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The short answer: size on roughly 20 BTU per square foot of SACC — the Department of Energy Seasonally Adjusted Cooling Capacity that appears on every portable AC sold in the US since 2017. That is about 6,000 SACC for 250 sq ft, 8,000 for 350 sq ft, 10,000 for 450 sq ft and 12,000 for 550 sq ft. Ignore the larger ASHRAE number in the marketing: the same machine can read 14,000 ASHRAE and 8,000 SACC. Add 10 percent for a sunny or top-floor room, and 4,000 BTU for a kitchen.
We do not physically handle the products we rank; picks come from aggregated owner reviews and expert consensus — see how we rank. Model shortlists are in best portable air conditioner.
How many BTU for my room size?
Measure length by width and read the SACC column. The 20 BTU per square foot figure matches the ENERGY STAR room air conditioner sizing guidance and assumes an 8 foot ceiling, average insulation and normal sun exposure. Then apply the adjustments — they are additive, and a west-facing top-floor bedroom with two people in it can easily need 30 percent more than the base figure. A unit that is 2,000 SACC short will run continuously and still lose ground on the hottest afternoons.
| Room size | Example | SACC needed | Often listed as (ASHRAE) |
|---|---|---|---|
| 100 – 150 sq ft | Small bedroom, office | 5,000 | 8,000 – 9,000 |
| 150 – 250 sq ft | Standard bedroom | 6,000 | 10,000 |
| 250 – 350 sq ft | Primary bedroom, den | 8,000 | 12,000 – 14,000 |
| 350 – 450 sq ft | Living room | 10,000 | 14,000 – 16,000 |
| 450 – 550 sq ft | Large living room | 12,000 | 17,000 – 18,000 |
| 550 – 700 sq ft | Open plan | 14,000 | 20,000+ |
- Very sunny room: add 10 percent.
- Heavily shaded room: subtract 10 percent.
- More than two people regularly: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
- Top floor under an attic: add 10 percent.
Why does the SACC number matter so much?
Because a single-hose portable creates its own problem. It blows room air across the condenser and out the window, and every cubic foot leaving has to be replaced by unconditioned air pulled in through gaps around doors, windows and outlets. The DOE test procedure introduced in 2017 measures that infiltration penalty along with duct heat gain, which is why SACC figures land well below the old ASHRAE laboratory ratings — commonly 55 to 70 percent of them. Two units advertising 14,000 BTU can differ by thousands of BTU of real performance.
Dual-hose designs take condenser air from outdoors instead of from the room, which removes most of the infiltration penalty and shows up directly in a higher SACC for the same physical size. The trade-offs are compared in single vs dual hose portable air conditioner.
Would a window unit be a better buy?
For most rooms with a suitable window, yes. A window air conditioner of the same nominal capacity cools more effectively and uses less electricity, because the condenser sits outside and there is no exhaust duct radiating heat back into the room. Portables win on three counts only: casement or sliding windows that will not take a window unit, HOA or lease rules that prohibit them, and the need to move the unit between rooms. Weigh that honestly before buying, because the running-cost gap is real.
The head-to-head is in portable AC vs window AC, venting requirements in do portable air conditioners have to be vented out a window, and electricity draw in how many watts does a portable air conditioner use. Bedroom-specific picks are in best portable AC for a bedroom. More in the heating and cooling hub.