Commissioning is often treated as a formality – switch it on, watch the meter turn, sign the certificate. That signature transfers a great deal of risk, and the tests you did not run become problems you own.
String open-circuit voltage (Voc). Measured for every string, compared against calculated expectation at the measured temperature. Deviations point to a missing module, a wrong module count, or a reversed connection.
String short-circuit current (Isc) or operating current. Every string, compared against its peers. This is your baseline – and having it recorded at commissioning is what lets you detect drift three years later.
Insulation resistance. DC circuits tested to the relevant standard (IEC 62446-1, or the applicable national code). Catches damaged cable insulation, moisture in connectors, pinched cables under module clamps. Low insulation resistance is both a performance issue and a safety one.
Polarity verification. Sounds trivial. Reversed strings still happen, and they cause damage.
Earth continuity and bonding. Every frame, rail and enclosure. Particularly important on ballasted systems where the mounting is not inherently bonded to the structure.
I-V curve tracing. Not universally offered, and worth requiring. A full current-voltage sweep of each string reveals mismatch, high series resistance, shunt problems and shading effects that a simple Voc/Isc check cannot. It takes longer and needs proper equipment, which is why it gets skipped.
The curve shape is diagnostic in itself: a rounded knee suggests series resistance, a sloped plateau suggests shunt losses, a stepped curve suggests mismatch or a bypass diode conducting.
Thermal survey at commissioning. Discussed elsewhere, but worth repeating here – damage from installation is common, and this is the moment it is still free to fix.
A capacity or performance test. A short period under defined conditions comparing actual output against expectation. IEC 61724-2 covers capacity testing. This is the test that ties to your performance guarantee, so the method should already be defined in the contract.
Equally important and more often deficient:
That monitoring access point causes more friction than any other. Plenty of owners discover years later that the monitoring account belongs to the installer, that they have view-only rights, and that changing contractor means losing the entire history.
Retain a meaningful portion of the contract value against completion of commissioning documentation, not just against physical completion. Otherwise the documentation arrives slowly, incompletely, or not at all – not from bad faith, usually, but because the crew has moved to the next job.
A practical approach: physical completion releases most of the payment, and a final retention releases on delivery of the complete commissioning pack plus the performance test result. It focuses attention remarkably well.
You do not need to understand every measurement to benefit from being present. Attend the string testing. Watch how methodically it is done. Ask what a particular reading means.
Tests conducted with an owner watching are, in my observation, conducted differently from tests conducted alone at the end of a long week.