British Thermal Units are like calories, a unit of heat measurement. A BTU is the amount of heat required to raise a pound of water one degree Fahrenheit. On air conditioners, it refers to the amount of heat it’s able to cool down a room. The range will be from 5,000 BTUs to 24,000 BTUs, but it pays to get the amount you need for your room.

Before we get to that, you need to be aware of the unit Energy Efficiency Ratio, or EER. That’s simple math, divide BTUs by wattage when the ambient air is 95 degrees F and you get the EER. The higher the number, the more efficient the unit and the more expensive it will be. The range for these will be from 8 to 11.5. You will often also see a SEER or Seasonal Energy Efficiency Ratio which is a little different. It’s supposed to capture how the device performs throughout the year. Energy Star rated AC’s must have an EER of 14 and most will have above 13. You can’t compare EER and SEER.

Lastly, the BTU will tell you how many square foot a unit will cool and under what conditions. An 8000 BTU Sharp AF-S85NX can cool 415 sq. ft. if there is an occupied room above it. Only 360 sq. ft. if there’s an insulated roof/attic above it and a mere 235 sq. ft. if there is an uninsulated roof/attic above it. As you can see, room configuration is significant.

Comparably, a 12,000 BTU Sharp AF-S125NX can cool 745 sq. ft., 650 sq. ft., and 415 sq. ft respectively.