How to choose an air conditioner: 7 things that decide it

1. Floor area and capacity (BTU)

Capacity is measured in BTU/h. For a room with a standard ceiling (up to 2.7 m) and normal insulation the rule of thumb is 9000 BTU for 15–25 m², 12000 BTU for 25–35 m², 18000 BTU for 40–55 m² and 24000 BTU for 60–75 m². South-facing rooms, large windows, attics and kitchens need one step up.

An undersized unit runs flat out and still falls short; an oversized one costs more and short-cycles in cooling mode, which hurts dehumidification. Inverters forgive a small margin, so when in doubt take the next size up.

2. Inverter compressor

An inverter varies compressor speed instead of switching it on and off. That means a steady temperature, lower electricity use in daily operation and quieter running. For a unit used regularly – especially for heating – inverter is the only sensible choice.

3. Energy class, SEER and SCOP

The label shows classes for cooling and heating, backed by SEER (seasonal cooling efficiency) and SCOP (heating). The higher, the less electricity for the same comfort. For a room conditioned daily, A++ or A+++ pays back through the bills; for a rarely used guest room A+ is enough.

4. Noise

For a bedroom look at the indoor unit minimum noise level – good models drop below 20 dB in quiet mode, practically inaudible. For a living room 25–30 dB is fine. The outdoor unit matters too if it will sit under a neighbour's window.

5. Winter heating

Modern inverter units heat efficiently down to about −15°C, some ranges to −25°C. If the unit will be the main heating, check the heating output at −7°C (not just the nominal figure) and the SCOP. For a house with radiators or underfloor heating an air-to-water heat pump may be the better choice.

6. Features people actually use

Wi-Fi control is useful if you want to switch the unit on before you get home. Purifying filters and ionisation help with allergies. Presence sensors, self-cleaning and "follow me" remotes are nice extras, not deciding factors.

7. Installation is part of the product

Even the best unit performs badly on a poorly evacuated line or in the wrong spot. Installation should be done by a certified technician with a vacuum pump, to the manufacturer's instructions – that also keeps the warranty. Installation prices are public and depend on capacity.