The cooling system of a portable air con works due to using a refrigerant gas undergoing changes between gaseous and liquid state, after being compressed inside the air conditioner itself and then expanded in the evaporator. Here, the air cools down until it reaches the desired temperature. At this point, it is conveyed into the room through a ventilation system, while the hot air, which is part of this thermo-dynamic cycle, is expelled outside the room through a special flexible pipe.
The drop in room temperature is achieved mainly by the ratio between the size of the room and the power of the appliance, measured in BTUs (short for British Thermal Units per hour). For adequate cooling in a room with a ceiling of up to 3 metres, you’ll need an output of 340 BTUs per square metre.
Therefore, for a 30 square metres room, choose a portable air conditioner capable of developing around 10000 BTUs. However, there are additional factors to consider: very high ceilings, southern exposure of the room, heat-generating appliances, and high average temperatures in the area add to count, meaning you would need additional BTUs.