How to Size an Air Conditioner (UK): kW, BTU & Heat Load

Correctly sizing a split or multi-split system is the difference between a customer who's comfortable and efficient and one who complains the unit "doesn't work". Sizing means matching the unit's capacity to the room's heat load— and that's driven by far more than floor area.

What drives the heat load

• Floor area and ceiling height — the volume of air to condition.
• Building fabric — a solid-wall period property loses and gains heat far faster than a modern insulated build.
• Glazing and orientation — south- and west-facing glass adds large solar gains on a sunny afternoon.
• Occupancy and equipment — people, computers, kitchens and server kit all add sensible and latent heat.
• Air infiltration — draughty buildings lose conditioned air continuously.

The rule-of-thumb starting point

For a quick sanity check, many UK installers use roughly 0.10–0.15 kW per m²of floor area for a typical room. So a 20 m² living room lands around 2.0–3.0 kW as a first guess. Treat this as a starting point only — it's useful to catch an obviously wrong specification, not to size the final unit.

The proper method: room-by-room load

A real calculation works each room individually, adding up the component loads — fabric loss/gain, solar gain through glazing, infiltration and internal gains from people and equipment — then applies a modest safety margin. The result is a required capacity in kW, which you match to the nearest suitable unit, allowing a little headroom without grossly oversizing.

Oversizing is a real trap: an oversized inverter unit short-cycles, controls humidity poorly and wears faster; an undersized one runs flat-out and never satisfies the room. The aim is a close capacity match.

Multi-split considerations

On a multi-split you size each indoor unit to its room, then check the outdoor unit against the combined load and the manufacturer's diversity/simultaneity data — you rarely need the outdoor unit to cover 100% of every indoor running at once. Pipe runs, height differences and refrigerant charge limits all feed back into the design.

Do it in seconds with Vento

Vento runs this room-by-room calculation the moment you enter each room's dimensions, glazing, insulation and orientation, then matches a capacity from a SEER/SCOP catalogue — so you never over- or under-size, and the sizing flows straight into the drawing and proposal. You can also try the public air conditioning size calculator for a quick estimate.