## 2 Materials and Methods
### Input Data for Simulations
#### 2.3.2 Solar Radiation Measurements and Uncertainty  
Some modeling tools shown in Table 1 are capable of sub-hourly simulations, whereas others are limited to an hourly resolution. Hence, all simulations shown here were performed using hourly averages of the global horizontal irradiance (GHI), diffuse horizontal irradiance (DHI), direct normal irradiance (DNI), ambient temperature (T\({}_{\text{AMB}}\)), and wind speed measured onsite. Measurements of broadband GHI, DHI, DNI, and albedo were collected onsite with spectrally flat class A sensors, with ventilation units underneath the GHI and DHI sensors (Figure 2b,c). The ventilation unit consists of a fan below the sensor base, which mitigates the accumulation of dust, snow, and ice on the glass dome. The DHI, DNI, and GHI are sampled every 10 s and are filtered according to recommendations published by the Baseline Surface Radiation Network (BSRN) before calculating the hourly averages that are input to the PV simulation software. The BSRN data quality checks ensure that the measurements are within both physical and reasonable limits. In some cases, our implementation is more stringent than the BSRN recommendations. For example, in our implementation, the sum of DHI and direct horizontal irradiance must always be within \(\pm 5\%\) of the GHI whereas the BSRN  
Figure 2: **(a)** Ground-level view of the bifacial HSAT system; **(b)** Onsite solar radiation monitoring tower where the broadband global horizontal (GHI), diffuse horizontal (DHI), and direct normal irradiance (DNI) measurements are recorded. **(c)** Onsite albedometer. **(d)** Two rear plane of array pyranometers (top and bottom circled in blue) on the fixed-tilt bifacial system; Sensors are placed 17 m from the nearest array edge. tolerance is \(\pm\)8% for most solar zenith angles. For more information on the quality checks, we refer the reader to [43]. The literature shows that the solar resource assessment (e.g., GHI) and the transposition of horizontal data to plane-of-array irradiance (\(G_{POA,Front}\)) are the highest sources of uncertainty when modeling PV energy yield [39; 44; 45; 46]. The present study therefore considers how the expanded uncertainty (U\({}_{\rm C}\)) of the onsite GHI, DHI, and DNI irradiance measurements impact the simulated energy yield and thus the comparisons to measured electrical data. We calculated the expanded uncertainty U\({}_{\rm C}\) of the DHI, GHI, and DNI measurements following principles in the Guide to the Uncertainty in Measurement and best-practice guidelines published within the field of solar radiation measurements [47; 48]. Measurement uncertainty of pyranometers and pyrheliometers is not constant but changes according to the prevailing environmental conditions (e.g., diffuse ratio, sun position, etc.) [49]. When the solar zenith angle is greater than 70\({}^{\circ}\), the largest source of uncertainty in the _GHI_ measurements is the cosine error. Since this cosine error is considered systematic, it could be significantly reduced by characterizing the angular-dependent response of the instrument and then applying a device-specific correction to the measurements [47]. However, such corrections were not performed here largely due to the complexity of executing such an approach with high accuracy. Figure 3 demonstrates how the U\({}_{\rm C}\) of the GHI is most heavily affected by the instrument's cosine response when the sky diffuse fraction is low (i.e., cloudless skies), and when the sun is close to the horizon (i.e., early morning, late afternoon). We estimate that the expanded (\(k=2\)) \(U_{\rm C}\) of the hourly averaged GHI is about \(\pm\)4% at solar noon on a clear summer solstice, and about \(\pm\)7.3% on a clear winter solstice. The uncertainty of the hourly averaged DNI, on the other hand, is most significantly affected by inter-hour irradiance variability. Figure 3: Uncertainty of the hourly GHI, DHI, and DNI measurements performed onsite from two example days in April 2019.