Cooling
Sizing is the most important decision in any air conditioning project, and the one most often got wrong. Get it right and your system is comfortable, quiet and cheap to run. Get it wrong and you’ll either sweat through the hottest days with a unit that can’t cope, or live with a system that’s cold, clammy and noisy. This guide explains how air conditioning is sized, what the kW figures mean, and why a quick calculation only ever gets you to a starting point.
What “kW” actually means on an air conditioner
Air conditioners are rated in kilowatts (kW) of cooling (and heating) output — the amount of heat they can move, not the electricity they consume. A 3.5kW unit can remove 3.5kW of heat from a room. Confusingly, it might only draw 0.8–1kW of electricity to do that, because it moves heat rather than creating it. When we talk about sizing, we mean matching that output to the heat load of your room.
The danger of getting it wrong
Undersized and the unit runs at full tilt and still can’t pull the temperature down on hot days. It’s noisier, works harder, wears faster and never quite delivers.
Oversized is just as bad, and more common because people assume bigger is safer. An oversized unit cools the air so fast that it reaches the thermostat setting and switches off before it has had time to remove humidity. The room ends up cold but damp and clammy, and the unit short-cycles — switching on and off repeatedly — which is inefficient, noisy and hard on the equipment.
The goal is a unit that’s correctly matched, so it runs steadily and gently, holding the room at a comfortable, dry temperature.
The quick rule of thumb
As a rough starting point, a reasonably insulated UK room needs around 0.15kW of cooling per square metre of floor area. So:
- A 12m² bedroom: roughly 1.8–2.0kW.
- A 20m² lounge: roughly 3.0–3.5kW.
- A 30m² open-plan space: roughly 4.5–5.0kW.
Our online calculator uses this as its basis and gives you a ballpark in seconds. But treat it as exactly that — a ballpark. The final figure can move a long way once the real-world factors come in.
The factors that change the answer
Two rooms of identical floor area can need very different units. A proper heat-load calculation accounts for:
- Windows and orientation. Glazing is a major heat source. A south or west-facing room with large windows can need 30–40% more capacity than its floor area suggests, because of solar gain through the glass.
- Ceiling height. You cool a volume, not an area. Tall ceilings, vaulted rooms and loft conversions need more than a standard 2.4m-ceiling room of the same footprint.
- Occupancy. People give off heat. A busy family lounge or a meeting room with several people adds a meaningful load.
- Equipment. Computers, TVs, kitchen appliances and especially server equipment are constant heat sources that push the requirement up.
- Insulation and construction. A modern, well-sealed home holds temperature and needs less; a draughty older property with solid walls needs more.
- Use of the room. A room used occasionally can be sized differently from one that must hold temperature all day, every day.
Why a survey beats a calculator
A calculator can’t see your windows, feel your draughts or count your kit. That’s why, for anything beyond a rough budget, we survey the space properly. We measure the room, assess the glazing and orientation, note the heat sources and the construction, and then specify a unit that’s right — not the biggest we can sell you, and not an optimistic minimum, but the correct match. It’s the single most valuable thing we do, because everything else — comfort, running cost, noise, lifespan — depends on it.
Sizing multi-room and commercial systems
When several rooms are involved, sizing gets more interesting. For a multi-split or a commercial VRF system, the outdoor unit doesn’t simply add up every indoor unit at full capacity, because it’s unlikely every room will be at maximum demand simultaneously. A diversity factor is applied — typically around 0.8–0.9 for mixed-use spaces — so the outdoor unit is sized for realistic peak demand rather than a theoretical maximum. This keeps the system efficient and the cost sensible, and getting it right is a job for an experienced designer.
Heating capacity matters too
Because your system is a heat pump, it has a heating output as well as a cooling one, and the two aren’t always identical. If you plan to use the system heavily for winter heating, we make sure it’s sized to deliver the warmth you need on the coldest days, not just to cool you in summer. For many homes the heating requirement is actually the deciding factor.
The bottom line
Sizing is where air conditioning projects are won or lost. The 0.15kW-per-m² rule gets you a quick estimate, but the real answer depends on your windows, ceilings, occupancy, equipment and construction — which is why we never finalise a specification without seeing the space. Right-sizing is the difference between a system you forget is there and one that annoys you every day.
Use our calculator for a starting figure, then let us confirm it with a free survey and no-obligation quote. You’ll get a system that’s matched to your rooms exactly — comfortable, quiet and efficient from day one.
