try numpy arrays instead

Author: cwhanse

pvlib/clearsky.py

@@ -626,9 +626,9 @@ def _calc_stats(data, samples_per_window, sample_interval, H):
         difference between successive data points
     """
 
-    data_mean = data[H].mean(axis=0)
+    data_mean = data.values[H].mean(axis=0)
     data_mean = _to_centered_series(data_mean, data.index, samples_per_window)
-    data_max = data[H].max(axis=0)
+    data_max = data.values[H].max(axis=0)
     data_max = _to_centered_series(data_max, data.index, samples_per_window)
     # shift to get forward difference, .diff() is backward difference instead
     data_diff = data.diff().shift(-1)