What Size Air-Conditioning Unit Do I Need?

Choosing the right size air-conditioning unit is one of the most important parts of the installation process. A unit that is too small will struggle to keep a room at the desired temperature, while one that is too large can waste energy and cycle on and off unnecessarily. For homeowners in Hampshire, Berkshire, Surrey, and Oxfordshire, selecting the correct size is essential for comfort, efficiency, and long-term reliability.

Why Sizing Matters

An incorrectly sized unit leads to problems such as:

• Inconsistent temperatures across the room
• Higher energy consumption than necessary
• Increased wear and tear on the equipment
• Poor humidity control, which can cause damp or dry air

Sizing is not just about square footage but about how each room is used and the building’s construction.

Factors That Influence Unit Size

Professional installers use several factors to calculate the correct capacity for each space:

• Room dimensions: Length, width, and height determine the cubic volume of air that needs cooling or heating.
• Insulation levels: Poorly insulated walls, single-glazed windows, or uninsulated roofs increase load requirements.
• Room orientation: South-facing rooms receive more sunlight and typically need larger units.
• Heat-generating appliances: Kitchens, home offices with computers, or entertainment rooms may need additional capacity.
• Occupancy: More people in a room increase the heat load.

Typical Cooling Capacities

As a rough guide, these are the typical cooling capacities needed for different room sizes:

• Small bedroom or home office: 2.0–2.5 kW
• Medium double bedroom: 2.5–3.5 kW
• Large lounge or open-plan living area: 5.0–7.0 kW
• Garden cabin or studio: from 2.0 kW, typically starting at £1,100 installed

These figures are averages and should always be confirmed with a professional site survey.

Local Examples

• In Basingstoke, a small home office required a 2.5 kW unit, installed for £1,200.
• A family in Reading fitted a 3.5 kW unit in a medium-sized bedroom at £1,950.
• In Guildford, a large open-plan living space needed a 7.0 kW system, which was part of a £4,800 multi split installation.
• A homeowner in Oxford installed a 2.0 kW system in a garden studio for £1,150.

These case studies highlight how unit size and cost vary depending on property type and usage.

Why Professional Sizing Is Essential

While online calculators can give estimates, only a professional site survey ensures accuracy. Certified engineers measure room volumes, assess insulation, and consider usage patterns. This ensures that the unit is:

• Neither underpowered nor oversized
• Installed in the correct position for airflow
• Matched to the electrical supply and system design
• Supported by a warranty based on correct installation

Risks of Incorrect Sizing

Getting the size wrong can be costly:

• A unit that is too small will run constantly, driving up energy bills and shortening lifespan.
• A unit that is too large will cool quickly but not dehumidify properly, leaving air feeling damp and uncomfortable.

This is why professional guidance is always recommended.

A Smarter Investment

At ClimateWorks, we size every system carefully during our surveys. From garden cabins in Hampshire to large family homes in Surrey, we ensure each installation is tailored to the property’s needs. This protects your investment and ensures comfort all year round.

References

• Energy Saving Trust – Heating and Cooling Guidance
• UK Government – Building Regulations
• Planning Portal – Air Conditioning Guidance

Author Bio

Dr Julian Carter is Technical and Compliance Director at ClimateWorks. With over 25 years of experience in air conditioning, refrigeration, and ventilation, he has delivered projects across Hampshire, Berkshire, Surrey, and Oxfordshire. Julian specialises in system sizing, compliance, and sustainable design. He also lectures at a UK university as a visiting professor, teaching building services engineering and energy efficiency. His blend of academic knowledge and industry practice ensures every ClimateWorks installation is designed for long-term performance and reliability.

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