What size air conditioner do I need?

Cooling capacity is measured in BTU per hour — British Thermal Units. It is simply a measure of how much heat the unit can move out of your room in an hour. The bigger the number, the more heat it shifts.

Almost everyone assumes that bigger is safer. It is not. An oversized air conditioner is a genuine problem, and it is the more expensive mistake of the two.

Start with floor area

As a rough starting point, allow roughly 600 BTU for every square metre of floor area. That gives you:

• Up to 15 m² — 9,000 BTU
• Up to 20 m² — 12,000 BTU
• Up to 30 m² — 18,000 BTU
• Up to 40 m² — 24,000 BTU

Measure the room and find your bracket. That is your baseline. Now adjust it.

Then adjust for the things that actually matter

Floor area alone is a crude measure, because two rooms of identical size can have very different heat loads. Move up one capacity step if two or more of these apply:

• The room takes direct afternoon sun, especially through west-facing windows.
• The windows are large relative to the wall area, or single-glazed.
• The ceiling is above three metres, which means far more air volume than the floor area suggests.
• The room is directly under an uninsulated roof.
• More than three people use the room at once — each adult adds roughly 100 W of heat.
• There is heat-producing equipment: a kitchen, servers, a workshop, display lighting.

Move down a step only if the room is genuinely shaded all day, well insulated, and lightly occupied.

Why undersizing costs you

An undersized unit never reaches the set temperature on the hottest days. It simply runs continuously at full output, which means maximum electricity draw for hours on end, constant compressor wear, and a room that is still uncomfortable. You pay the most for the least comfort.

Why oversizing costs you more

This is the part most people get wrong. An oversized unit cools the air very quickly, hits the thermostat setting, and shuts off. That sounds efficient. It is not, for two reasons.

First, an air conditioner removes humidity only while it is actually running. Moisture condenses on the cold evaporator coil and drains away, and that process takes sustained runtime. A unit that blasts the room cold in six minutes and stops has not run long enough to pull the moisture out. You end up with a room that is cold and clammy — technically at temperature, genuinely unpleasant.

Second, compressors wear out from starting, not from running. Every start draws a large inrush of current and puts mechanical stress on the system. An oversized unit short-cycles: on, off, on, off, all day. It will fail years earlier than a correctly sized one, and it draws that heavy starting current far more often.

One unit or two?

For a long or L-shaped space, two smaller units usually beat one large one. A single indoor unit can only throw air so far, so in a long room you get one cold end and one warm end regardless of capacity. Two 12,000 BTU units in the right positions will outperform one 24,000 BTU unit in a space like that, and you keep cooling if one fails.

Inverter or fixed speed?

If the room is used for more than a few hours a day, choose an inverter. A fixed-speed compressor is either fully on or fully off; an inverter varies its speed to match the actual heat load, so it holds a steady temperature, draws less current over a day, and runs quietly enough to sleep beside. The higher purchase price is usually recovered within a couple of seasons.

Fixed speed still makes sense for a room cooled only occasionally, or where you want the simplest possible machine to service.

When to stop calculating and just ask

The guide above will get you to the right capacity for most ordinary rooms. If the space is unusual — very high ceilings, a glass frontage, a commercial kitchen, a server room, or a space you are fitting out from scratch — the heat load needs calculating properly rather than estimating.

Call us with the room dimensions, the ceiling height, which way the windows face and what happens in the room. It takes a few minutes and it is free. Sizing it correctly at the start is far cheaper than discovering the mistake in August.