It's a tempting idea: buy one mobile air conditioner and cool the whole flat. Unfortunately, the physics doesn't cooperate, and understanding why will save you money and frustration. Here's the honest answer and the strategy that actually works.
The short answer
A single mobile air conditioner is designed to cool one room, and it can't effectively cool an entire apartment. Each unit has a cooling capacity (BTU) matched to a certain room size; spread that capacity across multiple rooms connected by open doors and hallways, and the cool air disperses faster than the unit can replace it. The result is a flat that's slightly less hot everywhere but properly cool nowhere.
Why open-plan cooling fails
To cool a space, an AC has to remove heat faster than the space gains it. A closed room is a contained job it can win. But open the doors to "let the cool air spread," and you've added the volume of the whole flat plus a constant inflow of warm air from unshaded rooms. The unit now fights an impossible battle, running flat out while the target room never gets comfortable and your electricity bill climbs.
The single-room strategy that works
The effective approach is to cool one room properly and treat it as your retreat:
- Pick the room that matters — the bedroom at night, the living room or home office by day.
- Close the door to contain the cooled space.
- Seal and shade the room so the unit isn't fighting heat and leaking cool air.
- Size the unit to that room so it can comfortably keep up.
One genuinely cool room beats a whole flat that's merely less hot.
Cooling different rooms at different times
Because a mobile AC is portable, you can move it to wherever you need it: the office during the workday, the bedroom before sleep. For many people that flexibility is enough — you rarely need every room cool at once. If you truly need multiple rooms cooled simultaneously, that's a job for one unit per room or a fixed system, not a single portable.
Reduce the load on the whole flat too
Even with a single-room strategy, whole-flat habits help: shade all the sunny windows from outside and ventilate at night so the rest of the apartment doesn't bake and bleed heat into your cool room. The cooler the surrounding flat, the easier your one unit's job.
The takeaway
Don't buy a mobile AC expecting whole-apartment cooling — buy it to make one room genuinely comfortable, and size it to that room. Check which correctly-sized units are in stock near you and reserve one for the room that matters most.