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Series Resistance

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Degradation Diagnosis

Series Resistance

Elevated series resistance is one of the most common and insidious forms of PV degradation. It develops gradually through connector oxidation, solder bond fatigue, cell cracking, and ribbon delamination — reducing power output while remaining invisible to simple voltage or current spot checks. IV curve analysis provides the most reliable means of quantifying series resistance and tracking its progression over time.

Equipment Options
Field Testing Guide
Reading IV Curves: A Field Technician's Guide
What a healthy IV curve looks like, how six common fault types deform it, and when alternative test methods can identify the same issue faster — or catch what the IV curve misses.
Emazys
Z300 HE
1500V PV tester with inrush current protection for HE and bifacial modules. Riso 0–99 MΩ, Voc, Isc, Rs, ground fault location. Bluetooth cloud-connected.
Emazys
Z300 PVT
1500V PV tester for conventional and PERC modules. Riso 0–99 MΩ, Voc, Isc, Rs, ground fault location. Bluetooth cloud-connected.
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Frequently Asked Questions

Series resistance creates a voltage drop proportional to current (V = I × Rs). At Pmax current, this directly reduces the terminal voltage and therefore power output. A series resistance increase of 0.5 Ω in a string running 8A of current causes a 4V terminal voltage drop — equivalent to roughly 32W of loss per string at that operating point. The loss compounds over operating hours and worsens with higher irradiance.

The most common cause is MC4 connector degradation — oxidation of the copper contact surfaces, inadequate crimp quality at installation, or physical stress from thermal cycling. Junction box terminals corrode similarly. Inside the module, solder bond fatigue and cell ribbon delamination increase cell-level series resistance. All of these develop gradually, making series resistance monitoring over time more useful than a single spot check.

Yes. Instruments like the Emazys Z300 PVT and Z300 HE measure series resistance directly using a dedicated Rs test function — without tracing the full IV curve. Thermal imaging is also effective: elevated series resistance dissipates heat at the resistance site, making hot connectors, junction boxes, and cell ribbons visible under load. Both methods are faster than IV tracing for initial screening and fault location.

Acceptable string series resistance depends on the number of modules, string length, and wiring gauge. A healthy residential string might show 0.1–0.3 Ω; a utility-scale string at 1500V with longer wire runs might show 0.5–1.0 Ω. The more useful comparison is against the commissioning baseline for that specific string — an increase of 0.2 Ω or more from baseline is worth investigating regardless of absolute value.