Choosing the right portable air conditioner is simple once you know your room size, how much sunlight it gets, and the type of window you need to vent through. A correctly sized unit cools more effectively, runs more efficiently and is less likely to struggle on hot days.
1. Measure your room
First, measure the length and width of the room in metres:
For example, a room measuring 4 m by 5 m is:
4x5=20m²
Use the table below as a practical starting point. Always check the stated recommended room size on the individual product page, as performance varies by insulation, ceiling height and sunlight.
| Room size | Typical room | Recommended portable AC size |
|---|---|---|
| Up to 15 m² | Small bedroom, study, box room | 7,000–8,000 BTU |
| 15–20 m² | Bedroom, home office | 8,000–9,000 BTU |
| 20–25 m² | Large bedroom, small lounge | 9,000–10,000 BTU |
| 25–30 m² | Lounge, larger office | 10,000–12,000 BTU |
| 30–35 m² | Large lounge or open space | 12,000–14,000 BTU |
| Over 35 m² | Large/open-plan room | Consider a high-capacity model or more than one unit |
BTU stands for British Thermal Unit: it describes how much heat an air conditioner can remove. Bigger rooms need a higher BTU rating.
2. Allow for heat and sunlight
Room size is the starting point, not the whole story. Consider choosing the next capacity up if your room has any of the following:
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Strong afternoon sun or large south-facing windows.
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Poor insulation, draughty doors or older glazing.
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High ceilings.
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Several people regularly using the room.
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Heat-producing equipment, such as computers, gaming PCs, cooking appliances or dryers.
As a guide, extra sun can justify around 10% more cooling capacity, while rooms regularly occupied by more than two people need additional capacity. A unit that is too small may run constantly without reaching a comfortable temperature; one that is properly matched to the space will cool more reliably.
3. Understand BTU ratings
Some portable air conditioners show more than one BTU figure. Where available, prioritise the stated room-size recommendation and the unit’s SACC/DOE cooling rating, rather than comparing a large headline BTU number alone. These figures better reflect portable-air-conditioner performance in real operating conditions.
If you are between two sizes, choose the larger option for a hot, sunny or poorly insulated room. For a shaded, well-insulated bedroom, the lower option may be sufficient.
4. Choose the right window seal kit
A portable air conditioner must send warm air outside through its exhaust hose. The window seal kit closes the remaining gap around the hose, helping stop warm outdoor air from coming back into the room. A good seal improves cooling performance and helps reduce wasted energy.
Before ordering, measure:
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The width of a sliding or sash-window opening.
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The height of the opening for vertical windows.
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The maximum and minimum adjustment range of the window kit.
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The hose diameter required by your chosen air conditioner.
Choose a kit that covers your full open-window measurement. Do not select based only on the glass size: measure the actual opening where the panel or fabric seal will sit.
5. Which window seal do you need?
| Window type | Best option | What to check |
|---|---|---|
| Sliding sash window | Adjustable rigid window-kit panel | Measure the full horizontal opening width |
| Vertical sliding window | Adjustable rigid window-kit panel | Measure the full vertical opening height |
| Casement window | Zipped fabric window seal | Measure the full window-frame perimeter |
| Tilt-and-turn window | Zipped fabric window seal | Check that the fabric can seal around the opening without blocking the hose |
| Sliding patio door | Extended door/window-kit panel | Measure the door opening height and confirm panel compatibility |
Rigid panels are usually best for sliding windows because they create a neat, stable route for the exhaust hose. Fabric seals are often the more flexible choice for outward-opening casement and tilt-and-turn windows.
6. Install for best performance
Place the unit close enough to the window for the hose to reach comfortably, without stretching, crushing or making sharp bends. Restricted airflow through the hose can reduce cooling performance.
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Keep the exhaust hose as short and straight as possible.
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Ensure the window panel or fabric seal has no obvious gaps.
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Leave space around the unit for air intake and circulation.
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Keep doors and windows closed while cooling.
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Avoid placing the unit directly in intense sun where possible.
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Empty any condensate tank if your model requires manual draining.
A portable air conditioner works best when it is cooling one closed room, rather than trying to cool an entire home or a large open-plan area.
Quick checklist
Before buying, make sure you can answer these five points:
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What is the room area in m²?
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Is the room sunny, poorly insulated or unusually warm?
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What BTU range is suitable for that space?
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What type of window or door will vent the exhaust hose?
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What are the exact measurements of the window opening?
Start by measuring the length and width of the room you want to cool—what measurements do you get? Once you have your measurements ready you can find your perfect portable air conditioner and window seal kit by pressing on the links.