Nonetheless, when it comes to choosing a portable AC , one of the most common mistakes consumers make is incorrectly sizing their unit. Portable air conditioners are sized according to BTUs, which stands for British Thermal Units. A BTU is a unit of energy that is commonly used in the power, steam generation, and heating and air conditioning industries. Generally speaking, a portable air conditioner's BTU rating describes its power and cooling capacity. Naturally, the higher the BTUs, the stronger the portable AC. Portable air conditioners have cooling capacities that usually range from 5,000 BTUs to as high as 30,000 BTUs, and the size needed to cool a room will depend on the area being cooled. Thus, an incorrectly sized portable AC with insufficient or excess BTUs will result in inefficient cooling.