How to Choose the Right AC Capacity? Getting It Wrong Wastes Energy and Leaves You Warm

Basic Floor-Area-to-Capacity Reference

In Hong Kong, common capacity choices by floor area are roughly: 50–70 sq ft (small bedroom) → ¾ HP; 70–100 sq ft (standard bedroom) → 1 HP; 100–150 sq ft (small living room) → 1.5 HP; 150–200 sq ft (large living room) → 2 HP.

These are only rough starting points. If you select a unit purely by floor area and then find the performance unsatisfactory, it is usually because other environmental factors that affect the cooling load were not taken into account.

• ~50–70 sq ft: ¾ HP (e.g. small bedroom)
• ~70–100 sq ft: 1 HP (e.g. standard bedroom)
• ~100–150 sq ft: 1.5 HP (e.g. small living/dining room)
• ~150–200 sq ft: 2 HP (e.g. large living room)

Environmental Factors That Affect the Actual Requirement

A west-facing unit receives heavy direct afternoon sunlight and has a much greater cooling load than an east- or north-facing unit of the same size. Top-floor units are also warmer because the roof absorbs heat from the sun. Units with very high ceilings (e.g. duplexes) have a greater air volume that requires more cooling.

The number of appliances running simultaneously also matters: computers, televisions, kitchen appliances, and human body heat all add to the load. Rooms with more occupants need proportionally more cooling capacity for the same floor area.

• West- or south-facing: cooling load is roughly 20–30% higher than the same-size north-facing room
• Top floor: roof absorbs solar heat, raising indoor temperature
• Ceiling height: the higher the ceiling, the greater the air volume
• Large glass windows: significant heat gain through the glazing
• Multiple occupants: human body heat increases the cooling requirement

The Effects of Too Large or Too Small a Capacity

An under-powered unit runs at full capacity continuously yet cannot reach the set temperature, wasting electricity and putting the components under excessive stress, shortening the unit's service life. An over-powered unit cools so quickly that the compressor short-cycles (starts and stops very frequently), which actually hampers effective dehumidification and also increases energy use and wear.

Bigger is not automatically better. The right capacity allows the system to run stably and efficiently. Before purchasing, it is far better to have an experienced technician assess the actual space rather than relying solely on a floor-area conversion chart.

Pre-Purchase Capacity Assessment Checklist

• Measure the room floor area (length × width, in sq ft or m²)
• Identify the room's orientation (west-facing has the greatest solar impact)
• Note the floor level (top floor requires a larger capacity)
• Measure or estimate the ceiling height
• List the major appliances that will be running at the same time
• Note the number of regular occupants
• Share this information with the installation technician for a professional capacity recommendation

Frequently asked questions