What size air conditioning unit do you need?

As a starting rule of thumb, a well-insulated UK room needs roughly 100 watts of cooling capacity per square metre of floor area. That means:

• 2.5kW suits a typical bedroom or home office, up to around 20–25m².
• 3.5kW suits a larger bedroom or a small-to-medium lounge.
• 5.0kW across two indoor units suits two average rooms sharing one outdoor unit.
• 7.1kW across three indoor units suits a small flat or several rooms across a floor.

Treat these as a starting point, not a specification: the rule of thumb ignores several things that change the real answer.

What pushes the number up

• Ceiling height. A room with a 3m ceiling has roughly 25% more air to cool than the same floor area at 2.4m, and the rule of thumb above assumes a standard UK ceiling height.
• Glazing and orientation. A south-facing room with large windows heats up far more through the day than a north-facing room of the same size, and needs more cooling capacity to keep pace.
• Insulation age. Older properties with solid walls and single glazing lose the fight against outside temperature faster than a newer, well-insulated home, which affects heating mode more than cooling.
• What’s in the room. A home office with two monitors and a desktop PC running all day adds a genuine, constant heat load that a spare bedroom doesn’t have.

Why bigger isn’t simply better

An oversized unit cools a room quickly, then switches off, which sounds good, but it means the unit spends less time actively removing moisture from the air. The result is a room that reaches temperature but still feels slightly humid. A correctly sized unit runs longer at a lower intensity, holding a steadier temperature and doing a better job of dehumidifying at the same time.

How we size yours

Our online quote asks about room dimensions, ceiling height, and how the room is used, and recommends a system based on that, not a single generic number. If anything about your room is unusual (a converted loft with sloped ceilings, or a conservatory, for example), it’s worth mentioning it in the notes so we can flag it before your installation date, not on the day.