Buying a new air conditioner starts with one question: how many BTU does this room need? This article sums up the table from the Electricity Generating Authority of Thailand (EGAT), with how to measure the room and worked examples. Or let the air-conditioner size calculator work it out.
In short
- Measure the room's width × length in square metres.
- Decide whether the room gets sun, then look it up in EGAT's table below.
- Choose a unit with Energy Label No. 5, and set it to 26–27 °C with a fan on.
What BTU means
EGAT explains that BTU/hr (British Thermal Unit per hour) is a unit of heat. For an air conditioner it means how much heat the unit moves out of the room in one hour: a 14,000 BTU/hr unit removes 14,000 BTU an hour. The bigger the number, the more cooling.
EGAT's size table
EGAT gives a column for normal rooms and one for rooms exposed to the sun, which are hotter and so cover less area.
| Unit size (BTU) | Normal room (m²) | Sunny room (m²) |
|---|---|---|
| 9,000 | 12–15 | 11–14 |
| 12,000 | 16–20 | 14–18 |
| 18,000 | 24–30 | 21–27 |
| 21,000 | 28–35 | 25–32 |
| 24,000 | 32–40 | 28–36 |
| 25,000 | 35–44 | 30–39 |
| 30,000 | 40–50 | 35–45 |
| 35,000 | 48–60 | 42–54 |
| 48,000 | 64–80 | 56–72 |
Worked examples
- Bedroom 4 × 4 m = 16 m². It falls in 16–20 (normal) and 14–18 (sunny): 12,000 BTU either way.
- 3.5 × 4 m = 14 m², sunny. It falls in both 9,000 (11–14) and 12,000 (14–18). Our calculator suggests the smallest size that fits, 9,000, and says 12,000 fits too.
- Normal room of 22 m². It falls in no row (12,000 stops at 20, 18,000 starts at 24). The calculator picks the next size up, 18,000, so the unit is never too small. That rule is ours, not EGAT's.
For a room that isn't a rectangle, such as an L shape, split it into rectangles and add them up, or use area of shapes. A room over 80 m² is beyond this table; ask an installer.
Why too big or too small is bad
EGAT says most people buy a big unit so the room cools faster, and calls that the wrong decision.
- Too many BTU: the compressor cycles often, efficiency drops, the room stays humid and uncomfortable, and the unit costs more than needed.
- Too few BTU: cooling is slow and never reaches the set temperature, the compressor overworks and wears out sooner, and the bill goes up.
Saving power
- Choose Label No. 5: EGAT says the more stars, the more it saves. Its example for a 12,000 BTU unit falls from 917.12 baht a month (No. 5) to 743.53 baht (No. 5, three stars), at 3.96 baht per unit; your own tariff may differ.
- 26–27 °C with a fan: EGAT says this saves up to 10% compared with 23–24 °C.
- Clean it every 6 months: EGAT says this keeps it working at full efficiency and saves up to 10%. DEDE also recommends cleaning twice a year.
- Use the timer: DEDE suggests switching off 15–30 minutes before you get up or stop using the room.
- Where to install: EGAT suggests avoiding west-facing walls that get the afternoon sun, and putting the outdoor unit in the shade, raised off the floor, away from roofs and corners with no airflow.
Note
This article is based on EGAT's web pages and the knowledge centre of Thailand's Department of Alternative Energy Development and Efficiency, read on 29 September 2026. The table is general guidance: a high ceiling, lots of glass, or many people and appliances may need a bigger unit, so have an installer check the room.
