A 415 W Module Replaced a 400 W One and Output Fell

Correct answer: B) The same current flows through every module in a series string, so the module with the lowest maximum-power current sets the ceiling for all twenty

A series string has one current running through all of it. That is the whole meaning of series, and it is the fact the datasheet comparison turns into money. The replacement reaches its 415 W by running at a higher voltage and a lower current than the module it replaced, and the higher voltage adds up happily along the string while the lower current does not. It becomes the ceiling.

Roughly what that costs: the tracker can no longer hold the string near 10.0 A without pushing the new module well past its own maximum power point, so it settles near 9.5 A, and the nineteen original modules each give about 40 V × 9.5 A = 380 W instead of 400 W. Add the replacement's 415 W and the string lands around 7,635 W against 8,000 W before. The repair cost roughly 365 W, which is most of one module's output. Buying a better module removed about a module.

Treat that figure as an illustration rather than a prediction. Real behaviour depends on the shape of the two IV curves and on where the tracker actually settles, so the loss could land either side of it. The direction is not in doubt, and the direction is the point: a series string is limited by its lowest-current member, and a nameplate in watts hides which way a module gets there.

The O&M point: this only happens in service. The crew who built the array bought twenty identical modules on one order. You are replacing one unit, years later, from a model that has been discontinued, and the person quoting you the replacement is matching on watts and physical size because those are the fields their system has. The match that matters is current, and the number is on both datasheets. When no close match exists, the options are a used module of the original type, re-arranging strings so the odd module sits with its own kind, or accepting the loss deliberately and writing down that you did.

Why the other options are wrong

A) The replacement module is faulty and should go back to the supplier

The module is doing exactly what its datasheet says: 43.7 V and 9.5 A, which multiply to its rated 415 W. Nothing about it is faulty. It is a perfectly good module in the wrong string, and returning it will get you another one with the same specification. This is worth separating carefully, because a fault report against a healthy part costs credibility with the supplier and does not fix the string.

C) The string voltage is now mismatched to the inverter, which is clipping the extra

Voltage is the half of this that did not cause the problem. Nineteen modules at 40 V plus one at 43.7 V is a slightly higher string voltage than before, a few volts across the whole string, and inverters have far more tolerance than that on their input range. Clipping is also the wrong shape of explanation: it limits output at a ceiling in high light, whereas this string is down across the board.

D) Nothing is wrong: a single month is too short a comparison and the difference sits inside normal variation

A month is a reasonable comparison when the reference is the strings beside it rather than last year's weather, and those strings saw the same sky. The difference also has a mechanism sitting in plain sight on the two datasheets, which is the distinction worth holding: attributing a gap to normal variation is a conclusion, not a default, and it is only available once you have looked for a cause and not found one.

References

• NABCEP OMAT JTA v.2026.5 — Domain 3, Task 1.C: "String configuration"
• NABCEP OMAT JTA v.2026.5 — Domain 3, Task 1.G: "Equipment data sheets and installation instructions". Both numbers the answer depends on are printed on the two datasheets; no standard is cited, because none is needed for how current behaves in a series circuit.