On this page
- What a BTU figure actually measures
- Why portable units carry a second number, SACC
- Reading a portable listing: a worked example
- The two figures are not interchangeable
- Matching a capacity figure to a room
- Adjustments beyond floor area
- What a capacity rating does not tell you
- How to use the numbers when comparing models
- Which to choose
- Questions people ask
What a BTU figure actually measures
BTU stands for British Thermal Unit. Frigidaire's size guide calls it a unit of measurement for heat energy, defined by how much heat it takes to shift one pound of water by one degree Fahrenheit. On a room air conditioner listing the figure is a rate: cooling capacity is stated in BTUs per hour, as ENERGY STAR explains in its sizing guidance.
So a window unit listed at 12,000 BTU is rated to remove 12,000 BTUs of heat from the air in an hour, under the conditions its maker rates it at. That is the whole meaning of the number. It says nothing about how much electricity the unit draws to do it, how loud it runs, or whether it fits the window you have.
Why portable units carry a second number, SACC
Portable air conditioners list a second capacity figure, and it is the one that matters when portables are compared with each other. The federal energy conservation standard for portable air conditioners defines SACC as seasonally adjusted cooling capacity in Btu/h, determined under appendix CC of subpart B of that standard. For variable-speed portable air conditioners, the standard uses SACC Full.
The two figures come from different procedures. A portable's BTU number is the capacity its maker states for the unit. SACC is a seasonally adjusted figure worked out under the federal procedure, and it is the figure the efficiency standard for portables is built on. A window unit sits in the window opening and rejects its heat outdoors, and its listing carries a BTU figure, so BTU is the basis for comparing window units with each other.
Reading a portable listing: a worked example
Whynter's manuals print both numbers on the same page, which makes the gap easy to see. The ARC-1030WN is listed as 12,000 BTU with 10,000 BTU SACC. The ARC-1230WN is listed as 14,000 BTU with 12,000 BTU SACC, and the ARC-1230WNH, the version with a heat mode, carries the same pair of figures. A separate manual for the ARC-1280MX and ARC-1280WX lists 13,000 BTU and 10,000 BTU SACC, with a stated coverage area of 500 sq ft.
On these models the SACC figure is always the lower of the two. That is normal, and it is why a portable advertised at 14,000 BTU should not be read as the equal of a 14,000 BTU window unit. The window unit's figure and the portable's SACC describe different measurements of capacity.
| Model | Stated BTU | Stated SACC |
|---|---|---|
| Whynter ARC-1030WN | 12,000 | 10,000 |
| Whynter ARC-1230WN | 14,000 | 12,000 |
| Whynter ARC-1230WNH | 14,000 | 12,000 |
| Whynter ARC-1280MX / ARC-1280WX | 13,000 | 10,000 |
The two figures are not interchangeable
Because the figures come from different procedures, one cannot be converted into the other. There is no formula that turns a portable's stated BTU into SACC, and no way to estimate one from the other. A listing that gives only a BTU figure for a portable leaves the reader without the number needed to place it against other portables.
The practical rule: rank and compare portables on SACC only. When a portable's listing does not include a SACC figure, the accurate statement is that the maker does not list one, and that unit cannot be placed against another portable's SACC. Compare window units on their BTU figure, since that is the capacity figure their listings carry. Mixing the two in a single ranking produces a list that looks precise and means very little.
Matching a capacity figure to a room
A capacity figure only becomes useful once it is matched to a space. ENERGY STAR publishes a chart that pairs floor area with the capacity needed, calculated for an 8-foot ceiling, and its guidance is blunt: bigger is not always better. An oversized unit cools the room before it has had a chance to remove humidity, which leaves the air feeling damp and clammy.
Read the chart as a starting point. For a window unit, the capacity column is directly comparable to the BTU figure on the listing. For a portable, look at the SACC figure and at the coverage area the maker states, rather than at the headline BTU number.
| Area to be cooled (sq ft) | Capacity needed (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
Adjustments beyond floor area
Floor area alone is a rough guide. ENERGY STAR lists adjustments for the conditions a room actually sees: reduce capacity by 10 percent for a heavily shaded room, increase it by 10 percent for a very sunny one, add 600 BTUs for each person beyond two who regularly occupies the room, and add 4,000 BTUs if the unit goes in a kitchen. After the adjustments, ENERGY STAR suggests rounding down to the nearest thousand.
