How to Choose a Portable Air Conditioner Size for Outdoor Use

How to Measure a Portable Air Conditioner’s Size

BTUs (British Thermal Units) are a common way to measure the cooling capacity of air conditioners. It’s often simpler to call it the air conditioner size – Higher BTU air conditioners are “larger” with more cooling capacity. Higher BTUs can provide more cooling for larger areas. This makes perfect sense indoors. Outdoor cooling has more challenges.

First, a bit about BTUs and energy ratings. Knowing this can help you choose a portable air conditioner size suitable and effective for your outdoor space.

Why Are There Different Kinds of BTUs?

All the BTU ratings are attempts to describe efficiency and power in a “simple” way.

Product descriptions for portable air conditioner sizes may list their BTUs in different ways. Descriptions may simply say BTUs. Or they might say DOE BTUs, SASS BTUs, or SASS / CEC BTUs. Don’t let this alphabet soup bother you, though.

Standard (ASHRAE) BTUs

When you see “BTUs” in a portable air conditioner description, it refers to ASHRAE – the standard of measuring a portable AC’s cooling capacity (efficiency and power) in an ideal lab setting.

But outdoor settings are a far cry from any lab setting. You use that AC in all sorts of temperatures and humidity levels. In comes SACC BTUs to the rescue.

SACC BTUs

You might find portable air conditioners that sit inside the area you want to cool described with the newer DOE “SACC” (Department of Energy Standard Area Capacity Cooling) BTU rating. These numbers are different from the standard BTU numbers for two reasons:

  1. Portable air conditioners that sit inside the area they are cooling need to use some of that cool air to cool the electric components of that AC unit.
  2. Portable AC units don’t recycle the cool air the way central or window air conditioners do. Instead, they pull in fresh, hot air.

Because SACC is closer to the way your portable air conditioner is likely to work outdoors – with areas that are less enclosed and less insulation and pulling in different temperatures of hot air – the SACC BTU rating is closer to what you would get than the standard BTU rating. Even so, this assumes the best insulation and enclosure since most portable air conditioners are used indoors.

Expect to need more BTUs for cooling outdoor areas, no matter what kind of BTUs describe a portable AC unit.

SACC ratings are lower that ASHRAE ratings because efficiency is lower under less than ideal conditions. It simply takes more energy to reheat fresh, hot air than it does to cool recycled air that’s already been cooled.

Learnmetrics.com describes this well. They use an example of two 14,000 ASHRAE BTU portable air conditioners. Remember, that’s 14,000 BTUs under ideal conditions.

The first14,000 BTU unit has a DOE SACC BTU rating of 7,700. The second 14,000 BTU unit has a DOE SACC BTU rating of 10,500. The second unit with the higher SACC rating is more efficient than the first in less than ideal conditions. That means the second unit would be more efficient in your more variable and more punishing outdoor space.

When considering two units with the same (ASHRAE) BTUs, the one with the higher SACC BTU rating will be more efficient in more variable conditions, such as for outdoor use.

The chart below gives typical coverage areas for both BTU measurements. So, whether the portable AC you’re interested simply lists standard BTUs or DOE SACC, SACC, or SACC / CEC, I hope this chart will help you compare units and get a general idea of the BTUs you need.

You can learn more about ASHRAE versus DOE SACC BTUs at learnmetrics.com.

Cooling Needs for Indoor vs. Outdoor Air Conditioner

The needs for outdoor cooling can be dramatically different than for indoor cooling. When you shop for a portable air conditioner for outdoor use, keep in mind that many variables affect the amount of cooling you’ll get, no matter how efficient or powerful a unit might be.

  • Climate and weather vary much more than indoors temperature and humidity does. Wind is also an issue outdoors. Winds can blow your cool air away but still air helps it stay right where you want it. HIGHER TEMPERATURES AND HUMIDITY NEED MORE BTUs.
  • There are wider variations in insulation of outdoor areas versus indoors. From well-insulated stone walls, ceilings and floors, to lattice walls and overhangs, to metal walls of a vehicle or thin fabric walls of a tent. LESS INSULATION NEEDS MORE BTUs.
  • The degree of enclosure varies tremendously in outdoor spaces, while indoor spaces are, well, indoors and mostly enclosed. Outdoor spaces range from enclosed garages, shops, vehicles or tents, to partially enclosed patios and balconies, to entirely open spaces like backyards and parks. LESS ENCLOSED NEEDS MORE BTUs.
  • Area size also varies more outdoor than indoors. Most every indoor area has a roof. But many outdoor areas are under wide open skies or might have only a lattice or fabric cover. Many outdoor areas don’t have a wall on one or more sides. This increases the area size dramatically! MORE OPEN AREAS NEED MORE BTUs.

EXAMPLE:

While a camping tent or van are small and totally enclosed, there is very little insulation. If you camp or park in direct sun, you also have higher temperatures to deal with. The small tent or van might need a unit with more BTUs than a larger balcony with 5 solid sides (top, bottom, and 3 walls) that faces away from the direct sun.

Resist the urge to go by BTUs alone. Cooling the outdoors is as much about preference as it is about science. So, use BTU ratings as a guideline only. The chart below can help you get a general estimate of the size / BTUs a portable air conditioner needs to cool a certain sized area.

BTU Chart for INDOOR Portable Air Conditioner Size

Manufacturers have different charts with different recommendations of BTUs for different sized indoor areas. This is because the best portable air conditioner size for any area depends on different factors. Varying charts make assumptions about those factors in their recommendations.

If BTU charts for indoor areas vary, how much more would those charts vary for outdoor areas! Because of that vast variability, there are no BTU charts for outdoor portable AC sizes.

