How to Choose the Right AC Size for Your Home in Cambodia

Most people buying a new air conditioner in Phnom Penh focus on brand, price, and whether to go inverter. The question of size — measured in BTU or horsepower — often gets answered by whatever the salesperson recommends or whatever fits the bracket on the wall.

This is a mistake worth avoiding. Getting the size wrong creates problems that no amount of good maintenance or sensible thermostat settings can fix.

Why Size Matters More Than Most People Think

Air conditioner capacity is measured in BTU per hour (British Thermal Units). In Cambodia, you'll also hear units described in horsepower (HP): a 1HP unit is roughly 9,000 BTU, 1.5HP is around 12,000–13,000 BTU, and 2HP is approximately 18,000 BTU.

The goal is to match the unit's capacity to the cooling load of the room — how much heat the unit needs to remove to maintain a comfortable temperature. If the capacity is well matched, the unit cycles on and off in regular intervals, maintains a stable temperature, and removes humidity steadily from the air.

An undersized unit runs continuously without ever reaching the set temperature. In Phnom Penh's heat, a unit that's too small simply can't overcome the thermal load — it runs flat out, the compressor never rests, electricity consumption is high, and the room stays warmer than you want. Components wear faster because of the constant operation.

An oversized unit reaches the set temperature too quickly and shuts off before it's had a chance to dehumidify the room properly. The result is a space that feels cold but clammy — the temperature drops but the humidity stays elevated. Oversized units also cost more upfront and often produce uncomfortable temperature swings between their short on cycles and long off periods.

In Cambodia's climate, where humidity management is as important as temperature control, the oversizing problem is particularly noticeable. A room cooled by an oversized unit often feels uncomfortable at the same thermostat setting that would feel pleasant with a correctly sized unit — because the air is still laden with moisture.

The Basic Calculation

A widely used rule of thumb is approximately 600 BTU per square metre for standard residential rooms in tropical climates. This assumes a typical ceiling height of 2.7–3m, a moderate level of sun exposure, and normal occupancy.

Using this rule:

• A 15m² bedroom → 9,000 BTU (1HP unit)
• A 20m² bedroom or studio → 12,000 BTU (1.5HP unit)
• A 25–30m² living room → 18,000 BTU (2HP unit)
• A 40m² open-plan living/dining area → 24,000 BTU (2.5HP unit)

These are starting points, not final answers. Several factors in Phnom Penh's specific conditions push the required capacity higher or lower.

Factors That Increase the Required Capacity

Direct sun exposure. A room with large west-facing windows that receives afternoon sun in Phnom Penh is dealing with a significantly higher heat load than a shaded north-facing room of the same size. For rooms with significant direct sun exposure, add 10–15% to the base calculation.

High ceilings. Older villas and some newer developments have ceilings at 3.5m or higher. The volume of air to be cooled is substantially larger even with the same floor area. For ceilings above 3m, recalculate based on volume rather than floor area, or add 10–20% to the base figure.

Poorly insulated construction. Concrete walls with little insulation — which describes many apartments in Phnom Penh, particularly older buildings — allow more heat to conduct through from outside. If your walls get noticeably warm to the touch during the day, factor this in.

High occupancy. People generate significant body heat. A room regularly used by four people needs more cooling capacity than one used by one. Each occupant beyond two adds roughly 600 BTU to the load.

Kitchen heat load. Open-plan kitchens that combine cooking and living areas are common in Phnom Penh apartments. Cooking generates substantial heat. If you're cooling a space that includes an active kitchen, this adds materially to the required capacity.

Upper floors of buildings. Heat radiates through the roof in direct sun. Top-floor apartments in low-rise buildings without proper roof insulation run noticeably warmer than mid-floor equivalents. Add 10–15% for top-floor rooms under direct sun.

Factors That Reduce the Required Capacity

Well-shaded rooms. A room with good shading — overhanging eaves, external blinds, trees — that receives minimal direct sun has a lower heat load. For consistently shaded rooms, the 600 BTU per square metre figure is likely sufficient without addition.

Good cross-ventilation when the AC isn't running. Rooms that ventilate well overnight retain less heat into the following day. This doesn't directly change the AC sizing calculation, but it means the unit is starting from a lower base temperature each morning.

