Cooling
Window and portable AC sizing: BTU, SACC and the room
A window unit is sized from a chart; a portable unit is sized from a chart and then corrected for the cooling it loses to its own hose. Here is how both work, with the numbers.
One room, one unit
A room air conditioner cools one space, so it is sized for that space alone. The method most retailers and the federal ENERGY STAR program use starts from the floor area, reads a capacity off a chart, and then adjusts it for the sun, the ceiling, the number of people and whether the room is a kitchen. It is simpler than sizing a central system because there are no ducts and no other rooms to share the air with, but the same principle applies: the unit should match the heat coming into the room, not exceed it by a wide margin.
The ENERGY STAR chart
The chart gives a capacity in BTU per hour for each band of floor area. The window AC size calculator uses it as published, and the BTU per square foot table lists it alongside the whole-house figures.
| Room area up to | Capacity | Typical room |
|---|---|---|
| 150 sq ft | 5,000 BTU/hr | Small bedroom, home office |
| 250 sq ft | 6,000 BTU/hr | Bedroom |
| 300 sq ft | 7,000 BTU/hr | Large bedroom |
| 350 sq ft | 8,000 BTU/hr | Main bedroom |
| 400 sq ft | 9,000 BTU/hr | Small living room |
| 450 sq ft | 10,000 BTU/hr | Living room |
| 550 sq ft | 12,000 BTU/hr | Large living room |
| 700 sq ft | 14,000 BTU/hr | Open living and dining |
| 1,000 sq ft | 18,000 BTU/hr | Small apartment |
| 1,500 sq ft | 24,000 BTU/hr | Large open plan |
The four adjustments
The chart describes a room with 8 ft ceilings, average sun and two people. Four adjustments, all from the ENERGY STAR guidance except the ceiling, which is a common rule, move it:
The kitchen allowance is the one people forget, and it is large. A 200 sq ft kitchen needs 6,000 BTU per hour from the chart and 10,000 with the allowance. Rooms with a lot of electronics, such as a home office with a gaming PC and two monitors, need their equipment heat added too; the equipment heat load calculator converts watts into BTU per hour.
Window units, worked
A 300 sq ft bedroom
A 15 × 20 ft bedroom with 8 ft ceilings, average sun and two occupants.
- Chart value for 300 sq ft: 7,000 BTU/hr; no adjustments
- Unit: 8,000 BTU/hr, the next common size; 6,000 would be 1,000 short
- Electrical draw at a CEER of 11: 8,000 ÷ 11 = 727 W, 6.3 A at 115 V
If the room faces north or is shaded by trees, the shade factor brings the need to 6,300 BTU per hour, and the 6,000 unit becomes the better buy: cheaper, quieter, and running long enough to keep the room dry.
At the large end, the calculator gives a 550 sq ft sunny kitchen and dining room with three people 12,000 × 1.1 + 600 + 4,000 = 17,800 BTU per hour, so an 18,000 BTU unit. Units above 15,000 BTU per hour usually run on 230 V in the US and need a dedicated outlet, and a 12,000 BTU unit on 115 V draws enough to deserve its own circuit. The cost of a new circuit belongs in the price comparison with a mini-split.
Why portable units need a correction
A portable air conditioner sits inside the room it cools, and so does its compressor and its hot condenser coil. It cools the condenser with room air and blows that air out of a window through a hose. A single-hose unit therefore pushes air out of the room continuously, and the same amount of warm outdoor air leaks back in through gaps around doors and windows, which the unit then has to cool. The hose itself is warm and radiates heat back into the room.
For years portable units were labelled with the same ASHRAE capacity figure as window units, measured in a way that ignored these losses. Since 2017 the Department of Energy has required a second figure, SACC (seasonally adjusted cooling capacity), that accounts for them. On the box, the SACC is typically a little over half the ASHRAE number for a single-hose unit and around two thirds for a dual-hose unit, which brings its own outdoor air in for the condenser through a second hose.
The portable AC size calculator sizes the room exactly as for a window unit, treats that figure as the SACC the unit must deliver, and divides by 0.55 or 0.65 to find the ASHRAE label to look for.
A 250 sq ft bedroom with a single-hose unit
- Chart value: 6,000 BTU/hr, the real cooling needed (SACC)
- ASHRAE label needed: 6,000 ÷ 0.55 = 10,909 BTU/hr
- Unit: 12,000 BTU/hr ASHRAE, which delivers about 6,600 BTU/hr SACC
The box says 12,000, and the room gets roughly what a 6,000 BTU window unit would give. A 10,000 BTU dual-hose unit would deliver about 6,500 SACC and do the same job.
Where portable units run out
The largest common portable units are about 14,000 BTU per hour ASHRAE, which is roughly 8,000 to 10,000 SACC. A sunny 400 sq ft living room needs 9,000 × 1.1 = 9,900 BTU per hour of real cooling, which is past what a single-hose unit can do and at the edge for a dual-hose one. The calculator says so rather than naming a size, and lists the options: two smaller units, a 10,000 BTU window unit, or a mini-split head.
Window, portable or mini-split?
| Window unit | Portable unit | Single-zone mini-split | |
|---|---|---|---|
| Cooling per dollar | Best | Worst | Lowest running cost |
| Installation | Fits a sash window, seasonal | Hose through a window kit | Professional, permanent |
| Noise in the room | Moderate | Highest (compressor inside) | Lowest |
| Efficiency | CEER 10 to 12 typical | Lower, by the SACC losses | SEER2 18 to 30 |
| Heats in winter | Some models | Some models | Yes, as a heat pump |
A window unit gives the most cooling for the money where a sash window allows it. A portable unit is the choice when window units are not allowed, at the cost of noise and efficiency. A mini-split costs more to install and less to run, heats as well as cools, and is the right answer for a room you use every day all summer. The guide to sizing mini-split zones covers that route.
Bigger is not better here either
A room unit that is much too large cools the room in minutes and switches off before it has pulled much water out of the air, the same short-cycling problem described in the guide to oversized systems. In a bedroom that means a cold, clammy room and a compressor that clicks on and off all night. Size to the chart and its adjustments, and take the next size up only when the result is clearly above a size, not when it is within a few hundred BTU. For whole-house central systems the guide on how to size an air conditioner covers the larger method.