The Solar Star attic fan runs best when the solar panel is in direct sunlight. The unit will run at a reduced speed in the shade (provided the sky is bright) but if one has heavy shading for a good portion of the day an "Add-on Solar Panel" for the Solar Star is a great idea.  An extra panel can be placed up to 15' (4.5m) away (possibly on the opposite side of the roof with all-day sun exposure) from the main roof unit to provide better all-day performance.