Knowing how to calculate heat load in a practical way is essential for correctly sizing a split air conditioner and avoiding choices that are too short (the unit is working too hard) or too high (unnecessary expense). I'll show you a straightforward, reliable method for quick use, with step-by-step examples and practical adjustments you can apply now.
1) Quick Concept
Heat load = amount of heat that needs to be removed from the room per hour. We measure it in BTU/h (a unit used in air conditioning). The "professional" calculation uses many details (insulation, facade, sunlight, infiltration, etc.), but here I show you a practical and safe method for sizing a residential or light commercial split air conditioner.
2) Practical Step-by-Step (Rule of Thumb)
Base (Simple Rule):
Use 600 to 800 BTU per m² under normal conditions.
600 BTU/m² → environments with little sunlight and reasonable insulation.
800 BTU/m² → rooms with high sunlight or high ceilings.
Complete practical example
Room 4.0 m × 5.0 m → area = 4.0 × 5.0 = 20.0 m².
We'll choose 600 BTU/m² (moderate conditions).
Base calculation:
600 × 20 = 12,000 BTU/h.
(Detail: 600 × 20 = (600 × 2) × 10 = 1,200 × 10 = 12,000)
Common adjustments (add to the base value)
Each additional person (over 2 people) → +600 BTU per person.
Electronic equipment (TV, PC, small ovens): add 300–1,000 BTU depending on wattage.
Nearby kitchen/oven → +2,000 to 4,000 BTU (calculation recommended).
Room with direct sunlight → +10% to +20% of the total.
High ceilings (>3 m) → +10%.
Poor insulation or many doors/windows → increase 10–20%.
Continuing the example:
Occupants: 4 people total → consider 2 additional people → 2 × 600 = 1,200 BTU.
Equipment: TV + PC ≈ 500 + 400 = 900 BTU.
Insolation: room with strong sunlight → +10% (applies to the subtotal).
Calculating the values:
Base = 12,000 BTU.
People = + 1,200 → subtotal = 12,000 + 1,200 = 13,200.
Equipment = + 900 → subtotal = 13,200 + 900 = 14,100.
Insolation 10% = 10% of 14,100 = 1,410 → final total = 14,100 + 1,410 = 15,510 BTU/h.
(If you prefer to round: 15,510 ≈ 15,000 to 16,000 BTU/h.)
3) Convert to kW (optional)
1 kW ≈ 3,412 BTU/h.
Example: 15,510 BTU/h → 15,510 ÷ 3,412 ≈ 4.55 kW.
(Verification: 3,412 × 4 = 13,648; remainder 15,510 − 13,648 = 1,862 → 1,862 ÷ 3,412 ≈ 0.55 → total ≈ 4.55 kW.)
4) How to choose a split system in practice
Commercial models come in standard capacities (8,000, 9,000, 12,000, 18,000, 24,000 BTU, etc.).
For the example (≈15,500 BTU/h), choose the 18,000 BTU (close to safe commercial capacity).
Rule of thumb: Choose a model just above the design (not too high) to ensure performance on hot days and with higher occupancy.
5) Quick Adjustment Chart (Practical Use)
Base: 600–800 BTU/m²
Extra person: +600 BTU/person
TV/PC: +300–800 BTU each (depending on the device)
Nearby refrigerator: +800–1,500 BTU
Kitchen/oven: +2,000–4,000 BTU
Direct sun (exposed facade): +10% to +20%
High ceilings (>3 m): +10%
6) Final Tips and Precautions
Don't undersize. Insufficient air overloads the compressor and increases consumption and maintenance.
Don't oversize. A device that's too large cools quickly and has short cycles, causing discomfort and poor dehumidification.
Evaluate insulation, doors, and windows — heavily ventilated spaces require greater adjustment.
For commercial spaces, kitchens, or areas with variable heat loads, consult a designer or use a technical calculation (Manual J, detailed heat load).
Choose appliances with a Procel/A efficiency rating and inverter technology for savings and better capacity adjustment.
7) Quick summary (step-by-step guide to apply now)
Measure the area of the room (m²).
Multiply by 600–800 BTU/m² → base.
Add adjustments for people, equipment, and usage.
Apply a percentage for sunlight/ceiling height/insulation.
Choose a commercial capacity close to and slightly above the calculated total.