How to Choose an Air Conditioning Unit: A Complete Buying Guide (2025)

Jim Fellows

Looking to beat the heat at home, in your workshop, or garden office? Choosing the right air conditioner can make all the difference. In this expert guide from Garden-Review, we break down everything you need to know before buying an air conditioning unit — from room size and energy efficiency to types, features and maintenance tips.

✅ Quick Overview: What Type of Air Conditioner Do You Need?

Type	Best For	Pros	Cons
Portable Air Conditioners	Homes, flats, offices, garden rooms	Easy to install, moveable, affordable	Needs venting via a window
Monoblock AC Units	Workshops, garages, commercial spaces	Powerful cooling, heavy-duty build	Higher cost, less portable
Wall-Mounted Split Systems	Large homes or permanent installs	Quiet, efficient, sleek design	Professional installation
Evaporative Coolers	Well-ventilated spaces, dry climates	Low cost, no vent needed	Less effective in humid areas

1. Measure Your Room Size

Air conditioning units are rated in BTUs (British Thermal Units), which reflect their cooling capacity. The bigger your room, the higher the BTU you’ll need.

Room Size vs Cooling Guide:

Room Size (m²)	Suggested BTU Range
Up to 10 m²	5,000–7,000 BTU
10–20 m²	7,000–9,000 BTU
20–30 m²	9,000–12,000 BTU
30–50 m²	12,000–18,000 BTU
50 m²+	18,000 BTU+

Pro Tip: If your room gets direct sunlight, add around 10–20% extra capacity.

2. Understand the Key Features

Not all AC units are created equal. Here are the most useful features to look for in 2025:

3-in-1 or 4-in-1 Modes: Look for units that cool, fan and dehumidify — or even heat, like the Sealey SAC12000.
Remote Control & Timers: Ideal for convenience and energy savings.
Eco-Friendly Refrigerant (R290): Lower environmental impact.
Auto Restart: Handy for power cuts or timed usage.
Quiet Operation: Important if using in a bedroom or home office.

3. Consider Energy Efficiency

Running an AC unit can increase your energy bill — so look for:

Energy Efficiency Ratio (EER) or Class Ratings (A+, A++)
Inverter Technology (in split systems): Maintains a consistent temperature using less energy
Programmable Thermostats & Sleep Modes: Reduce energy use when cooling isn’t needed

4. Ventilation Requirements

Portable and monoblock air conditioners need to vent hot air through a hose — usually out of a window. Most units come with a window kit, but check compatibility with your setup.

Sliding sash windows? No problem — but check the kit fits.
No window nearby? Consider a through-wall vent or an evaporative cooler instead.

5. Installation & Maintenance

Portable AC Units: Plug and play. Simple setup, no installation needed.
Split Systems: Professional fitting required — higher up-front cost, but quieter and more efficient.
Maintenance: Clean filters monthly in summer. Descale regularly if using a humidifier function.

6. Multi-Season Use

Want your unit to work year-round?

Choose a 4-in-1 air conditioner with heating.
Ideal for garage workshops, garden rooms, or loft conversions used in winter.

7. Budget Guide (2025 Prices)

Budget Range	What to Expect
Under £250	Basic portable AC units (5,000–7,000 BTU)
£250–£400	Mid-range 3-in-1 units (9,000–12,000 BTU)
£400–£600	Larger capacity + remote control features
£600–£1,500	Monoblocks or small split systems
£1,500+	Premium monoblocks / large split AC

8. Best Use Cases: What Suits Your Space?

Space Type	Best AC Type
Bedroom	Quiet portable unit (7,000–9,000 BTU)
Workshop / Garage	Monoblock or high BTU portable
Garden Office	4-in-1 unit with heat for winter use
Conservatory	High BTU portable with good airflow
Flat or Rental	Easy-install portable with window kit

Final Checklist Before You Buy

✅ Know your room size
✅ Match BTU to your space
✅ Check for vents / window placement
✅ Decide if you want heating too
✅ Think about portability vs permanence
✅ Compare energy efficiency & noise levels
✅ Stick to your budget — but plan for long-term value

Discover our 7 top Air Conditioning Units to beat the heat >