The table most people are looking for
| Room size | Standard room | Hot room (top floor, west-facing, lots of glass) |
|---|---|---|
| Up to 15 m² | 7000–9000 BTU | 9000 BTU |
| 15–20 m² (typical bedroom) | 9000 BTU | 12000 BTU |
| 20–28 m² (living room, studio) | 12000 BTU | 14000 BTU or two units |
| 28–35 m² | 14000–16000 BTU | Two units, or one per zone |
| Whole apartment | No single portable unit does this. One unit cools one room properly, with the door closed — that is the honest limit of the format. | |
BTU is simply a measure of how much heat the machine can remove per hour. More BTU means faster cooling of a bigger volume, and also a heavier, louder, thirstier machine — so the goal is a match, not a maximum.
The five things that change the answer in Lisbon
Square metres are the starting point, not the whole calculation. In practice these are the factors that actually push a room up a size:
- Top floor or under the roof. Heat accumulates all day in the slab above you. This is the single biggest adjustment, and it is common in Lisbon's older buildings.
- West or south-west facing. Afternoon sun on the façade until 20:00 in July does more than any other orientation.
- Large windows or a glass door. Glazing is where the heat comes in; a wall of glass can add the equivalent of several square metres.
- Ceiling height. The table assumes about 2.5 m. Older Lisbon flats with 3 m or higher ceilings hold considerably more air to cool.
- People and equipment. Two adults, a laptop and a TV all add heat. A home office or a bedroom for three shifts the number upward.
Insulation and shutters cut the other way. A well-shuttered room with the blinds closed since morning needs less than the same room left in the sun — which is why our heatwave preparation steps are worth doing before you worry about capacity.
Why oversizing backfires
An over-powered unit cools the air fast, hits the thermostat, and shuts off — over and over. Those short cycles never run long enough to pull humidity out, so the room ends up cold and clammy rather than comfortable, and the compressor takes the wear. In coastal Lisbon, where humidity is a big part of why 28°C feels unbearable, correctly sized beats oversized nearly every time.
One extra rule for portable units
A portable air conditioner sits inside the room it is cooling, so part of its own heat stays with you and the exhaust hose radiates a little warmth. In practice, treat a portable unit's rating as slightly optimistic compared with a wall split of the same BTU — and be strict about sealing the window. An unsealed opening lets hot air pour straight back in and can cost you a third of the cooling, which no amount of extra BTU will fix. That is why a proper window sealing kit comes with every unit we deliver.
Not sure? Tell us the room
You do not have to get this right on your own. Send us the room size, the floor and which way the windows face, and we will tell you which unit fits — including when the honest answer is that you need two. We rent 9000 and 12000 BTU portable units across Lisbon and Greater Lisbon at €40 a day, €210 a week, €350 for two weeks or €450 a month, plus one flat €25 delivery, setup and pickup fee and a refundable €100 deposit. Same-day delivery when you confirm before 16:00.
Thinking of buying instead? Our portable air conditioner comparison for Portugal lists the models worth the money at each BTU rating, and you can rent a portable air conditioner in Lisbon to try a size before committing.
Quick answers
How many m² does 9000 BTU cover?
About 15–20 m² in a standard room with 2.5 m ceilings. Closer to 15 m² if the room is on the top floor, faces west, or has a lot of glazing.
How many m² does 12000 BTU cover?
About 20–28 m², so a living room, a studio, or a large or particularly hot bedroom.
Is 12000 BTU always better than 9000?
No. In a 16 m² bedroom a 12000 BTU unit short-cycles, dehumidifies poorly and is noisier and heavier for no benefit. Match the unit to the room instead.
Can one portable unit cool a whole apartment?
No. One unit cools one room properly with the door closed. For several rooms you need several units, and there is a discount from three.
How do I convert BTU to kW?
Divide by roughly 3410. So 9000 BTU is about 2.6 kW of cooling and 12000 BTU about 3.5 kW. That is cooling output, not electricity consumption — actual power draw is far lower, typically around 1 kW for a 9000 BTU unit.