Portable vs Fixed Air Conditioning Units: Which Should You Choose?
Flexibility or performance? We compare portable and fixed air conditioning on cost, efficiency, noise and long-term value.
Choosing the right size air conditioner is essential. A unit that's too small will struggle to cool the room, while one that's too large wastes energy and controls humidity poorly.
What does BTU mean?
Air conditioner capacity is measured in BTUs (British Thermal Units) – the amount of heat a unit can remove from a room per hour. As a rough guide for UK homes, allow around 20 to 25 BTU per square foot, then adjust for the room's real conditions.
How to calculate the right size
Measure the length and width of the room and multiply them to get the floor area. Then multiply the area by 25 for a baseline figure.
Example: a 15 ft × 20 ft room = 300 sq ft.
300 × 25 = 7,500 BTU baseline.
300 × 25 = 7,500 BTU baseline.
This baseline applies whether you're considering wall-mounted units for a bedroom or a larger split system installation.
BTU guide by room size
| Room size | Approximate capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU |
| 150–250 sq ft | 6,000 BTU |
| 250–300 sq ft | 7,000 BTU |
| 300–350 sq ft | 8,000 BTU |
| 350–550 sq ft | 10,000–12,000 BTU |
| 550–700 sq ft | 14,000 BTU |
| 700–1,000 sq ft | Up to 18,000 BTU |
Accurate sizing matters just as much for commercial installations, where equipment and occupancy add significant heat.
Adjusting for real conditions
- Sunlight: add around 10% for sunny south- or west-facing rooms; reduce slightly for shaded rooms.
- Ceiling height: rooms above about 8 ft need extra capacity – ceiling cassettes or ducted systems can distribute air more evenly.
- Occupancy: allow roughly 400 BTU for each regular occupant beyond two.
- Kitchens and equipment: add 10–20% for heat from ovens and electronics.
Worked example
300 sq ft sunny room: 300 × 25 = 7,500 BTU
+ 10% for sunlight = 8,250 BTU → choose a unit rated around 8,000–9,000 BTU.
+ 10% for sunlight = 8,250 BTU → choose a unit rated around 8,000–9,000 BTU.
For properties with several rooms, a multi-split system lets you size and control each room individually.
Why correct sizing matters
An undersized unit runs flat out, struggles to reach temperature and wears faster. An oversized unit cools too quickly, switches on and off constantly and leaves the room feeling humid. A correctly sized system gives steady comfort, better air quality and lower running costs.
Northampton homes and climate
Summer temperatures in Northampton can make loft conversions, conservatories and offices with large windows uncomfortable. These spaces often need slightly more capacity than the basic calculation suggests, especially where insulation is limited.
Get it right first time
This guide gives a useful starting point, but a professional heat-load assessment takes everything into account. Book a free survey and we'll recommend the right size and type of system for your space.