When purchasing  a room air conditioner, sometimes called a "window unit", the rule of thumb on finding one large enough to cool properly durng normal conditions is to first calculate the square footage of the area to be cooled. Once that has been done, it will give the approximate capacity for the area to be cooled. For example, a 10' X 20' room which is 200 square feet will require approximately 6,000 BTU of cooling to condition it properly.

Air conditioners are rated in something called "tonage" and there are 12,000 BTU's per ton. A ton of cooling requires 400 CFM of air to cool properly. If an area is 200 square feet, it would require approximately 1/2 ton which is equal to 6,000 BTU to cool it properly and  to properly  remove humidity. If it is too large, it will be just as inefficient as one that is too small.

Air conditioners are rated in what is known as SEER which is the Seasonal Energy Efficiency Rating. The higher the SEER, the more efficient the appliance will be as well as a more tolerable noise level as well as a better compressor in some cases.

Also keep in mind that no matter how efficient or properly sized an air conditioner is, reguardless of it being a central air system as well as a room air conditioner it's important to keep the air filter clean by washing it monthly and also not to try to save money by leaving it off during the day. By not using it while no one is home, it will also cost more to operate it because of the increased heat load that is placed upon the equipment.