Choose air conditioning capacity from a room-by-room heat-load assessment, not floor area alone. The assessment should consider glazing area and orientation, shading, roof and wall construction, insulation, ventilation, occupancy, appliances, lighting, desired conditions and simultaneous use. Capacity is commonly stated in kilowatts or BTU per hour, but the label alone does not show how a unit will perform at the design conditions. Both undersizing and oversizing can reduce comfort.
Capacity is not electrical consumption
Cooling capacity describes the rate at which the system can remove heat. Electrical input describes the power it consumes. A 2.5 kW cooling rating therefore does not mean the unit continuously draws 2.5 kW of electricity.
BTU per hour is another capacity unit found in product descriptions. Converting units does not improve the design if the underlying room load is only a guess. Ask for the assumptions and design conditions behind the recommendation.
What a heat-load assessment should include
Sun through unshaded west-facing glazing can dominate a room's afternoon load. A loft bedroom beneath a roof can behave very differently from a shaded ground-floor room with the same area. Fabric, air leakage and ventilation also affect heat flow.
People, computers, cooking and lighting add internal heat. Usage timing matters because not every room reaches its peak at once. London flats may have high solar and communal heat gains, while detached Essex homes may have more exposed roof and wall area.
- Measure glazing and record its orientation and shading.
- Describe wall, roof and insulation where known.
- List normal occupancy and heat-producing equipment.
- State when rooms are used and which operate together.
- Consider passive cooling before final selection.
Problems caused by the wrong size
An undersized system may run at high output for long periods and still fail to maintain comfort in design conditions. It may also be noisier at the fan speed needed to chase the load.
A substantially oversized system can produce draughts, rapid cycling or unstable temperature and humidity control. Inverter modulation helps match changing loads, but it does not make an arbitrary oversize harmless. Minimum output and indoor airflow are relevant as well as maximum capacity.
How to compare selections
Request the room design load and the selected unit's available output at the relevant indoor and outdoor conditions. For multi-splits, ask how shared capacity is allocated when several rooms call together.
Review sound, airflow, seasonal efficiency and heating duty alongside capacity. A reversible system intended for winter heating also needs a heat-loss assessment and should not be assumed suitable solely because its cooling rating looks adequate.
Frequently asked questions
How many BTU do I need per square metre?
A generic area multiplier is not reliable enough for final selection. Solar gain, glazing, fabric, occupancy, equipment and ventilation can outweigh floor area.
Is a larger air conditioner safer for very hot days?
Not automatically. Excess capacity can harm airflow and control at ordinary loads. A designer should select against stated design conditions and explain any margin.
Can one unit cool two bedrooms?
Usually not evenly when doors are closed. Each occupied room and the air path need assessment rather than assuming cooling will travel through a hallway.
Does inverter technology remove the need for sizing?
No. Modulation widens the useful operating range, but every system still has minimum and maximum outputs, airflow characteristics and manufacturer limits.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.