Thick concrete external walls. Older Phnom Penh construction with thick masonry walls — particularly ground and mid-level floors — provides reasonable thermal mass that buffers temperature swings. These rooms are easier to cool and maintain once the temperature is down.

Choosing Between a 1.5HP and a 2HP Unit

The most common decision in a Phnom Penh apartment context is between a 1.5HP and 2HP unit for a mid-sized room. The size calculation often puts a 20–25m² room in an ambiguous zone.

The general guidance: if the room is on the upper floor, gets direct afternoon sun, or you regularly use it with multiple people, go with the 2HP unit. If it's a lower or mid-floor bedroom with limited sun exposure and typical occupancy, a 1.5HP unit is usually right.

The energy saving calculation from inverter technology is relevant here too. A correctly sized inverter unit will always be more efficient than a larger fixed-speed unit that short-cycles — so choosing correctly sized inverter equipment is the combination that delivers both comfort and low running costs.

What the Wrong Size Does to Maintenance Needs

Incorrectly sized units affect maintenance in ways that aren't immediately obvious.

An undersized unit running continuously accumulates internal contamination faster than a correctly sized unit cycling normally. The volume of air processed is higher, which means more dust and airborne particles reaching the evaporator coil and blower drum over the same period. These units typically need professional cleaning more frequently — four times a year rather than three — because they run so many more hours.

An oversized unit that short-cycles doesn't run its dehumidification phase properly. The result is a persistently damp internal environment that accelerates mould growth. If you've noticed recurring mould problems in your AC despite regular cleaning, an oversized unit is worth considering as a contributing factor.

Either way, correct sizing reduces the maintenance burden. A unit that's running as designed — cycling naturally, maintaining the target temperature without heroic effort — stays cleaner and lasts longer. Our guide to extending your air conditioner's lifespan in Cambodia covers the full picture of what determines how long a unit lasts, and correct sizing appears near the top of that list.

Multi-Split Systems for Larger Spaces

For homes or apartments with multiple rooms, a multi-split system — one outdoor unit serving two or more indoor units — is worth considering. The efficiency advantage is modest compared to separate single-split systems, but the reduced outdoor unit count matters in buildings where outdoor unit placement is restricted.

For large open-plan spaces that exceed what a single 2.5HP wall-split can handle efficiently, a cassette unit is often the better solution. Cassette units distribute air in four directions from a central ceiling position, which handles large floor areas more evenly than a wall-mounted unit blowing from one side of the room. The differences between cassette and wall-split unit maintenance are relevant here — cassette units have specific cleaning requirements that wall-split owners don't deal with.

Getting Advice Before You Buy

If you're replacing a unit and suspect the original was incorrectly sized — either running constantly without reaching temperature or cooling too quickly and leaving the room feeling humid — mention this to the technician doing your professional service. An experienced AC technician can assess the room and give you a practical sizing recommendation based on what they observe.

Before making a purchase, it's also worth reviewing the best AC brands available in Cambodia — not all brands offer the same capacity range, and some brands have better parts availability at the sizes you need. For upper-floor apartments with significant heat load, units from Daikin and Mitsubishi are particularly well suited because their compressors handle sustained high-load operation better than budget alternatives.

If you're renting and the existing unit seems perpetually overwhelmed, the landlord and tenant AC guide for Phnom Penh covers how to raise equipment adequacy with your landlord — because providing a unit that's materially undersized for the space is a legitimate issue to address at the tenancy level.

The Bottom Line

Sizing an air conditioner correctly costs nothing extra at the point of purchase — you're choosing between options at similar price points. But getting it wrong costs you in electricity every month for the life of the unit and creates problems that regular maintenance can only partially offset.

For most Phnom Penh apartments, 600 BTU per square metre is a sound starting point. Adjust upward for direct sun, high ceilings, upper-floor placement, or high occupancy. When in doubt between two sizes and the room has meaningful heat load, go with the larger unit — the short-cycling problem of oversizing is preferable to the constant-running problem of undersizing, and inverter technology mitigates the efficiency penalty of occasional short cycles far better than it mitigates running flat out indefinitely.

Once the right unit is in place, keeping it clean and running it at sensible thermostat settings is what turns a well-chosen unit into a well-performing one for the long term.