## 3 Results
### Bifacial Gain
All plots show bifacial gain as simulated by five software and from DC string measurements. The black error bars show the inner quartile range of daily bifacial gain measurements within each month. Datapoints when sun elevation is lower than 5\({}^{\circ}\) above the horizon are not included in these plots. (**a**) Results grouped by software (Table 1). (**b**) Results grouped by which 1-diode model and parameter set was used. PVsyst Parameter Set 1 is based on laboratory I-V measurements at multiple irradiances while PVsyst Parameter Set 2 is based on only STC measurements. tube (Figure 19). This slight differentiation in structural geometry may not have been simulated sufficiently in the software. Another notable discrepancy between the model and measurement is seen in April when a small spike in bifacial gain is observed on both the fixed-tilt and HSAT system. The trend in modeled bifacial gains from March to May indicate that the measured bifacial gain in April is overstated by as much as 1.5%. We believe the higher measured bifacial gain in April was caused largely by extraordinarily high pollen counts in late April 2019, which caused non-uniform soiling on the PV arrays, and ultimately more power loss in the monofacial than bifacial systems. We observed that the daily bifacial gains were between 10% and 20% from 23-26 April 2019, which corresponds to the dates of the soiling event (Figure 19). A significant rainfall event occurred on 27 April 2019 at which point more modest and typical bifacial gains resumed. This artifact highlights the challenge of curating high-quality data acquisition in large-scale PV test sites. Please note that the bifacial gain results from SolarFarmer are not presented in Figure 11 because the overestimation of \(G_{POA,Rear}\) shown in Figure 19 causes a 2-4% upward bias in bifacial gain as compared to the other tools, which use the PVsyst 1-diode model (i.e., PlantPredict and PVsyst). This result conflicts with our previous work [27], which showed SolarFarmer results mostly within 1% of the measured bifacial gain, while PlantPredict and PVsyst results were 3-4% low to the measured bifacial gain. The reason for this discrepancy is that the present work uses a 1-diode model parameter set based on laboratory measurements made at irradiances from 200-1000 W\(\cdot\)m\({}^{-2}\) (noted as 'Parameter Set 1' in Figure 11b) while our previous work used a parameter set based on measurements made only at STC (noted as 'Parameter Set 2' in Figure 11b). The improved agreement of 'Parameter Set 1' shown here is attributed to the fact that this parameter set more accurately predicts performance at low-light conditions (\(<\)400 W\(\cdot\)m\({}^{-2}\)) than does 'Parameter Set 2'. This finding is in agreement with the findings and recommendations of other authors [66; 67]. Figure 12 shows the simulated bifacial gain according to the \(G_{POA,Rear}\) to \(G_{POA,Front}\) ratio (Equation (2)). The agreement among the different software is similar regardless of whether the bifacial gain is calculated using the optical gain (Equation (2)) or the electrical performance (Equation (1)), with the exception of HSAT results in winter, where better agreement among software is seen using Equation (2). The larger discrepancies between HSAT simulations in winter using Equation (1) could be due to the different backtracking and shade-loss algorithms used by the software. Equation (2) results in better agreement with the fixed-tilt system measurements in winter, which indicates that inaccuracies in the shade-loss models are likely the result of the significant winter deviations shown in Figure 11.  
Figure 12b shows the results grouped by whether \(G_{POA,Rear}\) is calculated using a 2-D VF or 3-D RT approach. When visualized in this manner, it becomes clear that the 3-D RT approach follows the measured bifacial gain most closely for the HSAT simulation, within 0.5% of measurement for most months outside of winter.