How to size a portable AC for your room

Updated June 2026Independently researchedNo paid placement.

The standard BTU rule of thumb: what it is and why it’s incomplete

Most buying guides start with the same advice: multiply your room’s square footage by 20 to get the minimum BTU you need. A 200-square-foot room? That’s 4,000 nominal BTU. It sounds simple, but it leaves out a critical detail: portable air conditioners lose a big chunk of their cooling capacity because they have to exhaust hot air outside through a single hose. That exhaust heat doesn’t just disappear – it gets drawn back into the room from gaps around the window kit, plus the air conditioner itself puts out additional heat from its compressor and fan motor. The result: the actual cooling power that lands in your room is usually far lower than the shiny BTU number on the box. The 20 BTU rule works okay as a rough upper limit for nom-inal BTUs, but it can steer you wrong if you don’t check the SACC rating. Many shoppers end up with a unit that runs constantly and never quite cools the room because they relied on nominal BTU alone.

What is SACC and why it’s the honest number to compare

SACC stands for Seasonally Adjusted Cooling Capacity, and it was created by the U.S. Department of Energy to fix exactly this problem. It measures how much heat the unit actually removes from the room over a typical cooling season, factoring in the waste heat lost through the exhaust hose. Because of that built-in exhaust loss, SACC is almost always lower than the nominal BTU. For a single-hose portable model, SACC typically runs about 50–70% of the nominal BTU; for a dual-hose unit, it’s closer to 80–90%. Manufacturers still list the high nominal number on the front of the box because it’s what shoppers recognize, but the SACC figure is buried in the fine print or in the owner’s manual. Whenever you compare two models, ignore the headline BTU and compare SACC side by side. That’s the number that tells you how well the unit will actually cool a room of a given size.

Adjustment factors: when you need more cooling than the baseline

The 20-BTU-per-square-foot baseline assumes an average room with moderate sun exposure and standard 8-foot ceilings. Real life is messier. If your room has large south- or west-facing windows that get direct afternoon sun, add 10–20% to your target BTUs. High ceilings above 9 feet? The extra volume means the unit has to work harder – add roughly 5% for every extra foot of ceiling height. Very humid climates also tax a portable AC because the unit must remove latent heat (moisture) as well as sensible heat. If you live in a region like the Gulf Coast or the Midwest in summer, knock at least 10% more capacity. Kitchens produce extra heat from appliances, so bump the number up another 10–15% if the unit will sit in an open kitchen. These adjustments mean your final target SACC could be 30–50% higher than the baseline rule, especially in difficult rooms. Ignoring them is a common reason a “sufficiently sized” unit still leaves you sweating.

The single biggest mistake buyers make: going too small because they used nominal BTU

The most frequent complaint in owner reviews and on forums is: “My unit runs all day but never reaches the set temperature.” The culprit is almost always a unit chosen solely by nominal BTU. Someone sees a 10,000 BTU model that claims coverage up to 300 square feet, but the SACC might be only 6,000 BTU. That SACC is probably barely enough for a shady, 200-square-foot room, let alone a 300-square-foot space with a sunny window. Buying too small forces the compressor to run continuously, driving up electricity bills and shortening the unit’s life. It also struggles to dehumidify, leaving the room clammy. The fix is simple: find the SACC rating, and use that to size up. When in doubt, bump up one size – a slightly oversized portable AC will cycle on and off more efficiently and handle humidity better than an undersized one that never stops.

How to read a spec sheet to find the SACC figure

SACC is almost never printed on the front of the box because it’s the unflattering real number. Instead, look on the Energy Guide label (the yellow tag) or in the product’s detailed specs section on the retailer page. It’s often listed as “Cooling Capacity (SACC)” in BTUs per hour. If you can’t find it, search for the “DOE-rated” cooling capacity. Some manufacturers also list “ASHRAE BTU” (the old, inflated number) alongside SACC. Ignore the ASHRAE number for sizing. If a spec sheet only shows one BTU figure and doesn’t mention SACC, assume that figure is the nominal BTU and treat it with caution – look up the unit’s manual online to confirm. On amazon or big-box store listings, scroll to the “Technical Details” or “Specifications” section. There you’ll often find a line like “Cooling Capacity (BTU) - (SACC): 8,000”. That’s your number. If it says “Cooling Capacity: 12,000 BTU” with no SACC, it’s almost certainly the nominal figure.

Simple room-size lookup table for common scenarios

Using a nominal BTU baseline of 20 per square foot, here’s what a sensible SACC target looks like for typical rooms (8-foot ceilings, moderate sun):

• Up to 150 sq ft → look for a SACC around 5,000 to 7,000 BTU. Those are often nominal 8,000 to 10,000 BTU units. • 150 to 250 sq ft → target SACC 7,000 to 9,000 BTU, typically from a unit with a nominal rating of 10,000 to 12,000 BTU. • 250 to 350 sq ft → aim for SACC 9,000 to 12,000 BTU; you’ll see units with nominal 12,000 to 14,000 BTU. • 350 to 450 sq ft → SACC of 12,000 to 14,000 BTU or higher, from nominal 14,000 to 16,000 BTU models. If your room has strong sun or high ceilings, shift up one size bracket. For very humid climates, prefer the higher end of the SACC range. For dual-hose models, you can lean slightly lower because they lose less capacity to exhaust heat, but still use SACC as your guide.

Frequently asked questions