Every figure checked against its source on 5 November 2026
Which size of air conditioner is right for the room?
A room of 15 to 25 square metres with an ordinary ceiling wants roughly 2.0 to 2.5kW. That is the starting point in the table below, but a top-floor flat, a wall of glass or a tall Victorian room in Croydon can all move the answer up. A survey should always confirm the final choice.
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How do floor area and capacity line up?
| Room area | Cooling output |
|---|---|
| 10 to 15 m² | 1.5 to 2.0kW |
| 15 to 25 m² | 2.0 to 2.5kW |
| 25 to 35 m² | 2.5 to 3.5kW |
| 35 to 50 m² | 3.5 to 5.0kW |
These are iHeat's figures for standard ceiling heights, updated in May 2026. Multiply the room's length by its width in metres to get the area. Source: iHeat.
How do BTU and kW compare?
One kilowatt is about 3,412 BTU an hour, so a 3.5kW unit is roughly 12,000 BTU. Glow Green's table starts at 8,000 BTU for 9 to 14 square metres and rises to 25,000 BTU for 42 to 60, and the two tables do not match exactly, which tells you how approximate any table is. Source: Glow Green.
When should you go up a size?
- Glass. A floor-to-ceiling window in a new Croydon tower flat, or a south-facing bay in a Victorian house, adds heat the table ignores.
- Height. A tall room holds more air than its floor area suggests.
- Top floors and lofts. Heat arrives through the roof and rises from below.
- People and equipment. Computers, screens, lighting and bodies all add load, which is why offices are sized from occupancy rather than area. See office air conditioning.
Sources: iHeat; Glow Green.
Why not just buy the biggest?
Because an oversized unit cools the room in minutes, switches off, and repeats, which is less comfortable, leaves the air damp and wears the compressor. It also costs more to buy. A unit that is too small runs flat out on the days you need it. The right size holds the temperature on a hot afternoon on a middling fan speed.
Is heating sized differently?
Sometimes. Output drops as the outdoor air cools, and Kenley Airfield in this borough averages about 35 days of air frost a year. If the unit is your winter heating, check the heating output the maker quotes at a low outdoor temperature rather than the headline cooling figure. The heat pump page explains why output falls in the cold. Source: Met Office.
What about a server cupboard?
Ignore the floor area and total the power the kit draws, since nearly every watt turns into heat: a 900W server warms the room like a 900W heater. Our server room page works through the method. Source: Server Room Environments.
Questions people ask
Can one unit cover two rooms?
Only if they are open to each other. A door stops the air, so each closed room needs its own indoor unit.
Does insulation change the size?
Considerably. An insulated room keeps its temperature with far less effort from the unit, whereas single glazing or a bare roof can mean the next size up.
Is the number on the box the cooling output?
Usually, but check whether it is cooling or heating, as they differ, and remember a portable's figure is not directly comparable with a split's.
Where these facts come from
Useful to have ready
With these we can give you a straight answer on the first call.
- Your postcode
- Flat, house or business premises
- Who owns the outside wall, if you lease or rent
- Which room, roughly how big, and which way the window faces
- Any conservation area or listing you know of
Ready to go ahead? Air conditioning in Croydon covers what we fit, what we look after and where we travel.
Ready to have it fitted?
Air conditioning installation
A survey, a written price, then units, pipework and electrics fitted properly.
Home air conditioning
Bedrooms, top-floor flats and loft rooms, with heating thrown in.
Conservatory air conditioning
Turns a glass room you use in May into one you use in August and January.
More guides
How much does air conditioning cost?
Unit prices, labour, electrics, running costs and the extras flats add.
Planning permission for air conditioning: when you need it
The permitted development conditions, with Croydon's exceptions.
Can you have air conditioning without an outdoor unit?
Monobloc units explained: what they do well, where they fall short.
Air conditioning in a flat: who has to say yes?
Leases, freeholder charges, the one-unit-per-block rule and office conversions.
Is bedroom air conditioning worth the money?
The 26°C rule, what to try first, fixed or portable, and what a night costs.
Cooling a loft conversion: what actually works?
Why lofts overheat, what Part O misses, and where the units go.
Checking an installer on the F-gas register
Company certificates, personal qualifications, and who issues them.
Portable or fixed: which air conditioning should you buy?
A fair comparison, including when the portable wins.
Prefer a call back?
Leave your postcode and a line about the room, and we ring you. Or ring 020 8194 3009.