## 3 Results
### Frontside Plane-of-Array Irradiance (\(G_{POA,Front}\)) and Module Temperature (\(T_{MOD}\))  
PV project developers and investors are often interested in bottom-line figures, such as performance ratios, specific yields, and, in the case of bifacial PV, the bifacial gain. However, comparing simulations to measurements at such a high level is often not meaningful without first analyzing the performance of key submodeling steps. This section builds on this analysis, which started in Section 3.1 with an assessment of \(G_{POA,Rarr}\), and shows how the simulations compare to onsite front-side plane-of-array irradiance (\(G_{POA,Front}\)) and back of module temperature (T\({}_{MOD}\)) measurements. Figure 13 shows an overlay of the simulated and measured \(G_{POA,Front}\) on a cloudless day. The trends shown here are representative of the results from clear sky days within the test period. The plot is color coated by which combination of solar radiation measurements is used in the Perez transposition model. Lower \(G_{POA,Front}\) estimates (\(\approx\)40 \(W\)-m\({}^{-2}\)) are seen at midday when _DNI_ is used in the transposition  
Figure 12: Monthly bifacial gain on fixed-tilt and HSAT systems calculated with Equation (2). All plots show bifacial gain as simulated by seven 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 whether a 2-D view factor or 3-D ray trace approach was used to calculate back side irradiance. step. On an annual basis, the MBE is between 11 and 13 W\(\cdot\)m\({}^{-2}\) when _GHI_ is used in transposition, and when _DNI_ is used in the transposition step, the MBE is between \(-7\) and 5 W\(\cdot\)m\({}^{-2}\). These rather large differences in results using _DNI_ and _DHI_ versus _GHI_ and _DHI_ could be the result of measurement issues with either the _GHI_ pyranometer or _DNI_ pyheliometer. However, it is worth reiterating that weekly maintenance is performed at the weather station, and that the _GHI_, _DHI_ and _DNI_ data passed the quality checks recommended by the BSRN [46]. Additionally noteworthy in Figure 13 is how there are considerable deviations among the tools that use the same _DNI_ and _DHI_ data as input, a result that is similar to that of other authors [28]. Since the front-side irradiance constitutes 92% or more of the total irradiance for half of the timestamps in the bifacial simulations, one would suppose that software that uses _DNI_ in the transposition step would tend to underpredict DC power. However, this turns out to not always be the case. For example, MoBiDiG is one such tool that uses _DNI_, and this tool was shown in the CDF plots of Figure 9 to _overpredict_ DC power of all PV system types compared to the other five software. SAM is another such tool that uses _DNI_, but the results in Figures 8 and 9 show that SAM tends to predict marginally higher DC power values than the tools that use _GHI_ but only in HSAT simulations. The underprediction of _GPOA,Front_ by MoBiDiG and SAM could be compensated by their tendency to overpredict electrical performance at irradiance conditions \(<\)400 W\(\cdot\)m\({}^{-2}\), which is due to their use of the [PERSON] versus PVsyst 1-diode model (Figure A1). This demonstrates how analyzing the accumulated errors at various submodeling stages and their subsequent impact on energy yield is not always a straightforward process. Figure 14 shows the difference between module temperature and ambient temperature versus _GPOA,Front_. This difference between module and ambient temperature is known to be essentially linear with respect to the in-plane irradiance with a slope proportional to the U\({}_{0}\) coefficient used. The variations around the regression lines are due to the assumptions for convective heat transfer in the U\({}_{1}\) coefficient.