bought. Get it wrong and you will spend the next fifteen years arguing about a capacity number nobody measured properly.
Two stages, two purposes
Factory acceptance testing
FAT happens at the manufacturer’s facility, before anything ships. Its purpose is to catch defects while the equipment is still the supplier’s problem.
The important limitation is that FAT is partial. Tests are often sample-based and conducted under partial stimulation, because a BESS cannot be fully energised at a factory — there is nowhere for the energy to go. A manufacturing plant generally cannot absorb or export megawatts on demand.
What FAT does prove well:
- Physical and electrical inspection — build quality, wiring, terminations, enclosure integrity.
- EMS integration testing — that the system communicates correctly with site-level or grid-level control: setpoint commands for active and reactive power, state reporting for state of charge, state of health and alarms, scheduling, and curtailment response. The two control layers involved are explained in EMS versus BMS.
- Capacity and performance testing against contractual specification under controlled conditions — full charge and discharge cycles at rated power, capacity verification at reference conditions, and round-trip efficiency measurement.
- Protection and safety functions, including how the system responds to simulated faults.
Note the phrase reference conditions. Capacity verification is commonly done at around a 0.5C rate and ideally 25°C. That is a laboratory condition, and it is a fair basis for comparison — but it is not an Indian afternoon, which is why the same test at site can read differently. The relationship between rate, depth and measured capacity is covered in C-rate, depth of discharge and state of charge.
Site acceptance testing
SAT tests the whole integrated system where it will actually live, under real operating conditions. This is what you sign against, and it covers ground FAT structurally cannot:
- Performance testing — rated discharge power, ramp rates, round-trip efficiency, energy capacity and response times.
- Grid code compliance — frequency response, voltage regulation, reactive power capability, fault ride-through and power quality.
- Integration with everything else on site — transformers, protection, metering, communications, and any co-located solar.
- Real ambient conditions, which is where thermal management either works or does not.
Round-trip efficiency measured at SAT is the honest AC-to-AC number, including auxiliary loads like cooling. It is almost always lower than the cell-level figure on the datasheet, and it is the one that determines earnings — see round-trip efficiency and degradation.
The standards underneath
Indian testing practice is generally structured around the national standards — IS 16270, IS 17092 and IS 17387 — covering storage batteries, stationary storage systems and battery management system safety and performance respectively. Our guide to BIS standards for batteries sets out what each covers and which certifications are mandatory.
Grid code compliance testing at SAT connects to the connection requirements in CEA technical standards, including the round-trip efficiency and monitoring thresholds now written into regulation.
There is also a commercial wrinkle at this stage. Power injected between first synchronisation and completion of trial runs is infirm power, and CERC has proposed compensating it at normal deviation rates subject to a ceiling of ₹2 per kWh — covered in our guide to the Deviation Settlement Mechanism. Commissioning is not free, and long trial periods have a cost.
What to fix in the contract, before testing
The single most valuable thing you can do is settle these in writing before anyone runs a test:
- Test conditions, not just thresholds. At what C-rate, what temperature, what state-of-charge window? A capacity threshold without conditions is unenforceable.
- Who witnesses, and whether an independent engineer signs off.
- What happens on failure — retest rights, cure periods, liquidated damages, and rejection rights.
- Which measurements become the warranty baseline. SAT capacity is normally the starting point from which degradation guarantees are measured, so it matters more than any other single number.
- How long the trial run lasts, and who bears the cost of energy during it.
What this means for you
- If you are a developer: send someone to FAT. It is the last point at which rejecting equipment is a supplier problem rather than a logistics and schedule problem on your site.
- If you are a C&I buyer: you may not run a full grid-code SAT, but you should still insist on a witnessed capacity and round-trip efficiency test at your site, with conditions written down. That measurement is what you will refer back to in year eight.
- If you are procuring under a tender: align your acceptance criteria with the applicable Indian standards rather than inventing your own, and specify the test conditions explicitly. Our manufacturing operation tests to these protocols before shipment.
- If you want the test regime reviewed: our engineering team can walk through a proposed FAT and SAT protocol for a specific project — get in touch.
Testing standards, grid code requirements and regulatory provisions change by notification, and individual tender documents may impose stricter or different requirements. Treat this as an August 2026 snapshot and verify the applicable standards and protocols for your own project.