Picture a 4 metre by 3 metre spare room and a buyer who picks the biggest unit on the shelf to be safe. It cools the room in minutes, switches off, then starts again. A unit of around 2kW would have done the job, and that is the whole point of sizing it properly.

So what size air conditioner do you need for a room? Measure the floor, start from 100 watts per square metre, then add heat for sun, people and equipment.

Step 1: Work out the floor area

Multiply the length by the width in metres. A 4m by 3m room is 12 square metres.

Multiply that figure by 100 watts. You get 1,200 watts, or 1.2kW. This is your starting point for a room with a standard ceiling height and average insulation.

Step 2: Add the heat that comes in or gets made

Cooling capacity has to beat the heat the room gains. Add these allowances to your starting figure:

  • Large south- or west-facing windows: add 10 to 20 per cent
  • Each person who regularly uses the room: add roughly 100 to 150 watts
  • Each desktop computer or large monitor: add roughly 150 to 300 watts
  • A kitchen or open-plan cooking space: add around 1kW

Watch the loft. A converted attic room under a roof tiled in summer sun can run hot enough to need 20 per cent more than the floor area suggests.

Step 3: Adjust for the building itself

Ceiling height matters because tall rooms hold more air. Older High Wycombe terraces and Edwardian semis often have ceilings above 2.6 metres, so add about 10 per cent. A modern flat with good insulation and a low ceiling can sit at or just under the base figure.

Poor insulation or single glazing pushes the number up. Cavity wall insulation and double glazing pull it down.

Quick answers before you buy

What does BTU mean?

It is the older unit for cooling power. One kilowatt is about 3,412 BTU an hour, so a 2.5kW unit is roughly 8,500 BTU.

Can I use one unit for two rooms?

Only if the rooms are open to each other. A closed door stops the cool air from spreading.

Is bigger better?

No. Oversized units cycle on and off, waste electricity and leave the air damp.

Do inverter units change the sums?

They adjust their output to match demand, so they cope better with small errors. Sizing still matters.

What are the standard sizes?

Single split units commonly come in 2.0kW, 2.5kW, 3.5kW, 5.0kW and 7.0kW.

Step 4: Round to the nearest standard unit

Add up your figures, then pick the size that sits just above your total. Do not jump up two sizes. If your result falls between two units, choose the larger one for a south-facing room and the smaller one for a north-facing room.

A worked example

Take a home office of 5m by 4m with a south-facing window, two people and two computers.

  • Floor area: 20 square metres, giving 2.0kW
  • Sun through the window: add 15 per cent, giving 2.3kW
  • Two people: add 0.25kW, giving 2.55kW
  • Two computers: add 0.5kW, giving about 3.05kW

A 3.5kW unit is the right pick. A 2.5kW model would struggle on the hottest afternoons, and a 5.0kW model would be far too large.

Work through the same four steps for your own room before you phone anyone, then ask a local installer to check your figures on a site visit. That way, any quote for air conditioning in High Wycombe starts from numbers you understand.