The NECA-IBEW Electrical Training Center, maintained by IBEW Local 48 and the Oregon-Columbia NECA Chapter, is located on Airport Way in Portland Oregon, just a short distance south of the Columbia River. On the south and west sides of the building are photovoltaic arrays. The arrays have the same number and type of modules. The arrays were installed at the same time, wired by the same people with the same methods, and tests indicate they have the same efficiency. Both are in good working order. As expected, the west array generates about 90% of the south array's output on a sunny summer early afternoon. Here's the puzzle: Annual average output of the west array significantly exceeds that of the south array. I know why. Do you?