What to do if solar modules underperform on the flash test

What the flash test is telling you

A flash test measures module output under a controlled light pulse at standard test conditions and produces the power figure everything else is compared against. When your measured number comes in under the label, one of three things is true: the modules are genuinely below specification, the test was not run under conditions that make the numbers comparable, or the two parties are measuring different things and both believe they are right.

That third case is far more common than buyers expect. Differences in equipment calibration, cell temperature during measurement, the reference cell used, and even how long modules have been out of the dark can move a reading. A supplier who tested on a calibrated line at the factory and a buyer who tested on site with a portable unit can honestly disagree by a margin that looks like fraud from either side.

So the first job is not to write an angry email. It is to establish a number that both sides will accept, before positions harden.

What to do in the first 48 hours

The order matters here, because the actions that protect a claim all happen before the modules move.

• Stop installation immediately. Modules on a roof are no longer evidence, and almost every supplier position starts with the argument that installed goods were accepted.
• Photograph the shipment as it arrived. Container seal, packing, pallet labels, serial numbers and any transit damage. Date stamped, before anything is unpacked further.
• Record the serial numbers of the modules you tested. A claim about anonymous modules is unwinnable. A claim about listed serials from a recorded pallet is a different conversation.
• Notify the supplier in writing the same week. Most contracts carry a short notification window for quality claims. Miss it and the merits stop mattering.
• Hold the unpaid balance. If a balance is due, do not release it while the question is open. It is the only lever most buyers actually have.

None of this requires you to have decided who is at fault. It just keeps every option open while you find out.

How to retest so the result stands up

A retest is only useful if the supplier cannot dismiss it. Four things make it credible.

Ask the supplier to nominate or accept the laboratory before testing starts. A supplier who agrees in advance has effectively accepted the result. A supplier who refuses has told you something useful about how the claim will go.

Why modules come in under the label

Once you have a reliable number, the cause usually falls into one of a few buckets, and the cause decides the remedy.

Binning and mixed pallets. Modules are sorted by power. A shipment weighted to the bottom of the range, or mixing bins without disclosure, produces an average below what you expected while every individual module sits inside a defensible tolerance.

Transit damage. Microcracks from rough handling reduce output without visible damage. Electroluminescence imaging separates this from a manufacturing shortfall, and it also points the claim at the carrier or the cargo insurer rather than the supplier.

Specification substitution. A different cell technology or a lower grade module than ordered. This is the serious case, and it is also the one that verifying the manufacturer before ordering is designed to prevent.

Normal degradation and measurement drift. Modules lose a little output over time, and the mechanisms are covered in what causes solar module degradation. On new modules this should not explain a meaningful gap, which is why a large shortfall on fresh stock points elsewhere.

What you can actually claim

Your remedy comes from the contract, not from the disappointment. Read these three clauses before you decide what to ask for.

The power tolerance tells you whether a shortfall is a breach at all. Many supply contracts state a positive only tolerance, and modules inside it are compliant even when they are under the number you had in mind. The power warranty covers output over years rather than at delivery, so it is the wrong instrument for a delivery dispute but the right one if the problem shows up later. The inspection and rejection clause sets your window, your rights and the remedy, and it is the clause most buyers discover they never negotiated.

Realistic outcomes, in descending order of how often they happen: a price adjustment or credit note against the shortfall, replacement modules on the next shipment, partial rejection of affected pallets, and full rejection with return. Return is rare because the freight economics rarely work. What gets you to the better end of that list is documented evidence, a test the supplier agreed to, and money you have not yet paid. For the full playbook when the supplier disputes everything, see how to get recourse when a supplier ships bad panels.

How to stop it happening on the next order

Every one of these disputes is cheaper to prevent than to win. Four changes do most of the work.

Agree the test standard, the sampling method and the laboratory in the purchase contract, so there is nothing to argue about later. Buy pre shipment inspection on any order large enough to hurt, because catching a problem in the factory costs a fraction of catching it at your gate. Structure payment so a meaningful balance falls due after inspection rather than before shipment. And qualify the supplier properly in the first place, using the checks in what documents to ask a solar supplier for.

The reason buyers skip all four is cash. Paying a full advance is what a stretched balance sheet does when a deadline is pushing. Financing the order rather than funding it from working capital is what makes a sensible payment structure affordable in the first place.