Air Conditioner Sizing: The 400W-per-Pyeong Rule

The reference value for sizing an air conditioner in Korea is 400W per pyeong (1 pyeong ≈ 3.3㎡). Multiply that Korean Industrial Standard figure by the area you intend to cool and you get the required cooling capacity in kW; the "pyeong rating" printed on retail units is simply that kW figure translated back into a label. For the living room of a 32-pyeong apartment the calculated band runs from 6.4kW to 8.3kW depending on whether the balcony has been absorbed; for a 6-pyeong bedroom, 2.3kW. The number to read first in a catalogue is not the pyeong rating but the cooling capacity in the spec table.

The order for calculating required capacity

There are three steps. First, establish the actual area to be cooled. The buying guide from Nosearch suggests taking 50% of the supply area for an apartment or villa, and 65% of usable area for an officetel or detached house. Second, multiply a living room by a further 1.3. Cool air escapes through hallways, kitchens and open bedroom doors, so applying the bedroom standard leaves the unit undersized. Bedrooms are calculated at actual area. Third, multiply the area in pyeong by 400W to get kW.

Manufacturer spec tables convert that kW back into pyeong ratings. The capacity table published by Samsung Electronics Service lists 6-pyeong at 2.3kW (18.7㎡), 10-pyeong at 4.0kW (32.5㎡), 13-pyeong at 5.2kW (42.3㎡) and 16-pyeong at 6.4kW (52.9㎡). Multiply each pyeong figure by 400W and it lands almost exactly on the stated kW. The pyeong rating is not an independent standard — it is the 400W rule under another name.

What rating does a 32-pyeong apartment need?

Applying the published rules to apartment supply areas produces the table below. Living-and-kitchen area is taken as 50% of supply area, then multiplied by the 1.3 living-room correction, then by 400W.

This is where two answers diverge. With the 1.3x correction, a 32-pyeong apartment calls for 18–20 pyeong; without it, using only "supply area ÷ 2", the answer is 16 pyeong. That simpler arithmetic is why showrooms often recommend a 16-pyeong unit for a 32-pyeong apartment. Which value to take is decided by the space itself. If the balcony has been absorbed into the living room, if living room and kitchen are fully open to each other, if the unit faces southwest into long afternoon sun, or if it is a top floor, the upper figure matches the calculation. If the balcony remains, the living room faces north, and doors stay closed, the lower figure is enough.

Bedrooms are simpler: a 6-pyeong room needs 2.4kW, an 8-pyeong room 3.2kW, corresponding directly to the 6-pyeong (2.3kW) and 8-pyeong (3.2kW) entries. The Nosearch guide notes that agonising over a single pyeong of difference in this range means little, because inverter units modulate output to match load. That said, an undersized unit runs flat out without reaching the set temperature, while an oversized one cycles on and off in short bursts and dehumidifies poorly. Choosing within one step above or below the calculated value is the safe band.

Wall-mounted versus floor-standing: same rating, same output

Form factor is a question of airflow direction, not output. At the same pyeong rating, a wall unit and a floor unit deliver identical kW. The difference is the height the air leaves from. A floor unit pushes upward from near the floor and circulates a large volume; a wall unit sends air downward from height and cools a small room quickly. Wall-mounted line-ups tending to stop around 16 pyeong reflects airflow reach and product strategy more than any hard physical limit.

So the choice follows layout, not area. If living room and kitchen form one open volume above roughly 20 pyeong of cooled area, a floor unit fits; for a single room or studio under 10 pyeong, a wall unit fits both the arithmetic and the geometry. Covering a whole apartment from one living-room floor unit assumes doors stay open — otherwise splitting the total kW between a living-room unit and per-room wall units works better.

Grade 1 versus grade 3: what is the bill difference?

Air conditioner efficiency grades are not assigned from rated power draw but from the Cooling Seasonal Performance Factor (CSPF) measured under KS C 9306, which reflects part-load operation across a whole season. Comparing rated wattage alone therefore misses the real cost gap. Converting the savings figures published by the Korea Energy Agency into won exposes the relative weight of each factor. The agency states that cutting one hour of daily operation saves 40kWh and 4,880 won a month. That implies a unit price of 122 won/kWh, and 40kWh over 30 days works out to about 1.33kWh per hour of use. Assuming four hours a day (120 hours a month), monthly consumption is roughly 160kWh. On that basis:

The grade gap comes to about 1,320 won a month, or 15,860 won a year. Dropping the setpoint by 1℃ costs 1,366 won a month — matching the grade difference — and cutting an hour of daily use saves 3.6 times as much. The agency recommends indoor cooling at 26℃ or above, noting a 7% rise in power draw for every 1℃ lower, and adds that an uncleaned filter raises consumption 3–5% on average. Efficiency grade is decided once at purchase and cannot be undone, so it belongs in the calculation, but the items that actually move the bill are operating conditions. The structural gap between inverter and fixed-speed units is covered separately in how air conditioner electricity bills diverge, and for a product category using the same area-conversion logic, see the 1.5x standard coverage rule for air purifiers.

A pyeong rating is not a standard — it is a nickname for the 400W rule. Match kW and coverage in ㎡ first, and the rating follows.

Checking real efficiency by model number

To compare on published specifications, certification data beats catalogue copy. The Korea Energy Agency's efficiency management site allows searching grade and measured CSPF by model number. Even within one brand and one size class, CSPF varies by model year and line-up, so narrowing to two or three candidates and then checking by model number wastes no effort. Three items matter: rated cooling capacity (kW), coverage area (㎡), and CSPF grade. Once those fall inside the required kW and target area calculated above, the remaining features are a price question.

Take the upper size if, wait if

• Upper size fits — balcony absorbed into the living room, fully open living-and-kitchen layout, southwest exposure or top floor, household home during daylight hours
• Lower size is enough — balcony intact and doors kept closed, north-facing living room, low floor, separate wall units per room
• Worth postponing — when required kW overlaps within one step of the rating already installed. Filter cleaning and setpoint adjustment then move the bill more than an efficiency upgrade would
• Timing — the Nosearch guide notes prices bottom out between May and July when competition peaks; longer installation queues after peak season are worth weighing too
• Installation conditions — outdoor unit placement and pipe run length affect delivered capacity. The kW on the spec table assumes standard piping

References

• Cooling capacity by pyeong rating (Samsung Electronics Service)
• Air conditioner savings guide (Korea Energy Agency)
• Air conditioner buying guide: sizing, efficiency, features (Nosearch)