What size air conditioner do I need?
Rules of thumb for UK rooms: 2.5kW for a typical double bedroom, 3.5kW for most living rooms, 5kW for large open-plan spaces, then adjusted for glazing, orientation and the roof above. Loft rooms need more than their floor area suggests. Oversizing is a real fault, not a safety margin: an oversized unit short-cycles, dehumidifies poorly and wears its compressor.
Air conditioning capacity is quoted in kilowatts of cooling (you will also see BTU: 9,000 BTU is roughly 2.5kW, 12,000 BTU roughly 3.5kW). The right size matches the room's heat load: the heat coming through windows, walls and roof plus what people and equipment add inside.
Floor area is only the starting point. A 15m² north-facing bedroom with one window might need 2kW; the same floor area as a loft conversion under slates, or with a west-facing glazed wall, can need double. Orientation, glazing area, insulation and the roof above the room move the number more than the room's dimensions do, which is why we size from the fabric at survey rather than from a postcode and a guess.
Undersizing is the obvious failure: the unit runs flat out on the hottest days and never quite gets there. Oversizing is the sneaky one. A too-large inverter unit reaches temperature quickly, then cannot modulate low enough to idle, so it stops and starts. Short cycling wears the compressor, holds humidity high because moisture removal needs sustained running, and feels clammy despite the thermometer reading right.
Ask whoever quotes you to show the sizing logic: room dimensions, glazing, orientation, roof. If the answer is 'we always fit 3.5s', keep shopping. We show the calculation and will talk you out of the bigger unit when the room does not justify it.
More in choosing a system, or browse all 40 questions on the FAQ index.
Price your project
Two quick steps. We reply within one working day.
No account, no obligation. We only use this to prepare your quote.
Ready to price your air conditioning?
Tell us the rooms and the rough plan. We reply within one working day with an honest cost bracket, then a free survey and a fixed written quote.