One of the most common mistakes when buying a heater is choosing the wrong size. An undersized heater will struggle to warm your room, running constantly without ever reaching a comfortable temperature. An oversized heater wastes energy by cycling on and off too frequently and may cost more than necessary upfront.
Getting the size right means efficient heating, lower power bills, and a comfortable home. In this guide, we'll walk you through the simple calculations and considerations that lead to the perfect heater choice.
The Basic Formula: Watts Per Square Metre
The fundamental rule for heater sizing in Australia is straightforward: you need approximately 100 watts of heating power per square metre of floor space. This assumes standard ceiling heights of around 2.4 metres and moderate insulation.
📊 Basic Calculation
Required wattage = Room area (m²) × 100
For a room that's 4 metres by 5 metres (20m²): 20 × 100 = 2000W heater needed
Step-by-Step Sizing Process
Step 1: Measure Your Room
Measure the length and width of your room in metres. Multiply these together to get the floor area in square metres.
- Small bedroom: typically 9-12m²
- Average bedroom: 12-16m²
- Living room: 20-35m²
- Open-plan living/dining: 35-50m²
Step 2: Apply the Basic Formula
Multiply your room's area by 100 to get the base wattage requirement. This gives you a starting point that works for many Australian homes.
Step 3: Adjust for Your Specific Situation
The basic formula assumes ideal conditions. Real homes have factors that increase or decrease heating requirements.
🔑 Adjustment Factors
- High ceilings (above 2.7m): Add 10-15% per extra 30cm
- Poor insulation: Add 20-30%
- Large windows: Add 10% per major window wall
- External walls: Add 10% for each exposed wall
- Well-insulated/new build: Reduce by 10-20%
Quick Reference: Room Size to Wattage
For quick shopping reference, here's a general guide to heater wattage by room size:
- 5-10m² (small bathroom/study): 500-1000W
- 10-15m² (bedroom): 1000-1500W
- 15-20m² (large bedroom/small living): 1500-2000W
- 20-25m² (living room): 2000-2500W
- 25-35m² (large living area): 2500-3500W or multiple heaters
Factors That Affect Heating Requirements
Insulation Quality
Insulation is the single biggest factor after room size. A well-insulated room might need only 70-80W per square metre, while a poorly insulated older home could require 120-130W per square metre.
Consider your home's insulation status:
- Excellent: New builds, retrofitted insulation, double glazing
- Good: Ceiling insulation, weather-sealed doors and windows
- Average: Basic ceiling insulation, some drafts
- Poor: No insulation, single glazing, visible gaps around doors/windows
Ceiling Height
Standard Australian ceiling heights are around 2.4 metres. Higher ceilings mean more air volume to heat. For every 30cm above standard height, add approximately 10% to your wattage requirement.
Window Size and Orientation
Windows are significant sources of heat loss. Large windows, particularly south-facing ones that receive little direct sunlight in winter, increase heating requirements. Consider:
- Single glazed windows lose 10 times more heat than insulated walls
- Heavy curtains can reduce window heat loss by 40-50%
- North-facing windows may provide passive solar heating during the day
Room Position in the House
Rooms with multiple external walls require more heating than internal rooms surrounded by other heated spaces. Corner rooms with two external walls typically need 10-20% more heating capacity.
💡 Pro Tip
When in doubt, slightly oversize your heater rather than undersize. A 2000W heater with good thermostatic control can efficiently heat a room that technically only needs 1500W, but a 1500W heater will struggle in a room that needs 2000W.
Sizing for Different Heater Types
Different heater types have varying effectiveness, which can affect sizing decisions.
Radiant Heaters (Infrared, Bar Heaters)
Radiant heaters heat objects directly rather than the air. They're effective for personal heating but less suitable for whole-room heating. For radiant heaters used as personal warmth sources, you can often use lower wattages.
Convection Heaters (Panel, Oil Column)
These heat the air in the room and are ideal for whole-room heating. Standard sizing formulas apply well to these heater types.
Fan Heaters
Fan heaters distribute heat quickly and evenly. The standard formula works well, though they may feel warmer due to the moving air effect.
Multiple Heaters vs. One Large Heater
For larger rooms, you might wonder whether to use one powerful heater or multiple smaller units. Both approaches have merits.
Single Large Heater
- Simpler setup and control
- Often more cost-effective to purchase
- May create uneven heating in very large spaces
Multiple Smaller Heaters
- More even heat distribution
- Flexibility to heat only occupied areas
- Redundancy if one unit fails
- Higher total purchase cost
For rooms over 35m², multiple heaters often provide better results than a single unit.
Common Sizing Mistakes to Avoid
- Ignoring insulation: Two identical rooms can have vastly different heating needs based on insulation
- Measuring only floor space: Ceiling height matters significantly
- Choosing by price alone: An undersized cheap heater costs more to run and never heats properly
- Forgetting about air movement: Rooms with ceiling fans or strong drafts need additional capacity
- Oversizing dramatically: While slight oversizing is fine, a heater twice the needed size wastes money on purchase
When Standard Formulas Don't Apply
Some situations require special consideration beyond standard calculations:
- Open-plan spaces: Connected rooms should be calculated together
- Bathrooms: Higher wattage needed due to moisture and brief usage times
- Garages and workshops: Poor insulation typically requires 25-50% more capacity
- Sunrooms/conservatories: Large glass areas create variable conditions
Practical Example
Let's work through a real example. Consider a living room that measures 5m × 6m (30m²), with 2.7m ceilings, moderate insulation, and two external walls with large windows.
- Base calculation: 30m² × 100W = 3000W
- High ceilings (+10%): 3000 × 1.1 = 3300W
- External walls (+15%): 3300 × 1.15 = 3795W
- Recommended heater: 3500-4000W, or two 2000W heaters
🔑 Key Takeaway
Start with the basic 100W per square metre formula, then adjust based on your home's specific characteristics. When uncertain, consult with a heating specialist or err slightly on the side of more capacity.
Conclusion
Choosing the right heater size isn't complicated once you understand the basic principles. Measure your space, apply the formula, adjust for your home's specific factors, and you'll find the perfect heater that keeps you warm efficiently all winter long.