A pre-shipment inspection costs about $300 to $500 per man-day for a qualified inspector in Guangzhou. For a full container of solar equipment, one day covers it. That is the cheapest insurance you will ever buy.
The three checks that catch the most problems. First, the label check. The auditor pulls 10 to 20 random units and verifies the model number on the label matches the model on your purchase order, the test report, the MSDS, and the packing list. Mismatches here cause customs holds.
Second, the visual check. The auditor opens cartons and looks for cosmetic damage, wrong accessories, missing manuals, and signs of refurbished units sold as new. Solar panels with scratched backsheets or batteries with dented casings are red flags.
Third, the functional check. The auditor powers up a sample of units and runs a basic test. For inverters, that means checking the display boots and the output voltage is within spec. For batteries, that means a capacity test with a constant current load. A full capacity test takes hours; a sanity check that the unit holds voltage takes minutes and catches most failures.
A good PSI report includes photos of the actual units inspected, the test readings, and a clear pass/fail per check. A bad PSI report is a single page that says "passed" and nothing else. Do not accept the bad ones.