For air conditioning, why is it 20BTU/hr/sq ft?

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.

• The A/C doesn't just cool the air once. You need to remove all the heat energy that is entering the room. Computers, lights, people, pets, cooking all add BTUs each hour. Sunlight, heat flow through windows, walls, ceiling... It gets complicated very quickly. Sep 5, 2016 at 5:07