First, a quick primer on BTUs (British Thermal Units), the main measure of energy output in air conditioners. The higher the BTU, the more square footage you can cool. For example, an unit with 8,000 BTU is perfect for cooling about 300-400 square feet. More square footage means you’ll need a higher level of BTU to keep it comfortable. If your unit will be installed on an upper floor where heat rises and gets trapped, opt for a slightly higher number of BTU than you might otherwise in order to mitigate that stuffiness.