Investing in a high-quality portable air conditioner is a great and hassle-free way to cool down, warm up, or remove the extra humidity from any room in your home and office. What’s more, a portable air con can help you save up on your electricity and heating bills and boost the overall energy efficiency of any building. However, before getting one, you ought to know how big of an A/C unit would best suit your needs.

Small Portable Air Conditioner

Size Matters

The size of your air con relative to the size of the room it’s in is absolutely crucial to its performance and the results you’ll get. Yet, it is not so much the actual physical dimensions of your A/C system that are important here — although they, too, matter. After all, you want to be able to fit your unit through the doorway and in the room.

What we are concerned with here, however, is the energy capacity of the A/C.

BTUs Explained

The British Thermal Unit or BTU is a unit of heat that measures the amount of energy required to heat up 1 pound of water at sea level with 1°F.

Despite its somewhat misleading name, the BTU is also used in the U.S. to categorize air conditioner sizes. In particular, the measurement denotes how many BTUs per hour an A/C unit can remove from the surrounding air. You can easily find the BTU score of any air-con system clearly labeled in a prominent place on your A/C.

How to Calculate Your Required BTU

Now that you know what the BTU acronym stands for, it is time to do some math. Don’t worry, this is not rocket science. All you need to calculate your BTU requirement is the surface area of your room. If you don’t know what the surface area is, simply multiply the width of the room by its length and — voila — you have got yourself your square footage.

Moving on to the BTUs. The basic rule is that it takes around 20 BTUs for a standard portable air-con to cool down a floor space of one square foot. So, a room of 300 square feet or less would need around 10,000 BTUs. By the same token, a 500-square-feet room would need 10,000 to 14,000 BTUs, and a room of over 500 square feet — 14,000+ BTUs.

As you can see, this is not an exact science. The figures above are general approximations. In real life, just how big of an A/C unit you need would be affected by many other factors, including:

  • The outside weather and season of the year
  • The quality of the building’s isolation
  • How often and for how long the door and any windows stay open
  • Whether or not you have another A/C system in the room
  • The amount of sunlight that enters the room during the day
  • The height of the ceiling
  • How many people there are in the room at any given moment
  • Any running kitchen or electrical appliances
  • Whether or not you plan to move around your portable A/C unit to other parts of the building, or will only keep it in one room
  • The temperature in the adjacent rooms

Nevertheless, the BTU estimates above should give you a good idea of what to look for and where to start.

Allow for Necessary Adjustments

No two rooms or buildings are the same. Therefore, you may have to make some slight adjustments to the BTU estimates above. Here are some things you need to consider:

  • If the room has plenty of natural light coming in during much of the day, you should increase your BTU score by 10%.
  • In contrast, if the room is somewhat shaded and gets little to no sunlight, you should reduce your BTU requirement by 10%.
  • If the ceiling is higher than 8 feet, increase the BTU by 10% for each extra foot of height.
  • In case there will be two or more people in the room on a regular basis, add an extra 600 BTUs per person to the score.
  • Finally, if kitchen or other electrical appliances will be running in the room on a regular basis, increase the BTU score by 4,000.

In the end, you should have a pretty accurate estimate of the required BTU capacity of your air-con, or, in other words, how big of an A/C unit you need.

What If You Miscalculate Your BTU Requirement?

If your portable air-con unit has a lower BTU score than what your room requires, your A/C system will start to run non-stop cycles. That, in turn, will overwork the unit and shorten its shelf life. If the disparity between the size of the room and the unit’s BTU score is too large, the A/C could even break. What’s more, your room will remain too hot for your liking, and your electricity bill will skyrocket.

Nevertheless, don’t make the mistake of buying an air con with a BTU rating that is much higher than necessary. When it comes to A/C systems, too much is just as bad as too little.

Using an aircon that is way too powerful for the room it’s in will only reduce its lifespan. First, the unit will cool down the room too quickly. It will then turn itself off, only to power itself back on way too soon to restart the entire process.

This artificially shortened cycle will wear down the aircon unit and boost your electricity bills. It is also highly likely that the system may not be able to remove the humidity from the air.

How Big of an A/C Unit You Need: Wrapping Up

A portable air conditioner could be a great addition to your household or office space. There are many excellent brands and models out there to suit any budget, need, or personal preference. There are just a few things to consider to maximize the efficiency of your A/C and get your money’s worth.

First, make sure you understand how BTUs work and what they stand for. Second, know the surface area of your room. Third, calculate your BTU requirement, deducting or adding points depending on the characteristics and parameters of the room. If you follow these steps, you should know, once and for all, how big of an A/C unit you need.