TH> No need to - you have plenty of sun energy units there - even on
  > the coastline.
GT> How is this affected by altitude?  We live at 9000 feet in the
  > Colorado Rockies, and the sun gets IN-tense up here.{g}
The higher the better I would think.
Two factors:
(1) Number of average hours of sunlight.
(2) Quality of sun light.
Chances are there are less overcast and obscured days at your level.
On the coast beach areas tend to be foggy - not good for the solar
enthusiast. 
Your only concern is outside temperatures. Your box needs to be well
insulated on all sides, and good glass (two or better, three panes,
glazing) facing the sun. It would be a good idea to have an insulated
cover that swings over the glazing at night to keep the heat from
radiating out.
Keep me posted on your plans. 
 
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