Good question, @Paul_B! As the Spotlight Cam Battery version is designed to be powered by battery, it is not intended to provide a constant flow of this power to the lithium-ion quick release batteries. While the Solar Panel may be rated similarly to the power from a micro usb, the Solar Panel is not constantly receiving sunlight (night hours), nor is it constantly powering the Spotlight Camera. Your Spotlight Cam will use the batteries to power an event, in which the Solar Panel will provide the charge afterwards, to replenish the battery used.
Check out our help center article which provides insight on obtaining the maximum power from your Solar Panels.