How can I calculate the Btus needed for a single room A/C unit?
I found this link, which is for a whole house:  http://www.northernac.com/newcalc.htm

This one is for a single room.  Assume the room direction is for the northern hemisphere.  Reverse North and South if you're in the south.  http://www.bhg.com/home-improvement/advice/measuring-materials/window-air-conditioner-size-calculator/>

From the comment by mike:

This calculator is a bit better in that regard, but does not take into account climate: thisoldhouse.com/toh/calculator –  mike