TIm is right on. The one thing I would add is that a sizing tool makes assumptions that a PE should not. One big assumption is that the panel is cantilevered over a flat plane like a standard roof at a standard distance up off the roof. Where it "blows" up if the panels are cantilevered over or near a roof edge. The strength of the rack does not change, its the wind loading that changes near roof edges or with uneven spacing off the roof. I have seen a few installs where the installer mounted a rail near the ridgepole and then extended the panel above the ridgepole in order to stuff in an extra row of panels. The wind loading will be much higher on that section that sticks above the ridge pole. This is done less often since roof venting clearances got standardized by fire code. I have also seen the opposite where panels were extended out over the soffits.
The other odd effect from wind is that wind pushing against a vertical wall will flow up over the roof and create a zone of uplift on the edge of the roof. If you have seen roofing failures on commercial buildings, frequently the roofing will be lifted up at the edges. Standard roofing details are to increase the fasteners along the edges. Fire access rules usually means leaving a clear path along one side of the array and if the designer ha a choice its best to have that access on the upwind side of the roof.
