What BTU power for your air conditioner?
The BTU power of a mobile air conditioner is calculated according to the surface area in m², ceiling height, thermal insulation and sun exposure. Discover how to correctly size your device to avoid
Why is BTU Power Crucial for Your Air Conditioner?
BTU power (British Thermal Unit) determines the cooling capacity of your mobile air conditioner. Inadequate sizing leads to either excessive energy consumption or insufficient comfort. Choosing the right power guarantees energy efficiency, lower bills, and device durability.
BTU measures the amount of heat an air conditioner can extract in one hour. The larger the area to be cooled, the greater the required power. This metric, although of Anglo-Saxon origin, remains the international reference for mobile air conditioners.
The Importance of Correct Sizing
An undersized air conditioner operates permanently at maximum capacity, reducing its lifespan and increasing bills. Conversely, an oversized unit cycles quickly, consumes unnecessarily, and creates damaging power surges. [According to ADEME, a poorly sized air conditioner consumes 30 to 40% more energy](https://www.ademe.fr/particuliers-eco-citoyens/habiter/climatisation-refroidissement).
How to Calculate Required BTU Power?
BTU power calculation follows a formula based on three main variables: living area, ceiling height, and exposure factors. For a reliable estimate, apply 100 to 130 BTU per m² depending on your home's thermal insulation. A well-insulated 20 m² studio requires 2,000 to 2,600 BTU, while a poorly insulated 40 m² room may require up to 6,000.
Basic Formula and Adjustments
The elementary formula is:
BTU Power = Area (m²) × Thermal Coefficient
The coefficient varies according to three criteria:
- Excellent insulation (RT 2012, double-glazed windows): 100 BTU/m²
- Average insulation (old house, single glazing): 115 BTU/m²
- Poor insulation (uninsulated attic, thermal bridges): 130 BTU/m²
Then add +20% if the room receives more than 6 hours of direct sunlight per day, and +10% per additional occupant beyond the first (body metabolism = heat output).
Concrete Example: 30 m² Apartment
Parisian apartment, 30 m², average insulation, south-facing window with lots of sunlight, 2 occupants:
- Basic calculation: 30 m² × 115 BTU/m² = 3,450 BTU
- Sunlight adjustment: 3,450 × 1.20 = 4,140 BTU
- Occupant adjustment: 4,140 × 1.10 = 4,554 BTU
- Recommended power: 9,000 BTU (rounded up to the next commercial range)
What are the factors influencing the choice of power?
The ideal BTU power does not depend only on the surface area: thermal insulation, geographical orientation, natural ventilation, and the number of occupants significantly modify the requirements. A kitchen with an oven and stovetop requires 15% additional power. A north-facing room, shaded, can do with 10% less.
Thermal insulation: the decisive factor
Insulation determines the speed of external heat infiltration. A home built to RT 2012 standards (2012 regulation) loses three times fewer calories than a property from 1980. [According to INSEE, 45% of French main residences were built before 1990](https://www.insee.fr/fr/statistiques/5039991), with often insufficient insulation.
- Single-glazed windows: +20% power
- Standard double glazing: baseline calculation
- Triple glazing or high-performance windows: -10% possible
- Uninsulated attics: +30 to 40%
- Poorly sealed doors: +15%
Solar exposure and orientation
A south-facing façade in summer accumulates 800 W/m² of solar radiation. A 2 m² window can introduce 1,600 W of additional heat. Conversely, a north-facing room loses this thermal advantage in summer.
Adjustments by orientation:
- South/west exposure: +20 to 25%
- East exposure: +10 to 15%
- North exposure: -10%
- Interior room, without direct window: -5%
Number of occupants and activities
Each person generates 100 to 150 W of body heat. An office with a computer adds an additional 300 W. An operating kitchen can generate 2,000 W of radiant heat.
- Studio with 1 sedentary person: baseline calculation
- Living room with 4 people: +30%
- Kitchen with active appliances: +40%
- Office with IT equipment: +25%
Conversion table: BTU to Watts and standards
| BTU/h | Watts | EU energy class | Room (m²) |
|-------|-------|-----------------|-----------|
| 2 500 | 730 | A+ | 15-20 |
| 5 000 | 1 460 | A+ | 20-25 |
| 9 000 | 2 635 | A | 25-35 |
| 12 000 | 3 520 | A | 35-45 |
| 18 000 | 5 275 | A | 45-60 |
| 24 000 | 7 030 | A | 60-80 |
European vs British standards
The BTU remains an imperial unit, but the European Union has required labeling in Watts (W) for air conditioners since 2021. The conversion is simple: 1 BTU/h = 0.293 W.
Performance standards differ:
- EU 2021 standard: Minimum Seasonal Energy Efficiency Ratio (SEER) of 5.5 for class A
- UK standard (post-Brexit): Maintains previous SEER standards, slightly less stringent
- French NF standard: Requires testing under real conditions (35°C indoors, 45°C outdoors)
Practical conversion example
An air conditioner displaying 12,000 BTU/h = 3,520 W. On the EU energy label, it must indicate ≥ 5.5 SEER to be classified as A. [Que Choisir recommends checking the EU label to avoid marketing traps](https://www.quechoisir.org/article-climatiseurs-mobiles-nos-conseils-pour-bien-choisir-2160).
