Portable air conditioner size is measured in BTU, which tells you how much heat the unit can remove from a room. Too little BTU and the unit will run loudly without making much difference. Too much and you may pay more for a heavier machine that cycles awkwardly or feels excessive for the space.
For UK homes, the right size depends on floor area, ceiling height, sunlight, insulation, the number of people in the room and how well you can vent the hot exhaust air. The table below gives a practical starting point, then the adjustment notes help you avoid the common buying mistakes.
Contents
Key Takeaways
- Small bedrooms often need around 7,000 to 8,000 BTU.
- Medium living rooms often need around 9,000 to 12,000 BTU.
- Sunny, poorly insulated or open-plan rooms need more capacity.
- Venting the exhaust hose properly is as important as BTU size.
- A portable AC cools one room, not a whole house.
Portable Air Conditioner BTU Size Chart
Measure the room length and width in metres, multiply them to get square metres, then use this chart as a starting point. If the room has very high ceilings, strong afternoon sun or poor insulation, move up a band.
| Room Size | Typical BTU Range | Suitable For |
|---|---|---|
| Up to 12 m² | 5,000 to 7,000 BTU | Small bedroom or box room |
| 12 to 18 m² | 7,000 to 8,000 BTU | Bedroom or small office |
| 18 to 26 m² | 9,000 to 10,000 BTU | Medium bedroom or small living room |
| 26 to 35 m² | 10,000 to 12,000 BTU | Living room or larger bedroom |
| 35 to 45 m² | 12,000 to 14,000 BTU | Larger living space, if well sealed |
When To Size Up Or Down
Room area is only the first step. A south-facing room with large glazing may need more cooling than a bigger shaded room. Kitchens, loft rooms and upstairs bedrooms can also need more capacity because they gain heat from appliances, roof spaces or trapped warm air.
| Condition | Adjustment | Why It Matters |
|---|---|---|
| Strong sun or large windows | Move up one BTU band | Solar gain adds heat faster than the unit can remove it |
| Poor window seal around hose | Fix seal before upsizing | Hot air leaking back in wastes cooling |
| High ceiling | Consider more capacity | There is more air volume to cool |
| Several people or electronics | Add allowance | People and equipment add heat |
| Small shaded bedroom | Do not oversize heavily | Noise and bulk may matter more than extra capacity |
Why The Exhaust Hose Changes Everything
A portable air conditioner cools by moving heat from the room to outside. If the exhaust hose is badly fitted, crushed, too long or blowing into another indoor space, the unit will struggle no matter how many BTUs it claims.
Use a proper window seal kit where possible, keep the hose short and straight, and close doors to the room being cooled. Our guide on venting a portable air conditioner without a window covers awkward rooms in more detail.
Portable AC, Fan Or Air Cooler?
A fan moves air over your skin, so it can make you feel cooler but does not lower room temperature. An evaporative air cooler can help in some dry conditions but is less useful in humid weather and adds moisture to the air. A portable AC actually removes heat, which is why it needs a hose and uses more electricity.
UKHSA hot-weather advice recommends shading windows, opening them when outside air is cooler, and reducing heat sources indoors before relying only on appliances. A portable AC works best when those basics are already handled.
Running Cost, Noise And Practical Comfort
Higher BTU models usually draw more power and are heavier. A 12,000 BTU unit may cool faster than a 7,000 BTU unit, but it can also be louder and more awkward to move. Check power input in watts, not only BTU, because two machines with similar cooling output may use different amounts of electricity.
Noise matters most in bedrooms. A unit that is acceptable in a living room may be irritating beside a bed. Look for sleep mode, a timer and a realistic dB rating, but remember that compressor tone and fan pitch can be just as noticeable as the number.
Real-World Sizing Examples
Small Bedroom
A 10 to 12 m² bedroom may be comfortable with 7,000 BTU if the window is shaded and the hose is well sealed. If it is a loft room with afternoon sun, consider 8,000 to 9,000 BTU.
Home Office With Computers
A 14 m² office may need 8,000 to 9,000 BTU because screens, chargers and people add heat throughout the day. Door discipline matters because the unit is cooling the room, not the hallway.
