How big a room it will cool
Cooling output, how much heat a unit can pull out of a room, is quoted in kW (kilowatts), and most all-in-ones give about 2 to 3.5 kW, roughly 7000 to 12000 BTU (British Thermal Units). As a rough guide, 2 kW suits a bedroom or small office, 2.5 to 3 kW a room of around 25 square metres, and 3.5 kW around 30 square metres or a sunny room. For a bigger space, look at a split system instead.
Cooling only or heating too
Heat pump models run in reverse in winter, warming the room for around a third of the electricity a plain electric heater would use, while cooling-only versions suit rooms that already have heating. An inverter model slows its compressor down rather than switching it on and off, so it holds the temperature more steadily and runs more quietly. That matters more than usual here, because the compressor sits inside the room rather than in a box outside.
Two wall holes and clean filters
Fitting means coring two holes of around 160 to 200 millimetres through an outside wall, and heat pump models usually need a small drain for the water they collect in winter. In a listed building or conservation area, check whether the holes need consent before any drilling. Most draw roughly 0.5 to 1.4 kW while cooling, and the filters need washing every few weeks, with the outside grilles kept clear of leaves and nests.
