Glossary · Air conditioning
What size air conditioner do I need? BTU and kW sizing
Air conditioner BTU sizing means matching a unit's cooling capacity, quoted in BTU per hour, to the heat a room gains. One kW is 3,412 BTU per hour, so 12,000 BTU is about 3.5 kW. Daikin says typical UK homes need around 0.1 kW per square metre before adjustments.
Key points
- Stelrad says 1 watt is equivalent to 3.412 Btu/hr, so 1 kW is 3,412 Btu/hr and a 12,000 BTU unit is about 3.5 kW.
- Midea's UK PortaSplit page lists its cooling capacity as 12,000 BTU / 3.5kW.
- Daikin says typical UK homes need around 0.1 kW per square metre at average ceiling heights, before adjusting for the room.
- Daikin says oversized units cycle on and off, raising running costs and wear, while undersized ones struggle to reach comfort levels.
- Daikin's installation manual says to ask a dealer or qualified personnel to install, and not to attempt it yourself.
How air conditioner sizing works
When people talk about the size of an air conditioner they mean its capacity, not the box on the wall. Daikin says size means heating and cooling capacity in kilowatts, and that the kW rating decides how much heat the system can remove from a space per hour.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit Some makers quote BTU instead.
The two are the same measure in different units. Stelrad, a radiator maker, says 1 watt is equivalent to 3.412 Btu/hr.[3]Stelrad, Radiator FAQs Midea lists its UK PortaSplit cooling capacity as 12,000 BTU / 3.5kW.[2]Midea, PortaSplit The conversion matches: 12,000 divided by 3.412 is about 3,517 W.
Converting kW to BTU
Worked from Stelrad’s factor, multiply kW by 3,412 to get BTU per hour.
| Capacity | BTU per hour |
|---|---|
| 2 kW | 6,824 |
| 2.5 kW | 8,530 |
| 3 kW | 10,236 |
| 3.5 kW | 11,942 |
| 4 kW | 13,648 |
What size does a room need
Daikin gives a rule of thumb: typical UK homes need around 0.1 kW per square metre for average ceiling heights and insulation. It says to adjust for usage, climate and the building.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit Its reference table, with a 2.4 m ceiling, converted by us:
| Room | Area | Daikin capacity | BTU per hour |
|---|---|---|---|
| Small bedroom | 20 sqm | about 2 kW | about 6,824 |
| Living room | 35 sqm | about 3.5 kW | about 11,942 |
| Open lounge or kitchen | 40 sqm | about 4 kW | about 13,648 |
Daikin lists what pushes the figure up:[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit
- A higher ceiling. A 3 m ceiling raises the room volume by 25%, and capacity should rise with it.
- Poor insulation or sun. A well-insulated 20 sqm bedroom might need 2 kW, while a poorly insulated sun-facing room of the same size could need 2.5 kW.
- Large south or west windows, which often need a 10 to 20% increase.
- Kitchens and rooms with equipment, which often need 10 to 20% more.
- People. Add around 0.18 kW per person when more than two people use the room.
Daikin also says climate matters. A 30 sqm room might need 3 kW in Scotland and 3.6 kW in the South-East of England.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit Where several rooms need cooling, Daikin says separate smaller outdoor units can be more efficient than one large unit. See multi-split air conditioning and split system air conditioner.
Fitting and the rules
This is a job for qualified people. Daikin’s installation manual says to ask your dealer or qualified personnel to carry out installation work, and not to attempt it yourself, because improper installation may cause water leakage, electric shocks or fire.[5]Daikin UK, Installation manual: FTXM-M, CTXM-M, ATXM-M room air conditioner The manual also warns of toxic gas if refrigerant leaks and contacts fire, so see refrigerant for background.[5]Daikin UK, Installation manual: FTXM-M, CTXM-M, ATXM-M room air conditioner
GOV.UK lists putting in a fixed air-conditioning system among the work that might need building regulations approval. You do not need to apply yourself if you use someone registered with a competent person scheme.[4]GOV.UK, Building regulations approval Our air conditioning team surveys the room before recommending a size.
Example
Gurdeep in Otley has a 25 sqm lounge where four people sit in the evening. Daikin’s guide gives the example of a 25 sqm lounge with four people needing about 3 kW instead of 2.5 kW.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit In BTU, 3 kW is 10,236 per hour, using Stelrad’s conversion.[3]Stelrad, Radiator FAQs Gurdeep looks for a unit around that capacity rather than a 12,000 BTU unit chosen because the number sounds right.
What people get wrong
A bigger BTU number does not mean a better air conditioner. The right number is the one that matches the room.
Daikin says oversizing leads to higher running costs and noise, because an oversized unit cycles on and off. It says undersizing forces a unit to run constantly, which inflates energy bills and reduces its life.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit The other mistake is sizing by eye. Daikin lists ignoring insulation, window orientation or ceiling height as a common error, and says to use kW calculations rather than visual judgement.[1]Daikin UK, Air Conditioner Size Guide: Find the Right Unit
The bottom line
BTU and kW measure the same thing: 1 kW is 3,412 BTU per hour. Start from about 0.1 kW per square metre, then adjust for the room’s ceiling, sun, insulation, use and the number of people. Aim for the capacity the room needs, no more, and have a qualified installer confirm it on site. Efficiency ratings such as SEER come after the sizing.
Common questions
How many BTU is 3.5 kW?
About 11,900 BTU per hour. Stelrad says 1 watt is equivalent to 3.412 Btu/hr, so 3.5 kW is 3,500 x 3.412 = 11,942 Btu/hr. Midea lists a 12,000 BTU unit as 3.5kW.
How big an air conditioner do I need for a 20 square metre room?
Daikin's guide gives about 2 kW for a 20 sqm bedroom with a 2.4 m ceiling. That is roughly 6,800 BTU per hour. Daikin says to adjust for insulation, sun, ceiling height and how the room is used.
Is it better to buy a bigger air conditioner just in case?
No. Daikin says oversizing leads to higher running costs and noisy operation, because the unit cycles on and off. An undersized unit runs constantly, raising bills and reducing the unit's life.
Can I install an air conditioner myself?
Daikin's installation manual says not to. It tells you to ask your dealer or qualified personnel, and warns that improper installation may cause water leakage, electric shocks or fire.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Daikin UK. Air Conditioner Size Guide: Find the Right Unit. Checked 11 November 2026. ↩
- Midea. PortaSplit. Checked 11 November 2026. ↩
- Stelrad. Radiator FAQs. Checked 11 November 2026. ↩
- GOV.UK. Building regulations approval. Checked 11 November 2026. ↩
- Daikin UK. Installation manual: FTXM-M, CTXM-M, ATXM-M room air conditioner. Checked 11 November 2026. ↩