Three root causes, three owners
Underperformance feels like one problem to the owner, but it resolves into three very different ones. Workmanship is about how the system was built. Degradation is the panels ageing, which is normal. A fault is something that has actually failed. The reason the distinction matters isn’t academic: each points to a different owner and a different warranty — the installer’s workmanship cover, the manufacturer’s hardware warranty, or simply nobody, because nothing is wrong.
| Cause | Signature | When it appears | Who owns it | Typical fix |
|---|---|---|---|---|
| Workmanship | Underperforms from day one; below design | At / soon after commissioning | Installer (workmanship warranty) | Rework the install |
| Degradation | Slow, uniform decline (~0.5–1%/yr) | Over years | No one — expected | None (unless abnormal) |
| Fault | Step-change; one string/inverter below peers | Any time | Repair; maybe mfr. warranty | Repair or replace |
Workmanship: it was never going to hit target
Some systems never had a chance. Panels set at the wrong tilt or orientation, undersized or poorly terminated cabling, mismatched strings, shading that wasn’t designed around — these are build problems, and their signature is telling: the system underperforms from day one against its design, rather than declining from a good start. That is the installer’s responsibility, and it belongs under the workmanship warranty — but only if you can show it, which means having the string-level evidence from early on.
Degradation: slow and expected
Every panel ages. Modern modules lose roughly half a percent to one percent of output a year — a gradual, uniform decline across the whole array. This is not a fault and not anyone’s failure; it is expected and priced into a good financial model. The only version worth a claim is *abnormal* degradation — faster than the manufacturer’s warranted curve — and telling normal from abnormal, again, needs a measured baseline, not a guess.
Faults: something has failed
A fault is a break in the story: a dead or weak string, a failed optimiser, an inverter derating or dropping offline, a blown fuse, a bad sensor. Its signature is a step-change or a single string sitting persistently below its peers — not a slow drift. Faults are where the largest recoverable losses hide, and they are exactly what a plant-level total smooths over. Some are repairs you pay for; some fall under the manufacturer’s hardware warranty — which is another reason the diagnosis, not just the symptom, is what matters.
Why the diagnosis decides who pays
Get the cause wrong and you lose money twice: you might absorb a cost that was the installer’s or the manufacturer’s, or you might roll a truck to a system that is simply hot or lightly soiled and behaving normally. Get it right and every problem goes to the party that owns it, on the warranty that covers it. A string-level, weather-normalised inspection is what makes that call defensible rather than a matter of opinion.
How SPC diagnoses it
Solar Performance Cloud compares each string to a weather-normalised expectation and reads the signature of the shortfall — underperforming from day one (workmanship), a slow uniform decline (degradation), or a step-change and peers-diverging drop (a fault) — across every major inverter brand. AI grades every string every day; solar engineers verify what matters. The output isn’t “it’s low”; it’s “this string, this cause, this owner” — so the right party acts on the right warranty.
