When we talk about the size of an air conditioning system, we’re not talking about the dimensions of the unit, but its capacity to heat or cool a room. The capacity is measured in Kilowatts (kW). If you are interested in installing air conditioning, our guide will help you work out what size air conditioner is right for you.
How To Calculate Air Conditioning Size in the UK
How Many Kilowatts Do I Need Per Square Metre?
A simple calculation will help you to work out approximately what size air conditioner is right for you.
First, calculate the floor area of the space to be cooled or heated by multiplying the length of the space by the width. For example, a room with a length of 8m and a width of 6m will have a floor area of 48 sq. metres.
Next, you will need your ceiling height, and this will determine the number of watts to be used in the calculation.
2.4m height = 150 watts
2.7m height = 160 watts
3.0m height = 175 watts
If your ceiling were 2.7m in height, then this would be the calculation:
48 sq. metres x 160 watts = 7,680 watts or 7.68 kW.
The calculation above provides a general rule of thumb only. Accurate system sizing for commercial premises requires a full heat-load assessment taking into account occupancy levels, lighting, equipment heat gains, glazing, ventilation rates and building fabric performance.
Most modern systems also operate as heat pumps, meaning they provide both cooling and heating.
Heating output may differ slightly from cooling output, and performance can vary depending on external temperatures.
An undersized or oversized system can significantly impact comfort, efficiency and long-term running costs.
A professional air conditioning installer will ensure the system is correctly sized for year-round operation, not just peak summer cooling.
What Size Air Conditioning Do I Need? A Commercial kW Sizing Guide for UK Businesses
The output of your air conditioning system should suit the size of your room(s). Even the most expensive unit designed for a 10m² room won’t cool a 50 m² facility. Below is a table to help you work out what size air conditioner is right for you, but speak to a professional air conditioning engineer about what size air conditioner is right for your needs.
| Floor Size (m2) | System Size (2.4m Ceiling Height) | System Size (2.7 Ceiling Height) | System Size (3.0m Ceiling Height) |
| 10-15 | 2.0kW – 2.2kW | 2.0kW – 2.4kW | 2.0kW – 2.6kW |
| 15-20 | 2.2kW – 3.0kW | 2.4kW – 3.2kW | 2.6kW – 3.5kW |
| 20-25 | 3.0kW – 3.7kW | 3.2kW – 4.0kW | 3.5kW – 4.3kW |
| 25-30 | 3.7kW – 4.5kW | 4.0kW – 4.8kW | 4.3kW – 5.2kW |
| 30-35 | 4.5kW – 5.2kW | 4.8kW – 5.6kW | 5.2kW – 6.1kW |
| 35-40 | 5.2kW – 6.0kW | 5.6kW – 6.4kW | 6.1kW – 7.0kW |
| 40-45 | 6.0kW – 6.7kW | 6.4kW – 7.2kW | 7.0kW – 7.8kW |
| 45-50 | 6.7kW – 7.5kW | 7.2kW – 8.0kW | 7.8kW – 8.7kW |
| 50-55 | 7.5kW – 8.2kW | 8.0kW – 8.8kW | 8.7kW – 9.2kW |
| 55-60 | 8.2kW – 9.0kW | 8.8kW – 9.2kW | 9.2kW – 10.5kW |
Take Room Traits Into Account
When working out what size air conditioner is right, the above table provides a general rule of thumb, but there are other factors to consider.
One of the most important factors often overlooked is internal heat gain.
People, lighting and equipment all generate heat. For example:
- Each person can contribute approximately 100–150 watts of heat
- IT equipment, servers and printers generate continuous heat loads
- LED lighting produces less heat than older lighting systems, but still contributes
In offices, gyms, restaurants or server rooms, these factors can significantly increase the required capacity.
Traits To Consider
Depending on whether the room you’re looking to cover is shaded or in the sunlight, you may have to adjust the power of your unit. You’ll need 10 per cent less capacity for shaded rooms, and 10 per cent more capacity for rooms in the sunlight.
If your air conditioning system is for a kitchen or a room with other appliances that generate heat (such as an IT room), you will also need to increase the capacity of your unit.
Other traits to take into account are whether the room has windows and how much insulation the room has. Poorly insulated homes or buildings may also need a higher output to cool properly. Rooms where much of the wall is glass will heat up more than those that don’t, especially if the windows are south-facing. Rooms on higher floors will also need more cooling capacity as heat rises.
