Buying the wrong size is the most common mistake. A general rule is 20 BTUs per square foot . 5,000 – 8,000 BTU: For rooms 150 to 350 sq. ft.. 8,000 – 12,000 BTU: For rooms 350 to 550 sq. ft..

Buying an air conditioner online is convenient and often cheaper, but requires careful planning since you cannot physically inspect the unit or test its noise levels beforehand.

5 Tips for buying an AC unit online - Blog - Realty Executives

Look for the SEER (Seasonal Energy Efficiency Ratio) rating. Higher numbers mean lower utility bills. The current U.S. minimum is 14, while top-tier models reach 26 or higher.