GoKawiilWhen shopping for an air conditioner, BTU ratings are one of the first specs people consider when deciding which unit to buy. But if you're looking at a portable air conditioner, those numbers can get quite confusing. Unlike window units, portable models often have two BTU ratings, a higher rating from the American Society of Heating, Refrigerating and Air-Conditioning Engineers and a lower one from the Department of Energy.
What do they both mean, and why do they matter? Here's everything to know before you purchase a portable air conditioner for your home.
What is a BTU rating?
BTU stands for British thermal unit, a measurement of heat energy. In addition to air conditioners, heat pumps and space heaters also have BTU ratings.
"A British thermal unit is the amount of energy required to raise one pound of water 1 degree Fahrenheit," Bryan Adams, CNET's senior lab engineer and former HVAC configuration manager, explained.
As a rule of thumb, he says, BTUs directly correlate with the size of the space you need to cool.
Why do portable AC units have two BTU ratings?
Originally, portable air conditioners were sold with a single BTU rating based on ASHRAE laboratory testing standards. Those ratings were often very high, such as 14,000 BTUs, but they didn't always accurately reflect the unit's real-world cooling performance outside of a lab.
In response, the DOE introduced new testing procedures for portable AC units in 2017 to accurately measure cooling performance. The updated testing measures performance across two different temperature and humidity tests and accounts for unit inefficiencies to provide a more accurate, seasonally adjusted cooling capacity rating, or SACC rating.
"The Department of Energy had to step in and correct the HVAC regulatory bodies' classification of their cooling efficiency, and that's pretty unprecedented," Adams said.
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