The type of space may impact your calculation. Open-plan areas are the easiest to evaluate, whereas it may be more efficient to cool smaller offices individually. Hallways, corridors, and reception areas may not be occupied, so you may wish not to cool these areas.
Other Considerations
The location of the units will also need to be a consideration. Indoor units will need to be placed higher to allow cooler air to flow downwards. If this isn’t possible, a more powerful system will be needed to ensure the system runs efficiently. For the outdoor unit, a north or south-facing wall will be best to avoid the unit heating up and having to work harder throughout the day.
Electrical supply and noise levels should also be considered. Larger systems may require upgraded electrical capacity, and outdoor units must comply with local noise regulations, particularly in residential or mixed-use areas.
Choosing the Right Type of System
Capacity is only part of the decision. The type of air conditioning system must also suit the space. Options may include:
- Wall-mounted split systems
- Ceiling cassette units
- Ducted systems
- Multi-split systems
- VRF/VRV systems for larger commercial premises
A professional survey ensures the system design aligns with how the space is used and you know what size air conditioner is right for you.
Do’s & Don’ts
Do: Install A Wall Mounted Or Central Air Conditioning System
Installing a wall-mounted or central system rather than a window or portable air conditioning system could be more efficient and cool your room better. While portable and window units may provide temporary relief, professionally installed split or ducted systems offer significantly greater efficiency, quieter operation and improved temperature control, particularly in commercial environments.
Don’t: Get A System Too Small For The Space
A smaller air conditioning system won’t cool (or heat) the space efficiently. They will need to run for longer periods of time, and even then, they may not effectively cool the space. It will definitely increase your electricity bill.
Don’t: Get A System That Is Too Large For Your Space
It’s OK to err on the side of a slightly larger system for your room to ensure adequate comfort, but getting a system that is much too big for your space can also cause problems.
A larger system will cool the air in the room much more quickly and begin to cycle on and off. If it is constantly on and off, it may not run long enough to extract the humidity from the room, leaving users feeling uncomfortable.
Systems that are too large or small for the space also run inefficiently, leading to increased workload and ultimately, increased electricity costs. It’s essential that you know what size air conditioner is right for your space.
Why Correct Sizing Improves Energy Efficiency
Modern air conditioning systems use inverter technology, meaning they adjust output based on demand rather than switching fully on or off.
However, even inverter systems lose efficiency if incorrectly sized.
A properly sized system:
- Runs more efficiently
- Reduces energy consumption
- Lowers electricity bills
- Extends equipment lifespan
Oversizing or undersizing increases cycling, strain and long-term costs.
Commercial Air Conditioning Sizing for Offices, Retail & Hospitality
Commercial premises require more detailed sizing calculations due to higher occupancy levels, equipment heat loads and ventilation requirements. Offices, restaurants, gyms and retail spaces all have different cooling demands. Speak to one of our air conditioning engineers to find out what size air conditioner is right for you.
Frequently Asked Questions About AC Sizing
How many kW do I need per square metre?
As a rough guide, between 150–175 watts per m² depending on ceiling height and heat load. However, a professional heat load calculation is recommended.Is it better to oversize or undersize an air conditioner?
Neither. Oversized systems short cycle and fail to remove humidity. Undersized systems run constantly and increase energy bills.Do I need a different size system for heating?
Most modern systems are heat pumps. However, heating performance varies in cold weather and should be considered during system design.Does insulation affect air conditioning size?
Yes. Poor insulation increases cooling demand, meaning a higher-capacity system may be required.
Speak To A Professional
As ever, this is a simple guide to evaluate your needs when looking to install a new system or replace an existing air conditioning system.
Correct air conditioning sizing is critical for performance, efficiency and long-term cost control.
At Pinder Cooling, we carry out detailed site surveys and heat load calculations to ensure every system we install is correctly specified for the space, usage and building design.
Whether you’re upgrading an existing system or installing air conditioning for the first time, our experienced engineers can guide you through the process. Speak to a professional engineer who can advise what size air conditioner is right for you.
About Pinder Cooling
Pinder Cooling provides professional commercial air conditioning sizing and installation, and commercial refrigeration installation services across West Yorkshire and the North of England, including Leeds, Bradford, Manchester, Wakefield, Liverpool, Sheffield and Hull.