The BTU rating, or British Thermal Units, is the single most important variable in the effectiveness of a portable air conditioner. In A/C technology, BTUs relate to how much heat the device can remove from the air around it. As a rule of thumb, you need about 20 BTUs for each square foot of living space with eight-foot ceilings. So, for example, a standard 300-square-foot room would be fine with a unit rated from about 6,000 to 10,000 BTUs, depending upon ceiling height, amount of sunlight, and other factors. A 500-square-foot room needs a unit capable of about 10,000 to 14,000 BTUs, etc.