Shading on solar arrays can have a serious impact on efficiency. Surrounding trees, chimneys, plumbing vents, nearby structures need to be accounted for in the system installation and sizing.
The perfect solar roof points due south. Very rarely does a perfect solar roof present itself, obviously. Generally speaking, only one side of any roof is ample for solar generation even when pointing well off of due south.
Roofs with many valleys and dormers are less suited to optimal solar generation. Some houses with very complex roof structures might not be suitable at all for a solar array.