How it’s worked out
- The base capacity comes from the ENERGY STAR table (the U.S. government’s efficiency program), which goes by floor area in square feet. The calculator also accepts square metres (1 m² = 10.76 sq ft).
- It’s then adjusted the way ENERGY STAR says: 10% less if the room is heavily shaded, 10% more if it’s very sunny, 600 BTU more for each person from the third one who’s usually in the room, and 4,000 BTU more if it’s a kitchen. The 10% applies to the table figure; people and kitchen are added after.
- kW and frigories. 1 kWh is 3,412 BTU (EIA), and 1 frigorie per hour (the unit used in Argentina and Uruguay) is about 3.968 BTU per hour. These figures say how much the unit cools, not how much electricity it uses.
- What it uses = BTU ÷ the unit’s efficiency (its EER or CEER, which is exactly that: BTU of cooling per watt of electricity). It’s the same sum the U.S. Department of Energy’s buying guidance uses: a 12,000 BTU unit with a CEER of 14.7 running 750 hours uses 12,000 × 750 ÷ 14.7 = 612 kWh a year.
- Per day = watts × hours × 0.75. With the thermostat, the compressor isn’t flat out all the time: in the official table of Argentina’s energy regulator (ENRE), a 1,350 W air conditioner of 2,200 frigories uses 75% of its power in an hour.
The ENERGY STAR table
| Area (sq ft) | Area (m²) | BTU per hour |
|---|---|---|
| 100 to 150 | 9.3 to 13.9 | 5,000 |
| 150 to 250 | 13.9 to 23.2 | 6,000 |
| 250 to 300 | 23.2 to 27.9 | 7,000 |
| 300 to 350 | 27.9 to 32.5 | 8,000 |
| 350 to 400 | 32.5 to 37.2 | 9,000 |
| 400 to 450 | 37.2 to 41.8 | 10,000 |
| 450 to 550 | 41.8 to 51.1 | 12,000 |
| 550 to 700 | 51.1 to 65 | 14,000 |
| 700 to 1,000 | 65 to 92.9 | 18,000 |
| 1,000 to 1,200 | 92.9 to 111.5 | 21,000 |
| 1,200 to 1,400 | 111.5 to 130.1 | 23,000 |
| 1,400 to 1,500 | 130.1 to 139.4 | 24,000 |
| 1,500 to 2,000 | 139.4 to 185.8 | 30,000 |
| 2,000 to 2,500 | 185.8 to 232.3 | 34,000 |
ENERGY STAR works these capacities out for an 8-foot (2.4 m) ceiling. If yours is higher, it advises a more powerful unit but doesn’t say how much: in that case, have an installer work it out.
An example
A 4 × 4 metre bedroom (16 m², which is 172 sq ft), very sunny, where 3 people sleep:
- From the table, 150 to 250 sq ft: 6,000 BTU.
- Very sunny: 6,000 × 10% = 600 BTU more → 6,600.
- The third person: 600 BTU more → 7,200 BTU per hour, which is 7,200 ÷ 3,412 = 2.11 kW of cooling (or 1,815 frigories).
- With a new unit at the U.S. minimum (CEER 10.9): 7,200 ÷ 10.9 = 661 W. On for 8 hours: 661 × 8 × 0.75 = 3.96 kWh a day, about 119 kWh a month.
- With an ENERGY STAR certified one (CEER 14.7): 7,200 ÷ 14.7 = 490 W, about 88 kWh a month.
Efficiency: EER, CEER and W/W
All three measure the same thing: how much cooling the unit gives for each watt of electricity it uses. The higher, the less it uses to cool the same.
- In the U.S. and much of the Americas it’s given in BTU per watt-hour (for example 10.9 or 14.7).
- In Europe and the UK it’s given in W/W (kW of cooling over kW of electricity, for example 3.2). To convert it to BTU/Wh, multiply by 3.412.
- In the U.S., a window unit of 8,000 to 13,999 BTU must have a CEER of at least 10.9 to be sold, and 14.7 to carry the ENERGY STAR label (Department of Energy FEMP guidance, updated December 2024). The old non-inverter units in Argentina’s ENRE table work out at about 6.5.
To make it use less
- Set the thermostat as high as is comfortable; ENERGY STAR suggests about 78 °F (26 °C).
- On humid days, set the unit’s fan to low: slower air removes more moisture from the room.
- Don’t put lamps or TVs near the unit’s thermostat: their heat makes it run longer.
- A fan helps spread the cool air around the room.
- Make sure it’s well sealed in the window: according to ENERGY STAR, a badly installed one leaks as much air as a 6-square-inch hole.
A window or wall air conditioner is plugged straight into the wall, never into a power strip or extension cord. Split systems and large units go on their own circuit: they must be installed by an installer or electrician, who can also fine-tune the capacity for your home.
Sources
- ENERGY STAR (U.S. Environmental Protection Agency). Room Air Conditioners: sizing table, adjustments, 8-foot ceiling and usage tips.
- U.S. Department of Energy, Federal Energy Management Program, December 2024. Purchasing Energy-Efficient Room Air Conditioners: minimum and ENERGY STAR CEER, and the annual energy sum.
- U.S. Energy Information Administration (EIA). British thermal units (Btu): 1 kWh = 3,412 Btu.
- Ente Nacional Regulador del Gas y la Electricidad (Argentina). Consumo básico de electrodomésticos: power and hourly use of 2,200 to 4,500 frigorie air conditioners (in Spanish).