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Read up on the uxn project, which is a platform designed to be run 
minimally.  Can be run on gameboys.  offline only(/mostly?).  The
people behind the project work off the grid in a sailboat,
requiring them to meter their electricity and data usage carefully.  
Video edit on sunny days, etc.  Power usage by computers is remarkable.
MacBooks alledgedly use 6-12 W, but this little RPi uses 2.8.