Air Conditioner Buying Guide: BTU, Power, and Room Size

Pick an air conditioner that's too small for your room and you'll still be sweating in August — and shivering in January. Pick one that's too large and it'll short-cycle, spike your electricity bill, and leave the air feeling damp and clammy. The good news: getting the sizing right doesn't require an engineering degree. You need a tape measure, a few minutes, and the right framework.

This guide walks you through everything that matters when buying an air conditioner — BTU calculation, kW sizing, inverter technology, energy ratings, and heat pump capability — whether you're equipping a bedroom, an open-plan living space, or a commercial property in Bosnia & Herzegovina or the wider region.

What is BTU — and why does it matter?

BTU stands for British Thermal Unit, a measure of thermal energy. In HVAC terms it describes how much heat a unit can remove from (cooling) or add to (heating) a space per hour. The bigger or hotter or more sun-exposed the room, the more BTU you need.

European spec sheets almost always list capacity in kilowatts (kW) alongside BTU. The conversion is straightforward: 1 kW = 3,412 BTU. Both figures appear in every product listing you'll encounter.

Common capacity brackets on the market:

• 9,000 BTU / 2.6 kW — small bedroom
• 12,000 BTU / 3.5 kW — standard living room
• 18,000 BTU / 5.3 kW — large room or open plan
• 24,000 BTU / 7.0 kW — office, café, small warehouse

AC sizing table: room size to cooling capacity

The table below applies to standard residential and commercial spaces with ceiling heights up to 2.7 m, average insulation, and moderate sun exposure. Adjust for your conditions using the correction factors in the next section.

• Room size (m²): Up to 15 m² — Recommended cooling capacity: 1.5 – 2.0 kW — BTU: 5,100 – 6,800 BTU — Typical use: Small bedroom
• Room size (m²): 15 – 25 m² — Recommended cooling capacity: 2.0 – 2.6 kW — BTU: 6,800 – 9,000 BTU — Typical use: Bedroom
• Room size (m²): 25 – 35 m² — Recommended cooling capacity: 2.6 – 3.5 kW — BTU: 9,000 – 12,000 BTU — Typical use: Living room, office
• Room size (m²): 35 – 50 m² — Recommended cooling capacity: 3.5 – 5.3 kW — BTU: 12,000 – 18,000 BTU — Typical use: Larger living area
• Room size (m²): 50 – 70 m² — Recommended cooling capacity: 5.3 – 7.0 kW — BTU: 18,000 – 24,000 BTU — Typical use: Open plan, boutique
• Room size (m²): 70 – 100 m² — Recommended cooling capacity: 7.0 – 10.0 kW — BTU: 24,000 – 34,000 BTU — Typical use: Small premises, restaurant
• Room size (m²): Over 100 m² — Recommended cooling capacity: Multi-split or central — BTU: — — Typical use: Office, hall, warehouse

For spaces over 100 m², contact our team for a multi-split or central system recommendation.

How to estimate BTU — a quick sizing method

A reliable baseline formula for Balkan continental and Mediterranean climates:

BTU = floor area (m²) × ceiling height (m) × coefficient factor

A factor of 100–150 BTU per cubic metre covers most standard scenarios. In practice:

1. Measure the room length × width: e.g. 5 m × 6 m = 30 m²
2. Multiply by ceiling height: 30 m² × 2.7 m = 81 m³
3. Multiply by factor 125 (mid-range): 81 × 125 = 10,125 BTU
4. Round up to the nearest standard model: 12,000 BTU / 3.5 kW

When to increase capacity

• High sun exposure (south- or west-facing, large unshaded glazing): add 10–20 %
• Ceiling height above 2.8 m: calculate actual volume, not floor area alone
• Kitchen or high-heat room: add 20–25 %
• Poor insulation (older build, unaddressed thermal bridges): add 15 %
• Multiple occupants or computers in an office: add 400–500 BTU per person/device
• North-facing room with modern insulation and triple glazing: subtract 10 %

Inverter vs. standard air conditioner

This is the question we hear most. The short answer: inverter AC is almost always the better choice.

