People have a strong need to simplify things, and that is especially true when it comes to long technical terms, mainly used by people who know what those terms mean. Doctors are famous for throwing out big words that their patients don’t understand. Unfortunately, simplification often involves the use of acronyms or abbreviations. 

Such is the case with air conditioning. Those in the business throw out terms like BTU’s and EER like everyone knows what they mean. But that’s not the case, so this article explains in brief what the BTU rating of portable air conditioners and window units means to you.

Take a look at our Portable Air Conditioners With or Without Heat, or our Window Air Conditioners, and you will see that they all have a BTU rating.  BTU is short for British Thermal Unit, defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit at one atmosphere of pressure. So a BTU is an amount of energy, specifically heat. 

When used to describe an ac unit, "BTU" is really a misnomer, since what it refers to is the power of an ac unit, which requires a time reference. So with air conditioning we really mean BTU’s per hour, and we're talking cooling power.  But in common usage everyone just shortens it to “BTU.” 

The higher the BTU rating, the greater the ability to change air temperature. So a 12,000 BTU unit will be able to cool more air than a 10,000 BTU unit in the same amount of time. Now we’re getting somewhere.  How much more? Here’s the next potentially confusing thing. We measure rooms in square feet, which is really the size of the floor only. 

But rooms are three dimensional, they have height too. Portable air conditioners don’t cool the floor, they cool the air, which occupies the entire room. So while the ability to cool air is fairly linear with changing BTU (other factors come into play), the volume of air in a room depends also on the ceiling height. For example, a 20 foot square room with an eight foot ceiling has 3200 cubic feet of air, whereas one with a ten foot ceiling has 4000 cubic feet.

So what is this all about? OK, finally I’ll get to “what it means to you.”  Portable Air Conditioners  and Window Air Conditioners  have BTU ratings, and those ratings are correlated with the size room the unit should be able to adequately cool. Let’s say you have ten foot ceilings in your home, and you have a 289 square foot (17 x 17) room that is too warm. 

By the charts on our Products page, one can see that our 10,000 BTU portable air conditioners would be recommended for 300 square feet.  But those numbers are for standard eight foot ceilings. With ten foot ceilings, the volume of air is 20% higher, meaning the volume is equivalent to a 346 square foot room with eight foot ceilings. So a unit designed for 350 feet will be a better choice to cool your room efficiently, i.e. 11,000 BTU instead of 10,000.  

Now that is a useful tidbit to help you choose a portable air conditioner with the right BTU rating for your room!  The same holds true for window units as well.  (Our article on Energy Star ratings for window air conditioners goes into more detail). 

The “other factors” mentioned above should also be considered when determining how many BTU’s to get. Things like sun exposure, insulation, temperature outside, duct length, door size, electronics heat producers in the room, and whether or not you are supplementing central air, all come into play when choosing a BTU rating. 

Because of this, even with standard ceilings, it’s not a bad idea to upsize the unit a little over the size charts if you can afford it. I’ve seen arguments both ways on this, but in my opinion, upsizing should allow the unit to cool a room more quickly, allowing it to run less. It also gives the option of using it in a larger room or one with more of the 'other factors' if needed.  So, look again at our Portable Air Conditioners With or Without Heat, and our Window air Conditioners, and choose the right unit to meet your needs!