## 3 Results  
### Rear Plane-of-Array Irradiance (\(G_{POA,Rarr}\))  
The fundamental challenge in bifacial--as compared to monofacial--PV performance modeling is estimating \(G_{POA,Rarr}\). Therefore, the discrepancies in simulated bifacial energy production are likely to occur in the derivation of \(G_{POA,Rarr}\) values. Figure 5 shows one year of simulated \(G_{POA,Rarr}\) values as a function of the average simulated value. The dispersion of simulated values is nearly the same for the fixed-tilt and HSAT system. The range of simulated values among software correlates with the front-side irradiance (R\({}^{2}\) = 0.81-0.85). The range of simulated \(G_{POA,Rear}\) values is approximately 20 W\(\cdot\)m\({}^{-2}\) at 1000 W\(\cdot\)m\({}^{-2}\) frontside irradiance. In other words, the range of \(G_{POA,Rear}\) is about 2% of \(G_{POA,Front}\). SolarFarmer is highest in this comparison because its integrated approach does not currently consider the obstruction of sky diffuse irradiance caused by neighboring PV rows. Therefore, the ground-reflected irradiance between PV rows is over estimated. To our knowledge, this detail is currently being revised and is expected to be implemented in SolarFarmer versions greater than 1.0.191.2. A comparison of the modeled \(G_{POA,Rear}\) during five weeks (21 February-30 March 2020) where measured \(G_{POA,Rear}\) data are available on the fixed-tilt and HSAT system is shown in Figure 6. The simulated results include reflection losses at the PV glass-air interface according to the IAM model implemented in each software. The solid black 45\({}^{\circ}\) lines in Figure 5 represent unity to the measurements. The measurements are the average of two MS-40 pyranometers (EKO Instruments, Tokyo, Japan) mounted on the backside of the structure: east and west in the case of the HSAT, top and bottom for the fixed tilt (Figure 2d). The trendlines from seven of the eight software agree well with the pyranometer measurements. The mean absolute error (MAE) of these seven software is 2.3-5.2 W\(\cdot\)m\({}^{-2}\) for the fixed-tilt system and slightly higher at 3.5-6.7 W\(\cdot\)m\({}^{-2}\) for the HSAT system. We analyzed the model residuals as a function of the sun position \(G_{poa,front}\) and diffuse fraction and found no systematic trends, other than a tendency towards higher percentage errors in \(G_{POA,Rear}\) when \(G_{POA,Front}\) is low. The peak total irradiance (i.e., sum of \(G_{POA,Front}\) and \(G_{POA,Rear}\)) of both system types was approximately 1000 W\(\cdot\)m\({}^{-2}\) during the five-week period shown in Figure 6. When the magnitude of total irradiance is considered, the MAE of \(G_{POA,Rear}\) contributes roughly 0.5% uncertainty to the bifacial PV modeling chain. The 3-D MoBiDiG RT simulation over predicts \(G_{POA,Rear}\) in the HSAT scenario, which could be due to its use of the [PERSON] all-weather luminance model [59], which is unique among all software tested. The overall poorer model agreement with measurements in the HSAT scenario makes sense considering that tracking introduces additional complexity--and thus additional degrees of freedom for error--at two levels. First, the tracker algorithm implemented by the software is introduced into the comparison and second, the VFs in HSAT simulations are calculated for each change in tilt angle whereas the VFs in fixed-tilt simulations are calculated once for the entire simulation. Figure 5: Simulated rear plane of array irradiance as a function of the average of the eight simulation tools. The solid black line is unity to the average. An inclinometer sensor mounted on the back of the tracker continuously recorded the tracker roll angle during the test period. We found that the modeled tracker angle was within \(\pm 1^{\circ}\) of the measured angles 50% of the time, but deviations were as high as \(5^{\circ}\) during periods of backtracking. We used pvfactors to test the effect that this difference in the roll angle had on simulated \(G_{POA,Rear}\) values.