A standard (ON/OFF) unit runs at full power until it hits the target temperature, then switches off completely. The repeated hard starts of the compressor consume more electricity and accelerate wear.

An inverter unit continuously adjusts compressor speed to match the actual thermal load of the room. The result:

• 30–50 % lower electricity consumption compared to a standard unit of the same class
• Quieter operation (no sudden start/stop noise)
• Tighter temperature control (±0.5 °C)
• Longer compressor lifespan

Every unit in Green Tech's heating and cooling catalogue features inverter technology as standard.

Energy ratings: SEER, SCOP, and what A+++ actually means

Since 2021 the EU has used a refreshed energy label scale running from A to G for air conditioners (you'll still encounter the older A/A+/A++/A+++ scale on some products still in circulation).

• SEER (Seasonal Energy Efficiency Ratio) — cooling efficiency across a full season
• SCOP (Seasonal Coefficient of Performance) — heating efficiency across a full season

Higher numbers mean lower running costs:

• Rating: A+++ (old scale) — SEER (cooling): ≥ 8.5 — SCOP (heating): ≥ 5.1 — Assessment: Outstanding
• Rating: A++ — SEER (cooling): ≥ 6.1 — SCOP (heating): ≥ 4.0 — Assessment: Excellent
• Rating: A+ — SEER (cooling): ≥ 5.6 — SCOP (heating): ≥ 3.4 — Assessment: Recommended minimum
• Rating: A — SEER (cooling): ≥ 5.1 — SCOP (heating): ≥ 3.0 — Assessment: Meets standard

For B&H conditions — cold winters, hot summers — we recommend at minimum A+ with SCOP ≥ 3.8 if you plan to use the unit for heating as well as cooling.

Using your AC as a heat pump in winter

Modern inverter units with heat pump technology can heat effectively at outdoor temperatures as low as −15 °C or −25 °C (depending on model specification). A SCOP of 3.5 means the unit delivers 3.5 kW of heat for every 1 kW of electricity consumed — far more economical than direct electric resistance heating.

If you're looking at reducing total energy spend across your home or business, pairing efficient air conditioning with LED lighting consistently delivers measurable results — see LED vs. Traditional Lighting: Real Energy Savings for a concrete breakdown.

For larger commercial premises where both lighting and HVAC represent significant cost lines, the principles in Industrial LED Lighting for Warehouses and Halls are directly applicable to a combined energy efficiency review.

Other factors worth your attention

• Noise level (dB): Indoor units below 25 dB are suitable for bedrooms; below 20 dB for very light sleepers
• WiFi and smart control: Remote on/off scheduling pays back in energy savings — off when you leave, on 30 minutes before you arrive
• Air filtration: PM2.5 filters, ionisers, and antibacterial coatings matter for allergy sufferers
• Brand support and service network: Availability of spare parts and authorised service centres in your region affects total cost of ownership

Frequently asked questions

Can an air conditioner both heat and cool? Yes — all modern inverter units are reversible heat pumps. The same device that cools in summer heats efficiently in winter, typically delivering 3 to 5 units of heat energy for every unit of electricity consumed.

How much can I save with an inverter AC versus a standard model? In a typical household running a unit 6–8 hours per day across summer and winter, the difference is often 200–400 KM per year in favour of the inverter model, depending on your electricity tariff and usage intensity.

What energy class should I choose? A+ as a minimum for residential use. For commercial settings with 8+ hours of daily operation, A++ or higher pays back the premium purchase price within 2–4 years through lower bills.

One large unit or two smaller ones? Two independently controlled zones (e.g. living room and bedroom) are typically more efficient than one large unit because you only condition the space you're actually using. A multi-split system — one outdoor unit, multiple indoor units — is a practical middle ground.

How many BTU for a 60 m² open-plan space? Based on the sizing table: 18,000 – 24,000 BTU (5.3 – 7.0 kW). If the space receives heavy sun exposure or has ceilings above standard height, move toward the upper end of that range or request a specialist assessment.

Found the right capacity for your space? Browse the full heating and cooling catalogue on Green Tech, or send us an inquiry — our team will match you with the right model and connect you with the nearest partner location in your region. You can also find a partner store near you directly.