What Size Air Conditioner Do I Need?

Start with a rough rule of thumb

As a starting point, UK installers often allow around 0.15kW of cooling per square metre of floor area for a standard, moderately glazed room with typical insulation and a 2.4m ceiling.

That gives you a ballpark, not an answer. It's a sanity check against quotes, not a substitute for a proper heat-load calculation.

Indicative capacity by room size

• Up to 12m² (small bedroom, box room, home office): 2.0–2.5kW.
• 12–18m² (double bedroom, small living room): 2.5–3.5kW.
• 18–25m² (large living room, open-plan kitchen area): 3.5–5.0kW.
• 25–35m² (large open-plan space): 5.0–7.1kW, sometimes two indoor units.
• Conservatories: add 50–100% to the figure for an equivalent standard room.

Factors that increase the requirement

• Large glazing, especially south or west-facing.
• Roof or loft rooms with heat gain through the roof plane.
• High ceilings — capacity relates to volume, not just floor area.
• Heat-producing equipment: servers, kitchen appliances, gym equipment.
• High occupancy — each person adds roughly 100W of heat.
• Poor insulation or single glazing.

Why oversizing is a real problem

It's tempting to fit the biggest unit affordable, but an oversized system cools the air rapidly then stops, cycling on and off. Short cycles remove far less humidity, so the room feels cold and clammy rather than comfortable, and the compressor wears faster.

Undersizing has the opposite problem: the unit runs at maximum continuously, never quite reaching setpoint, using more electricity and shortening its life.

What a proper survey should cover

• Room dimensions including ceiling height.
• Glazing area, orientation and glass type.
• Insulation standard and construction type.
• Occupancy and equipment heat gains.
• Ventilation and door openings.
• Where the outdoor unit can go and the resulting pipe run length.