What risks does oversizing pose?
An oversized air conditioner cools too quickly, creating short and inefficient cycles. The unit stops before properly dehumidifying the air, leaving a damp feeling. Electricity consumption increases by 15 to 25%, and repeated current surges prematurely wear out the compressor.
Short cycles and energy inefficiency
When an air conditioner reaches the target temperature in 10 minutes, it shuts off. But dehumidification (moisture removal) is not complete. The room warms up, the cycle restarts. This thermal oscillation consumes more than a properly sized unit operating in steady state.
Premature compressor wear
Repeated startups impose current surges 3 to 5 times higher than normal operation. After 5 years, an oversized air conditioner can lose 30% of its efficiency, compared to 10% for a properly selected unit.
Financial and environmental overages
- Purchase surcharge: +30 to 40% for doubled capacity
- Annual energy surcharge: +150 to 200 € over 2,000 hours of operation
- Additional CO₂ emissions: +500 kg/year for 50% oversizing
Practical examples for 20, 30 and 40 m²
Small space: 20 m² well insulated
Profile: Parisian studio, double glazing, north-facing, 1 occupant.
- Calculation: 20 m² × 100 BTU/m² = 2,000 BTU
- Adjustment: none (good insulation, no direct sunlight)
- Recommended power: 5,000 BTU (1,460 W)
- Estimated consumption: 400-500 kWh/year (summer)
- Annual cost: €60-75 (average rate €15/kWh)
Medium space: 30 m² average insulation
Profile: Old apartment living room, single glazing, southwest exposure, 2-3 occupants, 6 hours direct sunlight.
- Calculation: 30 m² × 115 BTU/m² = 3,450 BTU
- Sunlight adjustment: +20% = 4,140 BTU
- Occupant adjustment: +10% = 4,554 BTU
- Recommended power: 9,000 BTU (2,635 W)
- Estimated consumption: 750-900 kWh/year
- Annual cost: €110-135
Large space: 40 m² poor insulation
Profile: 1980s house living room, uninsulated attic, single glazing windows, east-facing, 3-4 occupants, adjacent kitchen.
- Calculation: 40 m² × 130 BTU/m² = 5,200 BTU
- Sunlight adjustment: +15% = 5,980 BTU
- Occupant adjustment: +20% = 7,176 BTU
- Kitchen adjustment: +10% = 7,894 BTU
- Recommended power: 12,000 BTU (3,520 W)
- Estimated consumption: 1,100-1,300 kWh/year
- Annual cost: €165-195
Standards and Certifications to Check
Before purchasing, verify three elements on the product label:
1. EU Energy Class: Minimum A (SEER ≥ 5.5)
2. Eurovent Certification: Guarantees announced performance in independent testing
3. Noise Level: ≤ 65 dB(A) for acceptable comfort
[The official ADEME website offers an air conditioner comparator certified units](https://www.ademe.fr/particuliers-eco-citoyens/habiter/climatisation-refroidissement), classified by actual efficiency and environmental impact.
Conclusion: Proper Sizing Saves Money
Choosing the right BTU power is a profitable investment. Accurate calculation based on surface area, insulation, exposure, and occupants avoids purchase and operating costs overruns. A well-sized 9,000 BTU mobile air conditioner consumes 30% less than an 18,000 BTU model for identical comfort.
Take the time to measure your room, assess its insulation, and apply adjustments. You will save hundreds of euros over the appliance's lifetime.
FAQ
How many BTU for a 25 m² room?
For 25 m² with average insulation, count 25 × 115 = 2,875 BTU as a base. Add +20% if south-facing exposure (3,450 BTU). The recommended commercial power is 7,000 to 9,000 BTU depending on direct sunlight and number of occupants.
Is it possible to undersizing an air conditioner?
Yes, and it's just as problematic as oversizing. An undersized unit runs 24/7 without reaching the target temperature, consumes enormous amounts of energy and ages prematurely. Always favor slight oversizing (10%) rather than undersizing.
What is the difference between BTU and Watts?
BTU (British Thermal Unit) and Watt both measure energy. 1 BTU/h = 0.293 W. An air conditioner of 12,000 BTU/h = 3,520 W. The EU has mandated labeling in Watts since 2021, but BTU remains the international commercial reference.
Should you add power for an office with a computer?
Yes. A computer generates 300 to 500 W of heat. Add +25% to your initial calculation. A 15 m² office normally requires 1,500-2,000 BTU, but with active computer equipment, plan for 2,500-3,000 BTU.
How to improve the efficiency of an existing air conditioner?
Seal air leaks (door seals, windows), install external roller shutters, clean filters monthly, and close doors to non-air-conditioned rooms. These measures reduce requirements by 15 to 25% and lower your energy bill.
Mots-clés : climatiseur mobile, puissance BTU, calcul thermique, efficacité énergétique, confort thermique