The BTU Rating
BTU stands for “British Thermal Unit” and it is the most important thing to look for when deciding on any kind of air conditioner. The BTU rating is effectively a measurement of how much heat portable air conditioners can remove from surrounding air.
In general, you’d want to get 20 BTU’s for every square foot of living space in a room with an eight-foot ceiling (which is the standard). If your room came in at 300-square-foot then you’d want to get a unit with a BTU between 6,000 and 10,000, depending on factors such as the amount of natural light and ceiling height. For a 500-square-foot room you’d want a model with between 10,000 and 14,000 BTUs.