The UK experienced another sweltering summer in 2025, and given that our homes and offices are, historically, built to retain heat, spending time indoors during periods of stifling heat can become unbearable. Even when the sun goes down, the humidity can make it challenging to concentrate, work productively, relax and even sleep.
Fortunately, a properly sized air conditioner helps make the heat indoors infinitely more manageable and comfortable, even during the peak of summer. Whether you’re considering a portable air conditioning unit for a single room or exploring large-scale residential or commercial systems, getting the right size is vital.
This guide from Clinachill breaks down everything you need to know.
Why Air Conditioner Size is Important
Air conditioning unit “size” refers to its cooling power, usually measured in kilowatts (kW) or BTUs (British Thermal Units). The right capacity means your AC unit will cool the room effectively, maximise energy efficiency, and run reliably and quietly.
However, if the air conditioning system is too small, your unit will struggle to cope. Conversely, if it’s too large (for installed systems), it cycles on and off, wasting energy and possibly leaving air feeling damp. Consequently, a correctly sized air conditioning unit keeps you comfortable, and can be equal parts convenient, energy-efficient and durable.
For portable air conditioning units, though, bigger is better. Since these units are rarely used outside of the hottest UK months (certainly not all year round), more power means faster cooling. You’re unlikely to regret having an air conditioning unit with a higher BTU rating if you go portable.
What Makes Portable Air Conditioning a Good Choice?
Portable air conditioning units are a popular choice in UK homes for their convenience and flexibility. They are straightforward to set up and install, and they’re inherently more flexible for homeowners, renters, or anyone else needing to move cool air between rooms.
However, to be truly effective, they still need to be correctly sized. Start by measuring your space (length and width) to determine its size in square metres (m²).
Climachill’s Portable AC Recommendations (BTU Sizing)
- Up to 25m²: 12,000 BTUs for medium bedrooms, studies and lounges
- Up to 36m²: 18,000 BTUs for larger spaces, open plan living or large bedrooms
- Up to 15m²: 7,000–9,000 BTUs for small bedrooms or offices
It’s preferable to select a unit with more cooling power if you’re unsure, particularly for portable ACs. This usually means you’ll cool the room faster and avoid issues in peak summer.
The below table is a handy go-to reference if you’re considering a portable air conditioner but unsure of the size you require.
| Room size (m²) | Recommended kW | Approx. BTU | Room type |
| Up to 15 | 1.2–1.6 | 7,000–9,000 | Small bedroom or office |
| Up to 25 | 1.6–2.2 | 12,000 | Medium bedroom or study |
| Up to 36 | 2.2–2.9 | 18,000 | Large or master bedroom, lounge or family room |
Other Factors to Consider:
- Ceiling height: Increase BTU for taller rooms.
- Sun exposure: South-facing, sunlit, or conservatory-style rooms will need extra cooling power.
- Insulation: Poorly insulated spaces will require higher BTUs.
- Occupancy and electronics: More people and devices raise a room’s temperature.
If in doubt at all, always round up (not down) on portable air conditioning unit sizes. Oversizing helps ensure you never fall short of your cooling needs. Opting for large portable air conditioning units over small ones may be higher upfront cost-wise, but the long-term implications and benefits of your choice will be well worth it, especially if we experience repeated hot summers like recent years.
Why Consider a Portable Air Conditioning Unit?
Portable air conditioning systems are ideal when you need a flexible, non-invasive, temporary solution without the disruption of installing one into the walls. It may be unfeasible for some to opt for a wall-mounted system (for example, they may be renting and unable to make alterations to the property), so portable air conditioners offer a reprieve in that sense.
When considering systems, look for reputable brands, energy efficiency ratings, and features that would be complementary of a UK climate (such as effective moisture control and noise suppression features, to name a few).
Find Your Ideal Air Conditioning Unit from Climachill
Climachill has a wealth of experience providing reliable air conditioning installations, repairs and maintenance services for homes and commercial spaces across Sussex and the South East. We provide portable units ideal for room-to-room usage, and bespoke whole-house systems, with all maintenance and installations handled by our in-house engineers, ensuring you’re left with a unit and system that fits your needs and lifestyle perfectly.
We believe sizing an air conditioning unit doesn’t need to be complicated. However, if you require expert advice or wish to enquire about portable units and our installation services, contact us today. Our knowledgeable and friendly engineers are on hand to guide you to the perfect product(s) for your space so you can stay cool and comfortable when the uncomfortable weather inevitably rears its head.