How to Choose the Right AC Size for Your Room

Choosing the correct size air conditioner is one of the most important decisions you'll make when purchasing a cooling system. Get it right, and you'll enjoy efficient, comfortable climate control. Get it wrong, and you'll face years of frustration, discomfort, and unnecessarily high energy bills. This guide walks you through everything you need to know about sizing an air conditioner for Australian conditions.

Why Correct Sizing Matters

Air conditioner capacity is measured in kilowatts (kW), representing the amount of cooling or heating power the unit can deliver. Contrary to what you might think, bigger is not always better.

Problems with Undersized Units

An air conditioner that's too small for your space will:

• Run continuously trying to reach the set temperature
• Never adequately cool or heat the room on extreme days
• Use excessive energy while failing to provide comfort
• Wear out prematurely due to constant operation
• Leave you frustrated and uncomfortable

Problems with Oversized Units

Surprisingly, an oversized unit causes different but equally frustrating problems:

• Short-cycles (turns on and off frequently) without properly conditioning the air
• Fails to adequately remove humidity, leaving air feeling clammy
• Creates uncomfortable temperature swings
• Wastes energy and money on unnecessary capacity
• Costs more upfront for capacity you don't need
• Wears out faster due to frequent cycling

Basic Room Size Guidelines

As a starting point, here are general capacity recommendations based on room floor area with standard 2.4m ceilings:

• Up to 20m² (small bedroom, study): 2.0-2.5kW
• 20-35m² (large bedroom, small living area): 2.5-3.5kW
• 35-45m² (medium living area): 4.0-5.0kW
• 45-65m² (large living area): 5.0-7.0kW
• 65-85m² (very large space): 7.0-8.5kW
• 85m²+ (open plan, whole areas): 8.5kW+ or ducted/multi-split

However, these are simplified guidelines. Real-world sizing requires considering multiple additional factors that can significantly increase or decrease your requirements.

Factors That Affect Sizing Requirements

Professional installers use detailed calculations that account for many variables. Understanding these helps you make more accurate estimates and have informed conversations with installers.

Ceiling Height

Standard calculations assume 2.4m ceilings. Higher ceilings mean more air volume to cool:

• 2.4m ceiling: Standard capacity
• 2.7m ceiling: Add 10-15% capacity
• 3.0m+ ceiling: Add 20-25% capacity

Insulation Quality

Insulation dramatically affects how hard your AC needs to work:

• Well insulated: Ceiling, wall, and underfloor insulation keeps conditioned air in—may allow slightly smaller unit
• Moderately insulated: Ceiling insulation only—standard sizing applies
• Poorly insulated: Minimal or old insulation—add 20-30% capacity

Windows

Windows are the biggest source of heat gain in most rooms:

• Window size: Large windows or glass walls require significantly more cooling
• Orientation: North and west-facing windows receive the most afternoon sun
• Glazing type: Single glazing allows more heat transfer than double glazing
• Shading: External blinds, eaves, or trees reduce heat gain substantially

A room with large west-facing windows may need 30-50% more cooling capacity than an identical room with smaller, shaded windows.

Climate Zone

Australia's diverse climate means different regions have different requirements:

• Tropical (Darwin, Cairns): Focus on cooling capacity; humidity is a major factor
• Subtropical (Brisbane, coastal NSW): Hot summers require good cooling; mild winters need less heating
• Temperate (Sydney, Perth): Moderate requirements for both cooling and heating
• Cool (Melbourne, Hobart, Adelaide hills): Strong heating requirements; cooling for summer peaks
• Alpine (Snowy Mountains): Heating dominant; cooling rarely needed

Internal Heat Sources

Activities and appliances within the room add heat that the AC must remove:

• Number of occupants: Each person adds approximately 80-100 watts of heat
• Computers and electronics: A desktop computer and monitor can add 200-400 watts
• Kitchen appliances: Cooking generates significant heat—open-plan kitchens need more capacity
• Lighting: Older incandescent or halogen lights produce considerable heat

Room Usage Patterns

How you use the room affects requirements:

• Bedrooms: Usually only used evenings and nights when outdoor temperatures are lower
• Living rooms: Often used during peak afternoon heat
• Home offices: May run all day; consider computer and equipment heat
• Entertaining spaces: May have many people occasionally, requiring extra capacity for those times

Calculating Your Requirements

For a more accurate estimate than the basic guidelines, follow this simplified calculation method:

1. Calculate base capacity: Room area (m²) × 0.125 = base kW
2. Add for high ceilings: +10% for each 30cm above 2.4m
3. Add for poor insulation: +20-30% if minimal insulation
4. Add for large windows: +10-20% for standard windows; +30-50% for large west/north-facing glass
5. Add for hot climates: +10-20% for tropical or subtropical areas
6. Add for internal heat: +5-10% for home offices or kitchens

When to Seek Professional Advice

While these guidelines help you understand approximate requirements, professional sizing calculations are recommended for:

• Ducted system installations
• Multi-split systems
• Unusual building construction or layouts
• Large or open-plan spaces
• Rooms with extensive glazing
• Commercial or office environments

Professional installers use software that accounts for all variables, including detailed climate data for your specific location, to recommend precise capacities.

Common Sizing Mistakes to Avoid

• Copying your neighbour: Even identical houses can have different requirements based on orientation, shading, and insulation
• Using outdated recommendations: Building codes and AC efficiency have improved; old rules may not apply
• Ignoring heating requirements: If using reverse-cycle, consider both cooling and heating needs
• Forgetting about future changes: Planning renovations that add windows? Consider future requirements
• Choosing capacity based on price alone: A cheaper undersized unit will cost more in the long run

Practical Next Steps

Armed with this knowledge, you can:

1. Measure your room and note ceiling height, window sizes, and orientations
2. Use our guidelines to estimate your required capacity
3. Compare products in your target capacity range using our product comparison tool
4. Get quotes from licensed installers who can verify your calculations

Taking the time to size your air conditioner correctly ensures years of comfortable, efficient cooling. When in doubt, consulting with a professional installer is always worthwhile—their expertise can save you from costly mistakes and ensure you get the perfect system for your space.