Assessment: IV Curve Analysis Identifies Systematic String Mismatch — 9.2% Yield Loss in 6.8 MWp Solar Park

Assessment type: Yield assessment / Defect evaluation System size: 6.8 MWp Bifacial (Tracker) Region: Brandenburg, Germany Period: Aug – Oct 2024

Assessment summary:

6.8 MWp ground-mount system (tracker system, bifacial modules) with 9.2% yield shortfall vs. P50
IV curve measurement on 412 strings: 67 strings (16.3%) with mismatch losses >5%
Root cause: Modules of different power classes installed in the same string (flasher data: Isc deviation up to 0.38 A)
Secondary cause: 23 strings with elevated series resistance (corroded connectors)
Assessed damage: EUR 218,000 (cumulative yield loss + rewiring)

Why was an IV curve assessment commissioned?

An investor acquired a 6.8 MWp solar park in Brandenburg, Germany in 2023 (commissioned in 2020, bifacial modules on single-axis trackers, 412 strings of 28 modules each). The performance analysis after acquisition showed a specific yield of 892 kWh/kWp — 9.2% below the P50 forecast of 982 kWh/kWp. Since the yield shortfall had been consistent since commissioning, a systematic installation error was suspected.

Measurement scope

Between August and October 2024, all 412 strings of the system were measured using an IV curve tracer (Solmetric PVA-1500) at irradiance levels between 750 and 950 W/m². Each measurement was normalized to STC (1,000 W/m², 25 °C). Additionally, the manufacturer's flasher data (power class sorting) was compared against the modules actually installed.

What did the systematic IV analysis reveal?

Finding category	Strings	Share	Average loss
Normal (FF >72%, Pmax >95% STC)	322	78.2%	<2%
Mismatch losses 5–10% (reduced FF, stepped curve)	44	10.7%	7.1%
Mismatch losses >10% (severely stepped curve)	23	5.6%	13.8%
Elevated series resistance Rs (>0.8 Ohm, flattened curve)	23	5.6%	6.4%

Cause of mismatch: Incorrect sorting during installation

Cross-referencing the module serial numbers with the manufacturer's flasher data revealed that modules of different power classes had been mixed in 67 strings. The Isc deviation within a single string was up to 0.38 A (at a nominal Isc of 10.2 A) — corresponding to a mismatch of 3.7% from current differences alone. Combined with the series connection effect (the weakest module limits the entire string), the losses add up to the measured 7–14%.

Technical background: Bifacial modules have a higher factory Isc spread (±3%) than monofacial modules (±1.5%), as rear-side output introduces additional variance. With 28 modules in series, an Isc difference of just 0.2 A is sufficient to produce measurable mismatch losses. Correct sorting by Isc class is therefore particularly critical for large-scale bifacial installations.

What was the economic damage?

Damage calculation (assessed):

Item	Amount
Cumulative yield loss 2020–2024 (4 years x 9.2% x feed-in revenue)	EUR 156,000
Rewiring of 67 strings (sorting by Isc class)	EUR 38,000
Connector replacement for 23 strings (corroded)	EUR 9,200
Assessment costs (IV measurement of 412 strings + evaluation)	EUR 14,800
Total damage	EUR 218,000

Expert assessment: The installation error (missing Isc sorting) would have been identifiable at the time of acceptance — an IV curve measurement during commissioning would have immediately revealed the issue. The statute of limitations for warranty claims (5 years under German construction contract law) had not yet expired at the time of the assessment, and a claim against the EPC contractor was recommended.

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Last updated: 16 June 2026 | Author: Christoph S. Prestele, TUV-certified expert assessor | PV-BESS-Assessor.com