Why AC Sizing Matters
Selecting the correct air conditioner size is one of the most important decisions you can make for home comfort and energy efficiency. An undersized unit runs non-stop trying to cool a space it cannot handle, consuming more electricity while still failing to reach the set temperature. An oversized unit blasts the room cold in minutes, then shuts off before completing a full cooling cycle — this short-cycling leads to poor humidity control, uneven temperatures, and excessive on/off wear on the compressor.
BTU Rule of Thumb
The most widely used starting point is 20 BTU per square foot (about 215 BTU/m²) for rooms with standard 8-foot ceilings. A 200 sq ft bedroom thus needs roughly 4,000 BTU, a 400 sq ft studio around 8,000 BTU, and a 1,000 sq ft open-plan space about 20,000 BTU. These baseline figures must be adjusted upward for top-floor rooms (add 10–15%), rooms with high sun exposure (add 10%), poor insulation (add 10–15%), and additional occupants beyond two (add 600 BTU per extra person).
BTU vs. Tons vs. kW
North American AC units are rated in BTU/h or tons (1 ton = 12,000 BTU/h). Asian and European units typically express capacity in kW, where 1 kW ≈ 3,412 BTU/h. A 2.5 kW mini-split is equivalent to roughly 8,500 BTU/h, suitable for a bedroom or small living room. When comparing models, always check the SEER (Seasonal Energy Efficiency Ratio) or EER rating alongside the BTU number — a higher rating means lower operating costs for the same cooling output.
Frequently Asked Questions
A. Too small: runs continuously, wastes energy, never cools adequately. Too large: short-cycles, dehumidifies poorly, wears out faster.
A. Approximately 20 BTU/sq ft (215 BTU/m²) for standard rooms. Adjust up for top floors, south/west exposure, and poor insulation.
A. 1 kW = approximately 3,412 BTU/h. Divide BTU/h by 3,412 to convert to kW.