Don’t Use ASHRAE BTU to Calculate Room Size For Portable AC

ASHRAE ratings for air conditioner BTU output are not the right ratings to use when considering what size room a portable air conditioner can cool – or what size AC you need for the room size.

The reason is that portable ACs are not nearly as effective at removing heat from a room as a window AC or central air conditioning. The reason they don’t remove heat as effectively is explained below.

ASHRAE vs SACC Ratings – They Are Not the Same

For example, a 14,000 BTU window air conditioner can reliably remove 14,000 BTUs of heat from a room in an hour. But a portable air conditioner with a 14,000 BTU ASHRAE rating will only remove about 10,000 BTUs of heat. That’s why the Department of Energy conducted its ASHRAE DOE BTU test comparison to convert ASHRAE ratings to what the department calls the DOE SACC, or Seasonally Adjusted Cooling Capacity.

Note – Manufacturers are required to list the new SACC ratings. Some also still use the ASHRAE ratings too, which can make it confusing or at least make it appear the portable AC can cool more space. Always go by the DOE SACC ratings when shopping for a portable air conditioner.

What if it doesn’t say which ratings they are? Then you can consider them to be the new SACC ratings. For example, this Hisense portable AC is listed as a 10,000 BTU model, and the information says it can cool 550 square feet or less. And that’s what we would expect for a 10,000 BTU unit using SACC ratings. 

The key in choosing the right size portable air conditioner is to convert ASHRAE BTU per square foot to SACC BTU per square foot. That’s what our calculator does in the next section.

Portable AC BTU and Room Size Calculator

So, how do you calculate the room size that a portable air conditioner can cool and dehumidify? That is, how many portable air conditioner BTUs do you need for the room you want to cool?

The Pick HVAC BTU Calculator for Portable ACs is the answer. It will give you the BTUs of air conditioning you need based on the SACC ratings.

Here it is. A brief explanation of how to use it follows.

Window AC BTU Calculator

It’s pretty straightforward. Answer three simple questions, and the calculator will automatically determine the right portable AC size for the space. If it doesn’t generate an answer for you immediately, hit the Calculate button.

1). Room Size – Measure length times the width of the room to get square feet. A room 15’ x 20’, for example, is 300 square feet.

2). Insulation Condition – If you don’t know, use Average or Poor. Older homes usually have Poor insulation, especially if they have drafty windows and doors. New homes, those built since 2000, probably have Good insulation.

3). Sun Exposure – If your climate is generally very sunny or the room faces south or west, choose Very Sunny. Otherwise, choose from Heavily Shaded or Average, whichever fits your situation.

Again, the size of the portable AC you need to cool the room should automatically be calculated. If not, hit the Button to get your answer.

How Big are Portable ACs?

That’s something of a problem. They’re not very large. Using ASHRAE sizing, they are available in sizes from 6000 to 14000 BTU.

But using the calculator to convert ASHRAE ratings to the SACC ratings, the available sizes are 4000 to 10000. A few might have a SACC rating of 12000 BTU, but that’s rare.

So, using our Calculator, you’ll see that the largest space a portable air conditioner can cover is about 500 to 550 square feet. Yes, you can plug any size space into the calculator and get a result, but just keep in mind that portable units are limited in size.

For further research into DOE SACC BTU per square foot and the ASHRAE BTU per square foot comparison, see our guide titled SACC BTU DOE vs ASHRAE in Portable ACs. 

Why Are Portable Room Air Conditioners Less Effective?

It’s a simple answer. Actually, there are two reasons.

First, the unit is situated entirely in the room. It pulls in air, removes heat from it using refrigerant, and then sends the heat out through the exhaust hose. The exhaust hose connects to a panel in the window, so it can exhaust warm, humid air. However, some of the heat that is removed leaks back into the room before it can be exhausted. It leaks out of the unit and some transfers through the hose as it is passing through it.

Secondly, because air is exhausted from the living space, it creates a slight amount of negative air pressure indoors that causes outside air – warm, humid outside air – to be drawn inside through the home’s envelope.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree from Lone Star College and EPA & R-410A Certifications.

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