I live in a relatively old apartment complex near Manhattan Beach (Los Angeles, CA, USA). No A/C. I'm looking at buying a portable one, the kind where you exhaust the hot air out a tube out the window. I'm researching what capacity I would need to be comfortable.
The number that's thrown around is 20 BTU/sq ft (because in 'Merca, we measure A/C capacity in BTU/hr...). I don't understand where this number comes from. My calculations tell me it takes 3.253e-2 BTU per K per ft^3 to cool (or heat) air. I'll pretend that the A/C is going to evenly cool the entire air column equally. Which means, with my 8 ft ceilings, 20 BTU/sq ft would result in a temperature change of more than 75 K/degrees C. Obviously that's not happening. What am I missing?
Given my 8 ft ceilings and my ~500 sq ft living room and the fact that I live in a very pleasant climate and only need to cool my apartment by a few degrees, the recommended 20*500=10,000 BTU/hr air conditioner seems like major overkill.