400 W Nameplate, 318 W Measured: Are the Modules Failing?

Question

400 W Nameplate, 318 W Measured: Are the Modules Failing?

Core Knowledge Technical Domain 2 · Task 5.A

Asked to confirm whether a string has degraded, a technician measures one module on a clear July afternoon and reads 318 W. The nameplate says 400 W. An irradiance meter in the plane of the array reads 950 W/m², a back-of-module sensor puts the cell temperature at about 62 °C, and the datasheet gives a maximum power temperature coefficient of −0.35 %/°C. What do these numbers show?

Answer 1

Reveal answer and explanation

Correct answer: D) Output is within a few percent of what these conditions predict, so there is no evidence of degradation here

The nameplate is not a promise about a roof. It is the module's output at Standard Test Conditions, which the JTA names as a specification in its own right: 1000 W/m², a cell temperature of 25 °C, and a defined spectrum. A clear July afternoon supplies neither of the two numbers that matter here, and the measurement has to be brought to the same footing before it can be compared with anything.

Both corrections are on the datasheet. Irradiance first: 950 of 1000 W/m² scales the expected power to 0.95 of nameplate. Then temperature: the cell is 37 °C above the 25 °C reference, and at −0.35 %/°C that is a further 13% off. Multiply them out and 400 W becomes about 331 W of expected output under the conditions the technician actually measured in. Against 331 W, a reading of 318 W is about 4% low, which is the territory of light soiling, module tolerance and the accuracy of the instruments involved. It is not evidence of anything failing.

Temperature is the part people underestimate, because it is invisible. A module on a rooftop in summer runs far above air temperature, and the loss it causes is entirely normal, entirely expected and entirely absent from the nameplate. Comparing a hot afternoon measurement against a laboratory figure manufactures a 20% deficit out of physics that was always going to happen.

The O&M point: this is the measurement that most often produces a wrong answer with real consequences. A warranty claim built on an uncorrected reading gets rejected, and it costs the relationship with the manufacturer some credibility for the next claim, which may be a real one. Telling an owner their modules have lost a fifth of their output is not a small thing to be wrong about either. The correction is two multiplications and the inputs are on a datasheet you already have, which makes it one of the cheapest pieces of rigour available on a roof.

Why the other options are wrong

A) The modules have lost about 20% of their output and the string should be put to the manufacturer as a warranty claim: This compares a rooftop measurement against a laboratory specification and reads the gap as damage. 318 against 400 does come to about 20%, and essentially all of that gap is the irradiance and temperature difference between a July roof and a test lab. Performance warranties are written against STC-corrected output precisely because of this, so a claim submitted on the raw figure has not demonstrated anything the manufacturer is obliged to act on.
B) Output is about 4% below prediction, which points to a bypass diode conducting somewhere in the module: The arithmetic is right and the conclusion is an order of magnitude out. A residual of about 4% is what remains after the corrections, and that is a normal residual. A conducting bypass diode removes roughly a third of a module from the circuit, so it would show as something near 33%, not 4%. Reading a small residual as a discrete fault is how a clean measurement turns into an unnecessary module swap.
C) Nothing can be concluded in the field; confirming degradation needs a laboratory flash test at Standard Test Conditions: This is more cautious than the situation requires, and over-caution has a cost too. Correcting a field measurement to Standard Test Conditions is routine: the irradiance and cell temperature were both measured on site, and the temperature coefficient is printed on the datasheet. A flash test is what you escalate to when a corrected field measurement still shows a deficit worth arguing about, not the first step. Declining to interpret data you have is not rigour.

References

NABCEP OMAT JTA v.2026.5 — Domain 2, Task 5.A: "PV module specifications (e.g., Standard Test Conditions [STC], open circuit voltage, short circuit current)"
No external standard is cited for the correct answer. The correction uses only the module’s own datasheet values and the definition of Standard Test Conditions, both of which the JTA names directly. The temperature coefficient, the nameplate power and the reference conditions are properties of the module in front of you, not figures from a code.