## 1 Introduction
### Detailed System Performance Observations, Evaluation, and Analysis
Solar azimuth angles, on the other hand, varied across a wide range between about 40\({}^{\circ}\) (NNE) and \(-\)60\({}^{\circ}\) (WNW) between 9:00 and 17:00. The electric output power correlated well with both the Sun altitude angle and horizontal-plane irradiance, despite most of the power generation obtained consistently (throughout the days) from the north-oriented vertical wall of windows. It is important to note (Figure 6d) that the output power readings exceeding 150 W (out of the typical maximum-recorded output powers near 200 W) have been obtained consistently on different days, in a range of solar exposure conditions, between 10:00 and 15:00, and at a large range of horizontal-plane solar irradiance variations (between about 250-650 W/m\({}^{2}\)). This confirms the capability of this solar window atrium to collect energy efficiently in a wide range of solar incidence  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Time**} & **Energy stored (Wh)** & **Power** & **Solar exposure** & **Horiz. Irradiance** & **Sun Altitude** & **Sun Azimuth** \\  & **(W)** & **(M)/m\({}^{2}\))** & **(W/m\({}^{2}\))** & **(dogs)** & **(dogs)** \\ \hline
8:43 & 54 & 76 & 0.8 & 255 & 15.8 & 50.09 \\
8:54 & 77 & 92 & 0.8 & 286 & 1  
geometry conditions, at least during clear and sunny weather conditions. The dataset of Figure 6d has only been collected over several weeks (and therefore, not all possible independent time/solar irradiation variable combinations could be evaluated); it also contains some weather-related \"noise\" affecting the low-power readings. This dataset is, however, quite representative of the system power generation capabilities seen during most winter conditions on sunny, cloudless days. We included this additional two-dimensional surface-plot representation as \"a guide for the eye\" only, to demonstrate and visualize the time- and weather-related correlations of the output power. The irradiation data used for plotting Figure 6d have been grouped, so that all data points for the horizontal irradiation values measured inside small intervals between eg 150-200 W/m\({}^{2}\) were all assigned a 200 W/m\({}^{2}\) value for plotting. For all \"missing\" output power data points (due to the limited duration of study not covering all possible weather conditions) corresponding to some locations across the time-irradiance data grid used, these points were approximated to correspond to the minimum actually measured power values observed at boundaries of the corresponding data intervals. The amounts of electric energy generated by each individual sub-section of the atrium installation have also been monitored. During the daylight hours of May 28, the daily total energy generation by the two shaded east-wall windows was only 54 Wh, while the four east-roof windows produced 268 Wh, and the 8 windows on northern wall contributed 743 Wh; the 4 west-roof windows generated 191 Wh. On a different day (June 03, 2019, which was slightly sunnier than May 28), the 8 front-wall windows produced a total of 794 Wh of energy during the daylight hours. Therefore, the vertically-mounted north-oriented wall windows, which were often shaded temporarily by the people traffic near the shopping center entrance, contributed daily, on average, almost 100 Wh per window unit, in late-autumn conditions. Per unit window module, the daily energy generation from shaded windows (on eastern wall) was a factor of 3.4 smaller compared to the north-facing vertically-mounted units. Increased energy collection efficiencies are expected to be observed during the spring and summer months, due to both the stronger solar irradiation intensity levels, and (most importantly) the substantially longer daily sunshine durations. The Supplementary data file (Table S1) contains daily-total energy generation data for all window groups, as measured by the system after sunset each day, for the period between May-early September 2019.