Brazil BTU Calculator: What Size Air Conditioner Do I Need?
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Buying the wrong air conditioner costs you twice — once at the store and again on every electricity bill. This calculator applies the sizing rule used by installers and manufacturers in Brazil: 600 BTU/h per m² for a shaded room, 800 BTU/h per m² when direct sun hits most of the day, plus 600 BTU for each additional person and each heat-generating appliance. It then rounds the result up to a capacity you can actually buy and estimates what it will add to your bill.
How air conditioner sizing works
BTU stands for British Thermal Unit, and an air conditioner rated at "12,000 BTU" removes 12,000 BTU of heat per hour. The heat load of a room comes from four sources you can estimate quickly: its floor area, how much sun reaches it, how many people are inside, and how many appliances are dissipating heat there.
The rule of thumb used across the Brazilian market is a simple sum: floor area × 600 BTU/h per m² for a shaded room (or one that only gets morning sun), or × 800 BTU/h per m² when direct sun reaches it for most of the day. Add 600 BTU for every person beyond the first — the first occupant is already built into the per-m² coefficient — and 600 BTU for every heat-generating device such as a TV, desktop computer, console, fridge or electric oven. LED lighting and phone chargers are negligible and can be ignored.
One caveat: this rule assumes a ceiling height of up to 2.8 m. High ceilings, mezzanines, rooms directly under an uninsulated roof, and open-plan kitchens with a stove in use all need an extra 10% to 30% on top of the result.
Room size to BTU quick reference
The table below assumes the most common scenario — two people and one heat-generating device in the room — and rounds up to the capacities actually sold in Brazil (9,000 / 12,000 / 18,000 / 22,000 / 24,000 / 30,000 / 36,000 BTU). Notice how the same room can jump an entire size class purely because of which way the window faces.
| Room area | Shaded — BTU needed | Shaded — unit | Direct sun — BTU needed | Direct sun — unit |
|---|---|---|---|---|
| 9 m² (small bedroom) | 6,600 | 9,000 BTU | 8,400 | 9,000 BTU |
| 12 m² (double bedroom) | 8,400 | 9,000 BTU | 10,800 | 12,000 BTU |
| 15 m² (large bedroom) | 10,200 | 12,000 BTU | 13,200 | 18,000 BTU |
| 20 m² (small living room) | 13,200 | 18,000 BTU | 17,200 | 18,000 BTU |
| 25 m² (living room) | 16,200 | 18,000 BTU | 21,200 | 22,000 BTU |
| 30 m² (large living room) | 19,200 | 22,000 BTU | 25,200 | 30,000 BTU |
| 40 m² (living + dining) | 25,200 | 30,000 BTU | 33,200 | 36,000 BTU |
Inverter vs. conventional running costs
A conventional (on/off) unit runs its compressor at full power until the room is cold, shuts it off, then restarts at full power when the temperature rises again. An inverter unit instead varies compressor speed, dropping to a fraction of full power once the target temperature is reached. That is why the nameplate wattage — say 1,100 W on a 12,000 BTU inverter — is a peak figure reached only in the first minutes of operation.
Running 8 hours a day for 30 days, a 12,000 BTU inverter uses roughly 120 kWh per month against about 264 kWh for a conventional unit of the same capacity. At the Brazilian average residential tariff of R$0.95/kWh (taxes and the September 2026 yellow tariff flag included), that is about R$114 versus R$251 a month. The price premium of an inverter model typically pays for itself within a single summer of heavy use.
Every unit sold in Brazil also carries the ENCE energy label from Inmetro, with the most efficient models in each class earning the Procel seal. Compare units within the same capacity and technology, and look at the seasonal efficiency coefficient in W/W — the higher, the cheaper the unit is to run.
Common sizing mistakes
Undersizing is the mistake people notice first: the unit never reaches the set temperature, so the compressor runs essentially non-stop. The room still feels warm, the bill is high anyway, and the compressor wears out early.
Oversizing is the mistake people never notice. The unit cools the room in minutes and shuts down before it has run long enough to dehumidify the air, leaving the space cold and clammy. On conventional units, that constant short cycling is the harshest possible duty for a compressor. The right approach is to round up to the next commercial capacity above the calculated figure — exactly what this calculator does — and no further.
Two inputs are routinely underestimated. Sun exposure changes the result by 33% on its own, and it is the single most common reason someone buys a 12,000 BTU unit when they needed an 18,000. Electronics add up faster than expected too: a home office with a desktop, two monitors, a UPS and a printer easily contributes 2,400 BTU — around 20% of a 12,000 BTU unit.
Frequently asked questions
What room size does a 12,000 BTU air conditioner cover?
About 18 m² in a shaded room with one person and no electronics, or roughly 15 m² in the more realistic case of two people and one device. If direct afternoon sun reaches the room, the same unit covers around 12 m². Beyond that, the next commercial size is 18,000 BTU.
Should I buy a bigger unit to be safe?
No. An oversized unit cools too quickly and switches off before dehumidifying the air, leaving the room cold and humid, and the constant cycling shortens compressor life on conventional models. Round up to the next available capacity and stop there.
Does air conditioning use a lot of electricity?
It is among the biggest loads in a home, but modern inverter units are far cheaper to run than their reputation suggests. A 12,000 BTU inverter running 8 hours a day uses about 120 kWh a month, roughly R$114 at R$0.95/kWh. A conventional unit of the same size, used the same way, exceeds 260 kWh and R$250.
Why does the calculator estimate less than the nameplate wattage suggests?
The nameplate figure is peak compressor power, drawn only while the room is cooling down to the set temperature. An inverter then slows the compressor and runs at a fraction of that. Averaged over a session, a 12,000 BTU inverter draws around 500 W, which is why the estimate is 120 kWh per month rather than 264.
Sources
Reference estimate based on the 600–800 BTU/h per m² sizing rule used in the Brazilian market. Actual consumption varies with brand, model, ENCE efficiency rating, insulation, set temperature and local climate. Costs use the R$0.95/kWh national average residential tariff (taxes and yellow flag, September 2026); your own utility rate and the monthly tariff flag can change this significantly. For commercial projects or rooms with unusual heat loads, consult an HVAC professional.
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Also available in Portuguese: Calculadora de BTU: qual ar-condicionado eu preciso?