East and West facing roofs can lose 10 - 20% of their effectiveness and North facing roofs a little more. Even northerly orientations are still worthwhile as the panels still work under daylight conditions.
Daylight solar panels operate on overall lighting levels, so anywhere between 25 and 70 degrees from the horizontal will work well. Beyond these limits you can lose up to 20% of the solar panels effectiveness.
These seasonal and climatic changes are no longer as important because the design of modern daylight solar panels has improved so much. However, these factors can affect overall power output by as much as 30%.