12,000 btu cools 3529 watts if nature favors you, if not then that 12,000 btu might just cool down around half of that (rule of thumb of the other guy). the (+/-) ~40-50% margin of that is huge. climate has a huge role.
Yes ambient temperature plays a big role here, for example I have ac in my room and that is a 12000 btu, when is at 40 degress celsius, that 12000 is close to nothing because little by little that 40c degrees air comes inside, my room is not totally insulated, so ambient temperature will always come inside but just little by little and when the temperature ambient is at 24c then I can even leave my window opened hehe, if you plan ac then you need to totally insulate that room but still as the ambient temperature rises, less efficient will the ac be.