What’s BTU?
If you are looking for air conditioning, you may see ratings such as 9,000 or 12,000 BTU. But what do these numbers mean?
BTU stands for British Thermal Unit. In air conditioning, the rating tells you how much heat a system can remove from a room each hour. It helps you compare cooling capacity and choose a system that suits your space.
What does BTU measure?
A BTU is a unit of heat energy. One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.
For air conditioning, the useful measurement is BTU per hour, written as BTU/h. Product listings often shorten this to “BTU”.
For example, a unit rated at 12,000 BTU/h can remove around 12,000 BTU of heat each hour under its rated test conditions. Air conditioning cools a room by taking heat from inside and releasing it outside.
A higher rating means more cooling capacity. It does not mean the air coming out will always be colder.
What is the difference between BTU and kW?
You may also see air conditioning capacity listed in kilowatts, or kW. These are two ways of describing the same cooling output.
1 kW of cooling capacity is approximately 3,412 BTU/h.
| Cooling capacity in BTU/h | Approximate cooling capacity in kW |
| 7,000 | 2.1 |
| 9,000 | 2.6 |
| 12,000 | 3.5 |
| 18,000 | 5.3 |
| 24,000 | 7.0 |
These figures convert cooling capacity only. They do not show electricity use or the room size each unit will suit.
Does a higher BTU rating mean higher running costs?
The BTU rating alone cannot tell you how much a system will cost to run. Running costs also depend on its efficiency, how long it operates, your chosen temperature and your electricity tariff.
For example, a unit with 3.5 kW of cooling capacity does not necessarily use 3.5 kW of electricity. Cooling output and electrical input are separate figures.
Check the manufacturer’s electrical input and energy efficiency information when comparing models.
Is a higher BTU rating always better?
The best rating is one that matches the room’s cooling needs.
A unit with too little capacity may run for long periods and still struggle to reach your set temperature. A system that is too large can also cause problems. Depending on its controls and minimum output, it may switch on and off too often, leading to uneven temperatures and poor moisture removal.
Choosing a suitable size helps the system keep your room comfortable and work efficiently.
How many BTUs do I need?
Room size is a starting point, but other factors affect how much cooling you need:
- Ceiling height and room layout
- Window size and direct sunlight
- Roof and wall insulation
- The number of people using the room
- Heat from computers, lighting and other appliances
A sunny home office may need more cooling than a shaded bedroom of the same size.
Our guide to what size air conditioning unit you need explains these factors in more detail.
For bedrooms, lounges and home offices, we can help you choose a suitable domestic air conditioning system. Workplaces often have different needs, so our commercial air conditioning services include systems planned around the building and how it is used.
Get help choosing the right air conditioning
You do not need to work out the BTU rating on your own. Subcooled can assess your space, calculate its cooling needs and recommend a suitable system.
Request an air conditioning quote to discuss your property and the rooms you would like to cool.