## 4 Discussion and Conclusions  
We assessed eight bifacial PV simulation software against \(G_{POA,Rear}\), \(G_{POA,Front}\), T\({}_{\text{MOD}}\), DC P\({}_{\text{MP}}\), bifacial gain, and energy measurements made at a 420-kW\({}_{\text{P}}\) test site in Roskilde, Denmark. Our results show that state-of-the-art bifacial performance models add approximately 0.5% uncertainty to the PV modeling chain. This finding was demonstrated in the analysis of modeled and measured \(G_{POA,Rear}\) and DC P\({}_{\text{MP}}\) values. Although 0.5% may seem small, an uncertainty of 0.5% in a 500-MW power plant translates into 2.5 MW, which translates into substantial risk in economic models. Therefore, continued efforts in reducing error in bifacial PV simulations should continue. One suggestion is the implementation of models that describe how \(\mathit{Bif}_{\mathit{less}}\) parameters change with prevailing conditions rather than the use of static \(\mathit{Bif}_{\mathit{less}}\) parameters as used in this work. Our results further show that, outside of winter months, 2-D view factor fixed-tilt simulations are within \(\pm 1\%\) of the measured monthly bifacial gain. Simulations of single-axis tracker systems are less accurate with 2-D view factor simulations within approximately 2% and 3-D ray tracing within approximately 1% of the measured bifacial gain, respectively. When comparing modeled \(G_{POA,Rear}\) to measurements, the accuracy is highly dependent on the location and type of the optical sensor. We therefore recommend future research efforts to develop standardization and/or best practice guidelines for the placement of rear POA sensors. We observed significant differences between \(G_{POA,Rear}\) measurements made with thermopile pyranometers and Si-based sensors. As such, future work should aim to improve the PV community's understanding of how the spectral distribution of albedo impacts rear-facing pyranometer reference cell versus measurements, in the framework of bifacial PV performance assessment. Figure 16: Monthly and yearly errors in energy yield (MWh) predictions from six software simulating four PV system types. An objective of this study was to assess the accuracy of reduced-order bifacial PV simulations, and we found good agreement with measurements considering the uncertainty of the solar radiation input data. Because of this finding, and the fact that the rear irradiance contribution constitutes roughly 8% of the total irradiance for the site studied here, we suggest that the PV modeling community do not forget the importance of improving the accuracy of all parts of the PV modeling chain, including, but not limited to, shade loss models, cell temperature models, and diffuse sky models. A shortcoming of this validation study is that it was performed at one site, with its specific conditions and equipment. We therefore recommend that future work includes a comprehensive review of all bifacial PV simulation validation studies performed to date. Such a study can inform the decisions of PV project developers and investors of bifacial PV assets. Conceptualization, N.R.-L., D.B., J.V., and J.L. ; PV modeling, N.R.-L. and D.B. ; Uncertainty modeling, D.A.M. and N.R.-L.; Engineering and design of test site, J.V. and N.R.-L.; Data analysis and visualization, N.R.-L.; Research assistance, A.A.P. ; Data curation, N.R.-L., D.A.M, and J.V. ; Writing--original draft, N.R.-L.; Writing--review and editing, D.B., D.A.M., A.A.P., P.B.P., J.L., J.V. ; Supervision, J.V., J.L. ; Project administration, J.V., P.B.P., J.L. ; Funding acquisition, J.V., J.L., N.R.-L., and P.B.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the Danish Energy Technology Development and Demonstration Program (EUDP) under project contract 64018-0624. This publication is based upon work from COST Action CA16235 PEARL PV supported by COST (European Cooperation in Science and Technology). We thank the members of WG3 for the substantive discussions on PV performance modeling.