This emphasizes the fact that your outdoor cooling needs are unique, considering the wide variety of outdoor spaces and conditions. It also emphasizes that these charts are for GENERAL GUIDELINES ONLY.

AREA TO BE COOLED-
SQUARE FEET
AREA TO BE COOLED-
SQUARE METERS
STANDARD
BTUs
100 – 1509.3 – 14.05,000
150 – 25014.0 – 23.36,000
250 – 30023.3 – 27.97,000
300 – 35027.9 – 32.58,000
350 – 40032.5 – 37.29,000
400 – 45037.2 – 41.810,000
450 – 55041.8 – 51.112,000 (1 TON)
550 – 70051.1 – 65.014,000
700 – 1,00065.0 – 92.918,000
1,000 – 1,20092.9 – 111.521,000
1,200 – 1,400111.5 – 130.123,000
1,400 – 1,500130.1 – 139.424,000 (2 TONS)
1,500 – 2,000139.4 – 185.830,000
2,000 – 2,500185.8 – 232.334,000
Suggested BTUs to cool different sized indoor areas. Actual numbers vary based on climate and conditions of the area to be cooled.

Shorthand for Mobile and Portable Air Conditioner Sizes

In the world of air conditioner sizing, multiples of 12,000 BTUs are commonly referred to as a “ton.” This does not mean the air conditioner weighs 1 ton! A 1-ton air conditioner may only weight 150 pounds (68 kg).

A portable air conditioner size of 24,000 BTU is called a “2 ton” unit. A 36,000 BTU portable air conditioner size is called a “3 ton” unit, etc.

Sizes over 5 tons (60,000 BTU) are called “mobile” air conditioners. Mobile air conditioners of 100s of tons are available. They are invaluable for large special events and venues, especially considering the extra cooling needs for outdoor spaces (less insulated, more weather-exposed, and less enclosed).

What Affects the Portable Air Conditioner Size You’ll Need?

Environment

  • TEMPERATURE. Allow more BTUs for more open areas and areas with poor insulation. Consider ways to enclose and / or insulate those areas. Otherwise, you might need 2-3 times the charted BTUs for that area.
  • SUNLIGHT. It takes more BTUs to cool a hotter space than a cooler one. If the outdoor space in in direct sun, has no overhang / ceiling, or air temperature is regularly above 95ºF (36ºC) and the humidity is over 60%, you might need a high ambient air conditioner. High ambient ACs can handle these more punishing climates, but can be harder to find and more expensive. Check the operating limits of the unit you’re interested in or ask the manufacturer.
  • NUMBER OF PEOPLE. If more than two people will regularly be in the space, add at least 600 BTUs per person. Bodies add warmth to an area and more BTUs will be needed to cool an area with more people in it.
  • Consider your budget. Portable air conditioners are expensive relative to most other cooling methods. Larger units with more BTUs are more expensive. They can involve more maintenance and use more energy, too.

Insulation and Enclosure

Materials that enclose the space and how well that space is enclosed have a big effect on the efficiency of any portable air conditioner.

EXAMPLES:

  • A tent with insulated panels would maintain the cool temperature better than a tent with canvas or mesh panels. Fabric panels allow more of the cool air to escape. You can add insulation to many outdoor areas like vehicles and tents. Be aware that adding insulation cuts down on air flow – an important element of camping tents.
    – If you need great ventilation, consider other outdoor cooling methods. Evaporative cooling for dry climates or outdoor fans for any climate could work better for you.
  • A plywood shed may have less insulation than a framed garage. Both may have less than a concrete or brick warehouse.
  • A trailer might have more insulation in the walls than a shed, and would maintain the cool temperature better than the shed would.

You can counter poor insulation somewhat by placing the unit closer to the spots that need the most cooling.

Alternative Outdoor Cooling Methods

Might other cooling methods be more helpful?

  • Consider adding shade. Maybe a solid overhang or solid walls. Can you make use of the shade of any nearby structures or plants? Consider materials that block more heat (more insulation).
  • Consider outdoor fans. They are simple and less expensive to purchase and maintain.
  • Consider evaporative cooling for areas in drier climates.

Tips for Choosing a Portable AC

  • Read descriptions carefully, Refer to the BTU chart as a general guide to size your outdoor AC.
  • Be sure your electricity source can handle any surges of power when the compressors turn on.
  • Look for portable and mobile units rated for outdoor use. They have special safety features that stand up to dust, dirt, wind, rain. Be very cautious about using an indoor rated unit for outdoor use. Use it only in a covered, dry, and protected location inside the area to be cooled. BE sure electrical connections are also in covered, dry, and protected areas. It’s helpful to consult with an electrician or HVAC installer to review your setup before purchasing or renting a unit.
  • When you purchase or rent a larger portable or mobile air conditioner, or one that might challenge your budget, connect with a sales consultant. They can advise you on mobile / portable air conditioner sizing and setup for your needs. Tell them:
    – Size of the area you need cooled
    – Description of the area – Is it a shed, tent, trailer, etc. Does it have an overhang or ceiling, how many walls, are there openings for the AC’s hoses, and is there an electricity supply?
    – Weather, climate, sun exposure
    – Number of people who will use it

Remember to consider your specific needs and consult the BTU chart as a starting point. With the perfect portable AC size and some thoughtful setup, you CAN have a cool and comfortable outdoor space.

Other Outdoor Cooling Methods

If you have a more open outdoor area that needs cooling, consider outdoor fans, misting systems, or misting fans which combine misting systems and outdoor fans into a single unit.

Swamp coolers (desert coolers) use a similar technology as misting systems (evaporation cooling), but in a contained unit. They are sometimes called air conditioners, but use a different technology than true air conditioners.

Outdoor shades are a nearly universal solution that combine well with every other cooling option.

Take a look at our comparison page to learn which outdoor cooling solutions may be best for you.