8 min read · Updated 2 July 2026
Air Conditioning Buying Guide: How to Choose the Right Unit
Portable, split or evaporative? BTU, energy ratings and running costs explained in plain English so you buy the right AC first time.
British summers are getting hotter, and air conditioning has gone from a luxury to something many UK homes now genuinely want. But the choice of units is bewildering, and the wrong one wastes money and disappoints. This guide walks you through everything that actually matters.
1. Pick the right type
The type of unit is the biggest decision you'll make. Each suits a different situation:
- Portable AC — no installation, wheel it between rooms, vent through a window. Best for renters and single-room cooling.
- Split system — permanent wall-mounted unit, quiet and efficient, heats in winter too. Best for a long-term solution if you own your home.
- Window unit — sits in a sash window, frees up floor space, but needs a compatible window.
- Evaporative cooler — cheap to run, no hose, but adds humidity and only works well in dry, ventilated spaces.
2. Match the BTU to your room
BTU (British Thermal Units) measures cooling power. Too few and the unit runs flat-out without ever cooling the room; too many and you waste money and get clammy, under-dehumidified air. As a rough guide:
- Up to 18 m² (small bedroom / office): 7,000–9,000 BTU
- 18–28 m² (average living room): 9,000–12,000 BTU
- 28–42 m² (large or open-plan): 12,000–15,000 BTU
- Add ~10% if the room is very sunny, or has lots of glass or electronics.
3. Check the energy rating and running cost
AC can be thirsty. Look for an A or better energy rating, and check the wattage — a typical 9,000 BTU portable draws around 1kW, costing roughly 25–30p an hour to run at current UK electricity prices. Split-system inverters are far more efficient over a season than portables.
4. Don't ignore noise
Noise is the number-one complaint about portable AC. Anything under 50 dB is comfortable for a bedroom; 45 dB or lower is excellent. Split systems are far quieter indoors (often under 25 dB) because the noisy compressor sits outside.
5. Consider year-round value
Many split systems and some portables reverse to provide heating, which can be cheaper than gas central heating for a single room. A dehumidify mode is also useful in a damp British winter. If you'll only use the unit a few weeks a year, a mid-range portable makes more sense than an expensive install.