Persistent expected-vs-measured gap

Over time, the plant produces a different amount than its expected or modeled energy. The difference shows up as a low ratio of measured to expected energy, or a low performance index, in monitoring reports. Before blaming the plant, first rule out weather, availability, clipping, curtailment, soiling, degradation and instrument problems. If the gap remains while the plant runs normally, the expected value itself may be wrong. A flawed PAN file is one cause. Find the exact PAN file used, then compare it with data sheets, independent test data and third-party files. If the gap appears mostly in hot periods, check whether the inverter DC voltage is stuck at its minimum. If the operating voltage stays above the Vmp measured from an IV curve, this points to inverter voltage floor losses. If the leftover gap follows air mass, season, atmospheric water or aerosols, spectral effect losses are likely. In this case the plant can look like it underperforms or overperforms. Use spectral correction only when it makes the residuals more regular and matches measured or modeled atmospheric conditions. Do not use it to explain away any unexplained gap.

<!-- review notes

• sentence 1: support check: partial - A persistent gap between expected and measured performance and the EER/performance index ratios are described, but nothing says the gap appears as a low ratio in monitoring reports.
• sentence 5: support check: partial - The correlation with air mass, season, water and aerosols and the under- or overperformance appearance are stated, but 'likely' is stronger than 'often correlated'. -->