Frigidaire's size guide is careful about the limits of this arithmetic, noting that its guidelines do not account for vaulted ceilings or for how well the room is insulated. Rooms with large west-facing glass, thin walls or an open stairway behave differently from the plain rectangle in the chart. More on that is in why some rooms are harder to cool than others.
What a capacity rating does not tell you
Two units with the same capacity can differ widely in everything else. Efficiency is a separate rating, and for room air conditioners it is CEER, covered in CEER explained. Noise, airflow direction, whether the compressor is variable speed, and whether the unit has a heat mode are all separate from capacity. ENERGY STAR notes that most of its certified room air conditioners use variable speed technology, which varies compressor speed to hold a temperature instead of switching fully on and off.
How the unit is fitted into the window counts too. ENERGY STAR points out that a room air conditioner fitted badly can leak about as much air as a 6-square-inch opening in the wall. For a portable, that means the window kit and the hose connections. For a window unit, it means the side panels and the seal around the frame.
How to use the numbers when comparing models
Start with the type. Window units are compared on BTU, portables on SACC. Then match the figure to the room, apply the adjustments, and only after that look at efficiency and features. The site's own lists follow that order: best window air conditioners ranks window units by their BTU cooling capacity, while best portable air conditioners ranks portables by SACC. Lists built around a single capacity band, such as best 12000 BTU portable air conditioners, use the same rule.
If a listing shows a large BTU number and a much smaller SACC, that is not an error or a downgrade. It is two different measurements of the same machine, and the smaller one, SACC, is the figure portables are compared on. The room air conditioner terms glossary covers the other labels you will meet on a spec sheet.
Which to choose
| If you | Look for |
|---|---|
| If you are comparing two portable air conditioners | Compare their SACC figures only. A portable whose maker does not list a SACC figure cannot be ranked against them. |
| If you are comparing two window units | Compare the BTU figure on each listing, since that is the capacity figure window units carry. |
| If a portable listing shows only a BTU number | Treat it as a unit with no SACC figure on record, and do not set its BTU number against another portable's SACC. |
| If your room's square footage is already known | Use the ENERGY STAR capacity chart for a starting figure, then check the coverage area the maker states for the model. |
| If the room is very sunny or heavily shaded | Adjust the chart figure by 10 percent up or down before settling on a capacity. |
| If the unit will go in a kitchen | Add 4,000 BTUs to the chart figure, as ENERGY STAR advises, then round down to the nearest thousand. |
| If two models share the same capacity | Use efficiency and features to separate them. Read CEER explained and the window AC buying checklist. |
Questions people ask
Is a higher BTU figure always better?
No. ENERGY STAR warns that an oversized unit cools a room before it has removed the humidity, which leaves the space feeling damp and clammy. A capacity that matches the room, with the adjustments for sun, shade, people and cooking, is the better target.
Why is the SACC figure lower than the BTU figure on a portable?
The two numbers come from different procedures. The BTU figure is the capacity the maker states, while SACC is a seasonally adjusted cooling capacity determined under appendix CC of subpart B of the federal standard for portables. They are separate measurements, not a large number and a smaller version of it.
Can a portable's BTU rating be converted into SACC?
No. There is no conversion and no way to estimate one figure from the other. A portable is compared on its SACC figure, and a listing without one cannot be placed against portables that have one.
Which figure applies to a window air conditioner?
The BTU figure. Window units sit in the window opening and their listings carry a BTU cooling capacity, so BTU is the basis for comparing them with each other and with the ENERGY STAR capacity chart.
Does the ENERGY STAR capacity chart work for portables?
The chart's capacity column is stated in BTUs per hour, so it lines up directly with a window unit's figure. For a portable, look at the SACC figure and at the coverage area the maker states, such as the 500 sq ft Whynter lists for the ARC-1280MX and ARC-1280WX.
Does a higher SACC mean a more efficient unit?
Not by itself. Capacity and efficiency are separate ratings. Efficiency for room air conditioners is expressed as CEER, and the two figures should be read side by side when you compare models.
Where does the room size figure come from?
From the maker's listing. When a listing states a coverage area in sq ft, that is the maker's own figure for the model, and it is worth checking against the ENERGY STAR chart for the same floor area.