Sizing your room correctly for an air conditioner is critical. The first problem is when you undersize a unit and it does cool adequately during times of higher ambient temperature. The other issue is when you oversize the a/c, and the units cut off before removing the humidity sufficiently from a room. Regarding comfort, size your room, office, or any other space, for the following parameters: 24 degrees with a humidity level of 30-50% for maximum comfort.
First, calculate the total floor area of the space you want to cool. Remember to include all the floor area that you expect to cover with a single unit. These basic calculations don’t take additional people, windows, and other issues into account.
Next, find a calculation sheet that has the typical cooling load requirements for your country. The cooling requirements can vary significantly between two countries, or even two cities. Use the floor space calculation from above to determine your basic cooling requirement.
Lastly, reduce your calculated figure by 10% for each of the following: heavily shaded house, heavy curtains, insulation. For each additional person in the room, increase 600btu per person. Also, you can subtract 30% of the cooling requirement of the unit will only run at night. Other issues like sun and poor insulation will drive up the requirement.