Choosing the wrong size air conditioning unit is one of the most common mistakes in domestic installations — and it affects both performance and running costs. Here is how proper sizing works and what to watch out for.
Why Size Matters
An undersized unit runs continuously at full capacity on hot days and still cannot reach the target temperature. An oversized unit reaches temperature quickly but then cycles on and off frequently — this is inefficient, creates temperature swings, and in cooling mode leaves the room humid because short run cycles do not allow adequate dehumidification. The correct size unit reaches target temperature, then runs at reduced capacity to maintain it.
The Heat Load Calculation
Correct sizing starts with a heat load calculation — not a simple room area rule of thumb. The calculation accounts for:
- Room dimensions: Floor area and ceiling height give volume
- Glazing area and orientation: South-facing glass admits 3-4x more solar radiation than north-facing glass at peak summer sun. A south-facing conservatory needs much more cooling capacity than its floor area alone would suggest.
- Insulation quality: A modern well-insulated home has much lower heat ingress through walls and roof than a Victorian solid-wall property. Newer properties need smaller units for the same room area.
- Occupancy: Each person generates approximately 100W of heat. A 10-person office generates 1kW of heat from occupancy alone.
- Equipment heat load: Computers, servers, lights and kitchen appliances all generate heat. A server room calculation is almost entirely equipment heat load.
Common Sizing Rules of Thumb — and Why They Are Misleading
You will see rules like “100W per square metre” or “2.5kW for bedrooms, 5kW for living rooms.” These are very rough starting points that can be significantly wrong for specific rooms. A 15 sq metre bedroom with a large south-facing window may need 3.5kW. A 20 sq metre bedroom on a north-facing wall in a well-insulated modern house may need only 2.0kW. Using a simplistic rule rather than a proper calculation risks both undersizing and oversizing.
What We Do
We carry out a proper heat load calculation for every installation as part of the free site survey. We measure the room, assess glazing area and orientation, check wall construction and insulation, and account for occupancy and equipment. The result is a specific kW requirement that drives the unit specification rather than a rule of thumb.
Call 07833 053749 or request a free site survey. Correct sizing from the outset avoids performance problems and unnecessary running costs.