How to choose a portable air conditioner

The most common mistake is buying on price rather than power. Cooling is measured in BTU per hour (British Thermal Units) or in kilowatts: the higher the figure, the larger the space it can cool. Buy too little and the unit runs flat out without ever reaching temperature; buy too much and it short-cycles, dehumidifies poorly and wears the compressor.

As a rough rule for a UK room with normal insulation, allow around 340 BTU per m² (roughly 30 BTU per sq ft). A portable needs a touch more than a fixed wall split because the exhaust hose costs some efficiency.

• Up to 15 m² (bedroom, study): 7,000-9,000 BTU, about 2.0-2.6 kW.
• 15-25 m² (medium room): 9,000-12,000 BTU, about 2.6-3.5 kW.
• 25-35 m² (living room): 12,000 BTU (3.5 kW) — the most popular size.
• Over 35 m² or high ceilings: 14,000-18,000 BTU, or a second unit.
• Add 10-20% if the room gets strong afternoon sun or has large windows.

A portable lives inside the room, so noise matters as much as power. The number to look for is the sound level in dB(A): below 40 dB(A) is comfortable for sleeping, 45-55 dB(A) is a noticeable background hum, and above 60 dB(A) it intrudes on a conversation or the television.

Monobloc units (one box) typically sit at 50-65 dB(A) because the compressor is inside the room with you. A split-type portable such as the Midea PortaSplit — sold in the UK as the electriQ PortaSplit — drops to around 39 dB(A) in quiet mode because the compressor rests on the window, outside the living space. If the unit is for a bedroom, this figure should weigh more heavily than any other.

A 3.5 kW portable draws roughly 1.0-1.3 kW of electricity when cooling hard. At a typical UK unit price of about 28p per kWh, three hours a day through summer works out at around £25-£35 a month, depending on your tariff and how long it runs.

Check the cooling energy rating (ideally A or better on the current scale), the SEER — the higher, the more efficient — and whether it has an inverter compressor, which avoids start-up spikes and smooths the bill. An efficient unit costs a little more up front but earns it back if you run it for long stretches.

Portables need no installer and no fixed outdoor unit: they push warm air out through a hose to the window. The difference is in how they do it. A monobloc uses a single flexible hose; a split-type portable separates the block that goes to the window from the part that cools, which improves performance and cuts noise.

Before buying, check the window kit fits your window type (tilt-and-turn, sliding or sash). This is the point that causes the most grief and that almost nobody checks until the unit is already in the flat.