Open-Plan Living Room
A 35 m² space may need 12,000 to 14,000 BTU, but a portable unit may still struggle if the area is open to stairs or a kitchen. In that case, zoning the room or using the unit near the seating area may be more realistic than expecting whole-space cooling.
Buying Checklist Before You Choose
- Measure the room in square metres.
- Check window type and whether a seal kit will fit.
- Choose BTU capacity for the hottest room you will use most.
- Check wattage, noise rating and unit weight.
- Confirm drainage requirements and whether the unit has a dehumidifier mode.
- Do not expect one portable AC to cool upstairs, downstairs and multiple rooms at once.
If your main issue is humidity rather than heat, our guide on what size dehumidifier you need may be more relevant. If you are choosing products, see our guide to the best portable air conditioners.
Mistakes That Make A Portable AC Feel Undersized
Many people buy a larger portable air conditioner when the real problem is the room setup. A single-hose unit expels hot air outdoors, which can pull replacement air in through gaps elsewhere. If the room is not reasonably sealed, the unit may be constantly fighting new warm air.
| Mistake | What Happens | Better Approach |
|---|---|---|
| Hose hanging out of an open window | Hot air leaks straight back in | Use a proper seal kit |
| Cooling with doors open | The unit tries to cool the hallway too | Cool one enclosed room |
| Starting too late at night | Walls and furniture have already stored heat | Pre-cool before peak heat if practical |
| Ignoring sunlight | Solar gain overwhelms the unit | Shade windows before the room heats up |
| Buying only by BTU | Noise, drainage and bulk are overlooked | Compare the whole specification |
Bedrooms Need A Different Decision Than Living Rooms
For a bedroom, noise and controllability matter almost as much as BTU. A slightly smaller unit that can run steadily, switch to a quieter night mode and fit neatly near the window may be more liveable than a more powerful machine that is too loud to sleep beside.
Open-Plan Rooms Need Realistic Expectations
Portable air conditioners are not central air conditioning systems. In an open-plan room, the unit will cool the area closest to it first and may struggle to reduce the temperature evenly across the whole space. If stairs are open to the room, warm air from upstairs can keep feeding the cooling load.
Expert Insights From Our Heating Engineers
Our engineers see many portable AC complaints caused by installation rather than the unit itself. A correctly sized machine with a poor hose seal can perform worse than a smaller unit installed carefully.
The best approach is to size for the main room, reduce solar gain, seal the exhaust properly and use the unit before the room overheats. Waiting until a bedroom is already baking at night makes any portable unit work harder and noisier.
Frequently Asked Questions
How Many BTU Do I Need For A Bedroom?
Many small to medium bedrooms need around 7,000 to 9,000 BTU, depending on room size, sunlight, ceiling height and insulation. A shaded 10 m² room may need less than a loft bedroom of the same size with strong afternoon sun.
Is 9,000 BTU Enough For A Living Room?
A 9,000 BTU portable air conditioner can work in a smaller living room, but larger or open-plan spaces often need 10,000 to 14,000 BTU. If the room opens onto stairs or a kitchen, cooling the whole area may be unrealistic.
Can A Portable Air Conditioner Cool More Than One Room?
Usually not well. Portable air conditioners are designed for one enclosed room with the exhaust vented outside. If doors are open, the unit has to cool a much larger air volume and warm replacement air enters from elsewhere.
Should I Buy A Bigger Portable AC Than The Chart Says?
Sizing up can help in sunny, hot or poorly insulated rooms, but do not jump too far without reason. Larger units cost more, take up more space and can be noisier. Fix window sealing and shading before relying only on extra BTU.
Does A Portable AC Need A Window?
It needs a way to exhaust hot air outside. A window is the simplest option, but some rooms use patio doors, wall vents or other arrangements. The important point is that the hot exhaust air must not leak back into the room.
What Is The Difference Between BTU And Watts?
BTU describes cooling capacity, while watts usually describe electrical power input. A higher BTU unit can remove more heat, but running cost depends on wattage, efficiency, thermostat settings, room heat gain and how long the compressor runs.
Summing Up
Choose portable air conditioner size by room area first, then adjust for sunlight, insulation, occupancy and the quality of the exhaust hose setup. A well-installed 9,000 BTU unit in the right room can outperform a larger machine with a poor window seal, so treat BTU and setup as one decision.
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