A BTU (British Thermal Unit) is a unit of heat energy. One BTU equals the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Used to rate air conditioners and heating systems.
How BTUs Apply to HVAC
Air conditioner capacity is measured in BTU/hour — how much heat the unit can remove per hour. A window unit might be 5,000-12,000 BTU, while a central system can be 24,000-60,000 BTU (2-5 tons). One ton of cooling = 12,000 BTU/hour.
General rule: you need approximately 20 BTU per square foot of living space, adjusted for climate zone, ceiling height, insulation quality, sun exposure, and number of occupants.
Sizing Matters
An undersized unit runs constantly without reaching the set temperature. An oversized unit cycles on and off too frequently ("short-cycling"), wasting energy and failing to dehumidify properly. Correct sizing is critical for comfort and efficiency.
Real-World Example
A 300 sq ft bedroom: 300 × 20 = 6,000 BTU base. Add 10% for a sunny room: 6,600 BTU. A 6,000-8,000 BTU window unit would be appropriate. For a 2,000 sq ft home: 2,000 × 20 = 40,000 BTU (3.3 tons) for a central system.
Frequently Asked Questions
How many BTU do I need per square foot?
The baseline is 20 BTU per sq ft. Adjust upward for hot climates, poor insulation, lots of windows, high ceilings, or kitchens (add 4,000 BTU). Adjust downward for shaded rooms, cold climates, or well-insulated spaces.
What does SEER rating mean?
SEER (Seasonal Energy Efficiency Ratio) measures how efficiently an AC converts electricity to cooling. Higher SEER = more efficient. Current minimum is 14 SEER; high-efficiency units reach 20-25 SEER. A higher SEER unit costs more but saves on electricity.
Are BTU and watts related?
Yes. 1 watt = 3.412 BTU/hour. A 1,500-watt space heater produces about 5,120 BTU/hour. To convert: BTU/hour ÷ 3.412 = watts.