When 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.
The correction comes down to how portable AC units actually operate. Unlike a window air conditioner, a portable unit is located entirely inside the home and vents heat outside through a hose that connects to a window. During the cooling process, it can pull in some air that it has already cooled and push it outside, lessening its efficiency.
“Because of all of the energy you waste recooling or venting air that you already treated, the true [BTU] number is usually between 25% and 40% lower [than the ASHRAE rating],” Adams said.
This is why portable AC units often feature two BTU ratings. One reflects the older ASHRAE rating, while the updated DOE rating shows the more accurate cooling performance. Some older models only feature ASHRAE ratings, which is why newer models have both ratings to avoid confusing buyers.
How many BTUs do you need?
Choosing the right BTU rating depends on the space you want to cool. The most important factor is room size, but sun exposure, insulation and ceiling height should also be considered.
If you’re not sure which BTU rating you need, an online BTU calculator can help. Ultimately, one BTU rating is not better than another. The right choice depends entirely on your space. If you choose one that is too low for your needs, it won’t properly cool a room, while one that is too high could result in wasted energy and increased costs.
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