Sizing an air conditioner: kW per m² and m³, insulation and sun

The first question most people ask when buying air conditioning is how powerful it needs to be. The honest answer: floor area matters less than sun, insulation and what happens in the room. This guide shows you how to make your own estimate, so you can sanity-check a quote.

What sizes are available?

Domestic split systems come in fixed steps, roughly 2 to 7 kW of cooling per indoor unit. UK retailers often quote capacity in BTU per hour instead: 1 kW is about 3,412 BTU/h, so a “9,000 BTU” unit is about 2.6 kW and a “12,000 BTU” unit about 3.5 kW. The labelling and ecodesign rules apply to air conditioners up to 12 kW ¹. The energy label gives the design load for cooling (Pdesignc) and heating (Pdesignh) – the loads at which SEER and SCOP are calculated ². Use those figures to compare models, not the maximum output in the brochure.

Where does the heat come from?

A room’s cooling load is the sum of a handful of sources. The figures below are assumptions for a rough estimate on a hot summer afternoon; an installer can calculate it more precisely.

The biggest variable is almost always the sun. Internal blinds and curtains help much less than external shading, because by then the heat is already inside.

Example 1: bedroom

14 m², moderately insulated, 2 m² window facing west with no shading, two people.

• Sun: 2 × 300 = 600 W
• Base load: 14 × 40 = 560 W
• People: 2 × 100 = 200 W
• Equipment: 50 W
• Total about 1.4 kW → the smallest split units are more than enough.

Example 2: living room with lots of glass

35 m², well insulated, 8 m² of south-facing glass, four people, 200 W of equipment.

With external shading, this room needs a unit little more than half the size. Shading is often the cheapest first step.

m² or m³?

Retail rules of thumb work in square metres, which is fine for typical ceiling heights of 2.4 to 2.7 m. With a gallery, high Victorian ceilings or a loft conversion under a sloping roof, the volume – and especially the roof area – matters: a poorly insulated roof in full sun is a big heat source. Treat such rooms as “loft room” in the table.

Why bigger isn’t better

• Short cycling. Even an inverter can only turn down to a certain minimum. If that minimum exceeds the heat load, the unit keeps switching on and off, which costs efficiency and wears the compressor.
• Poorer dehumidification. An air conditioner removes most moisture during long, steady runs. Short cycles leave a room cool but clammy.
• More noise, higher price. Bigger outdoor units generally have a higher sound power level, which feeds into the MCS 020(a) noise assessment that permitted development relies on in England ³.

A unit that is slightly on the small side and runs flat out for a few hours on the hottest afternoons is usually more comfortable and more efficient than one that is heavily oversized.

Heating: check Pdesignh

If you also want the unit to heat, the deciding factor is the room’s heat loss in cold weather rather than its cooling load. The label gives Pdesignh at the −10 °C design temperature for the average climate ². An installer calculates heat loss room by room; if you are applying for the £2,500 Boiler Upgrade Scheme grant for an air-to-air heat pump, the installer has to be MCS-certified ⁴. If Pdesignh is below the heat loss, your existing heating covers the coldest days – not a problem if the unit is a top-up.

In well-insulated homes the summer cooling load usually decides the size; in older, poorly insulated homes the winter heat loss often does.

Multi-split or several single splits?

To cool several rooms, you can use one outdoor unit with several indoor units (multi-split) or separate systems. With a multi-split, each indoor unit is sized for its own room; the outdoor unit doesn’t have to equal the sum of all indoor units, because rarely do all rooms need full output at once. A multi-split saves wall space, but if the outdoor unit fails, every room loses cooling.

Summary

1. Start with shading: it reduces the required capacity more than anything else.
2. Estimate the cooling load with the table above and compare it with Pdesignc on the label.
3. Don’t buy much bigger than your estimate; a correctly sized inverter unit is quieter and cheaper to run.
4. If you want heating, get a heat loss calculation and compare it with Pdesignh.

If heating is your main aim, also look at air-to-water systems in our heat pump overview.