Q Where is the solar panel on this light?
A The solar panel is located within the housing of the light at the bottom where it receives most light.
Q Could these be easily attached to the posts of decking? Also do they work in the winter?
A You could strap these to decking posts if you wanted to without too much trouble. In a good south facing location you'll still get something out of these in Winter - certainly more then other comparably priced lights.
Q Are they metal or plastic?
A The lower light body is made from plastic, the top of the light, pole and hook are made from metal.