What Size Air Conditioning Unit Do I Need? A Practical Guide
Undersized systems don't cool. Oversized systems short-cycle and waste energy. Here's how to work out the right capacity for any room.
Getting the capacity of your air conditioning system right is one of the most important factors in whether it works well and runs efficiently. Both undersizing and oversizing have real costs. Here's how to think about it.
The basics: BTU and kW
Air conditioning capacity is measured in BTU (British Thermal Units) per hour or in kilowatts. They're equivalent: 1 kW = approximately 3,412 BTU/hr. Modern systems in the UK are usually described in kW — 2.0kW, 2.5kW, 3.5kW, 5.0kW and so on.
The simple rule of thumb
A basic starting point is to allow approximately 100W (0.1kW) of cooling capacity per square metre of floor area, multiplied by ceiling height. A 4m × 5m room with a 2.4m ceiling has a volume of 48m³, giving a starting point of approximately 48 × 0.1 = 4.8kW. This would point to a 5.0kW system.
Why the simple calculation is often wrong
The volume-based rule assumes average conditions. Room type, glazing area, orientation and insulation can shift the requirement dramatically. A south-facing conservatory with polycarbonate roof needs 2–3× the capacity of the same floor area in a north-facing bedroom. A well-insulated modern flat needs significantly less than an uninsulated Victorian terrace.
Factors that increase the required capacity
- •South or west-facing rooms receiving afternoon sun
- •Large areas of glazing or skylights
- •Poor insulation (pre-1980 construction, single glazing)
- •High occupancy (each person adds approximately 100W of heat load)
- •Significant IT or electrical equipment (adds heat load)
- •Loft conversions (direct roof sun exposure)
- •Conservatories (extreme solar gain)
- •Garden offices and outbuildings (often poorly insulated)
The consequences of getting it wrong
An undersized system will run continuously at maximum output without achieving the target temperature on hot days. An oversized system will reach the target temperature too quickly, leading to 'short cycling' — repeatedly switching on and off — which reduces efficiency, increases wear and often means the air isn't properly dehumidified (the system doesn't run long enough for moisture to condense properly).
The most common sizing mistake we encounter is undersizing conservatories — people specify a small unit, it runs continuously in summer without ever reaching the set temperature, and they conclude that air conditioning 'doesn't work' in conservatories. A properly sized system absolutely does.
Why a professional survey matters
A reputable installer will calculate the heat load properly, considering all the factors above, before specifying a system. This is a significant part of what you're paying for at the survey stage — not just a sales visit but a genuine engineering calculation. Be wary of any installer who quotes a system size without visiting your property.