An air conditioner's BTU rating is a measurement of its cooling capacity, the amount of heat it is capable of extracting from the air over the course of an hour. Thus, a 6500 BTU air conditioner is stronger than one rated at 5000 BTUs — but that doesn't mean it's better.
British Thermal Units
BTU is an abbreviation for British Thermal Unit, a measurement of heat. One BTU is equivalent to 1.055 joules, explained in everyday terms as the heat put off by a match, or the heat required to raise the temperature of a pound of water from 39 to 40 degrees Fahrenheit. Both heaters and air conditioners are given strength ratings in BTUs per hour, though one refers to the heat put off and the other to the heat extracted.
5000 BTU Air Conditioner
A 5000 BTU air conditioner is capable of cooling a room of 100 to 150 square feet, assuming average amounts of sunlight and no more than two people occupying it at the same time for extended periods. This is the lowest end of the range in air conditioner ratings, with very few weaker air conditioner units sold. For a small room, however, this is exactly what is needed.
6500 BTU Air Conditioner
A 6500 BTU air conditioner can best cool a room with an area between 125 and 275 square feet. This recommendation can fluctuate depending on room conditions. If the air conditioner will be placed in a room that regularly receives direct sunlight, nothing larger than 250 square feet will do, though that number jumps to 300 if the room is heavily shaded. If the room is often shared by three people, dimensions between 150 and 250 square feet are ideal.
Why It Matters
Matching an air conditioner's strength and the size of the room in which it's installed is very important. If installed in too large a room, the air conditioner will run continually, raising your electricity bill without ever cooling the room to the desired temperature. If the room is too small for the air conditioner it will short cycle, cooling the air so rapidly that it cannot reduce the humidity enough, leaving the atmosphere clammy and damp.