Window vs. Portable AC: Why the Same BTU Cools Differently

Every room air conditioner has a cooling capacity measured under the ASHRAE test standard: the unit sits in a test chamber at 80°F and 50% humidity indoors, 95°F outdoors, and the heat it removes is measured. That is the big number on the front of the box, 8,000 or 12,000 or 14,000 BTU. For a window unit it is a fair description of what you get, because the hot half of the machine is hanging outside.

For a portable unit, the whole machine is inside the room, and the ASHRAE test ignores the consequences of that. So the DOE developed a new test procedure, finalised in 2016 and required in manufacturers' capacity claims from 2017, that produces a Seasonally Adjusted Cooling Capacity, or SACC, which now also appears on the yellow EnergyGuide label. SACC counts the heat that leaks back into the room from the hose and the case, the hot outside air that gets pulled in to replace what the unit exhausts, and weights the result over a realistic mix of outdoor temperatures. For a typical single-hose portable, SACC comes out at 60–70% of the ASHRAE number. Dual-hose units do a little better.

The label on a "14,000 BTU" portable will therefore say something like "9,500 BTU (SACC)" in smaller print, and the SACC is the number that describes what the room will feel.

A single-hose portable draws air from the room, blows some of it over the cold coil back into the room, and blows the rest over the hot condenser coil and out the window through the hose. That second stream is room air you already paid to cool, leaving the house at a rate of 150–300 cubic feet per minute. The room is now at a slight negative pressure, so an equal volume of outdoor air, at 95°F and humid, is pulled in through every gap around the doors, windows, and outlets. On a hot day the unit can spend a third of its capacity cooling the air it just sucked in.

Then there is the hose. It carries 130°F air and is usually a thin plastic accordion tube running five feet across the room. It radiates heat the whole way, and the longer and more uncoiled it is, the worse. The compressor and its motor also sit inside the room, adding a few hundred BTU of their own. A window unit has none of these problems: the condenser, compressor, and exhaust are all outside the wall, and the only thing entering the room is cold air.

Work out the room's cooling load first, the same way regardless of unit type: about 20 BTU per square foot, adjusted for sun, insulation, climate, ceiling height, people, and kitchen. Our sizing guide walks through it and the calculator does it in one step. Then match a window unit to that number directly, and match a portable by its SACC, not its ASHRAE rating. In practice that means a portable roughly one to two classes larger than the window unit you would otherwise buy.

For a 300 sq ft bedroom with a 6,000 BTU load, that is a 6,000 BTU window unit or a portable sold as 10,000 BTU ASHRAE with a SACC of at least 6,000. Notice that the "bigger" portable is not actually oversized; its real output is the same as the window unit's. Buying a portable by its ASHRAE number instead (an 8,000 for this room, say) gets you about 5,300 BTU of real cooling, and a room that never quite gets comfortable in the afternoon. Buying a much larger one to compensate brings the usual oversizing problems, since the SACC figure already accounts for the losses.

The rooms above 450 sq ft in the table have no good portable answer, because the largest common portables (14,000 ASHRAE) deliver about 9,500–10,000 SACC. For a 500 sq ft room or larger you are into two-portable territory, a large window unit, or a mini-split.

A dual-hose portable pulls outside air in through one hose to cool the condenser and sends it back out the other. Room air stays in the room, so there is no negative pressure and no hot infiltration. The hoses still radiate heat and the compressor is still inside, so a dual-hose unit is not as good as a window unit, but its SACC typically lands at 70–80% of ASHRAE instead of 60–70%. If you must go portable and have the window space for the double bracket, dual-hose is worth the premium.

On the window side, the newest development is the inverter or U-shaped window unit, which uses a variable-speed compressor and straddles the sash so the window can close down onto it. These are dramatically quieter than a conventional window unit (the compressor is on the outside of the closed window) and, like a mini-split, can throttle down to a fraction of full output instead of cycling. If noise is what pushed you toward a portable, an inverter window unit is usually the better fix. See the mini-split guide for why variable speed matters so much for comfort.

Because a portable delivers less cooling per watt, it costs more to run for the same comfort. A 6,000 BTU window unit with a CEER of 12 draws about 500 W. A 10,000 BTU ASHRAE portable that delivers a similar 6,500 SACC draws around 1,000–1,100 W. Over a summer of eight hours a day for 100 days at $0.16 per kWh, that is roughly $65 for the window unit and $135–$145 for the portable, and the gap grows in hotter climates where the infiltration penalty is worse. Over a five-year life the running-cost difference alone exceeds the purchase price of the window unit.

If you have a double-hung window and are allowed to put something in it, a window unit is cheaper to buy, cheaper to run, quieter for a given capacity, and takes up no floor space. That covers most bedrooms, offices, and living rooms in most houses, and it is what the bedroom and home office pages assume.

A portable earns its place when a window unit is impossible or forbidden: casement or crank windows, sliding windows too narrow for a bracket, a lease or HOA that bans anything visible from outside, a window that is the only fire exit, or a room you need to cool this week and next month somewhere else. Dorm rooms are the classic case; the dorm room page covers the small-room end. In all of those cases, buy by SACC, keep the hose as short and straight as you can, seal the window kit carefully with foam tape, and prefer dual-hose if the budget allows.

If you own the place and expect to cool the room for more than a couple of summers, a single-zone mini-split beats both. It costs more to install but runs at a fraction of the electricity, makes almost no noise indoors, and heats in winter.

Run your room through the free BTU calculatorto get the cooling load. If you are buying a window unit, buy that size. If you are buying a portable, find the SACC figure on the EnergyGuide label (or in the specifications, sometimes listed as "DOE" or "SACC" BTU) and choose the smallest unit whose SACC meets your load; as a rough guide, divide your load by 0.65 and round up to the next class to get the ASHRAE size to look at, so a 6,000 BTU room points you at a portable sold as 10,000 BTU. If that lands above 14,000, a single portable is not going to do it, and it is time to look at a window unit, two portables, or a mini-split. The BTU to watts converter helps when a unit lists its capacity in watts, as many imported portables do.