How to Choose Air Conditioner Capacity by Room Area and Heat Gains (Los Angeles Guide)

Choosing the right air conditioning capacity is not only about square footage. In Los Angeles, the same room size can require very different cooling power depending on sun exposure, insulation, ceiling height, occupancy, and appliances. The goal is to match the cooling load (heat coming into the space) with an AC unit sized in BTU/h or “tons” (1 ton ≈ 12,000 BTU/h). Start with room area. A common baseline is 20–25 BTU per square foot for an average home. Example: a 400 sq ft room typically needs about 8,000–10,000 BTU/h. Then adjust for heat gains: add capacity for large west- or south-facing windows, intense afternoon sun, poor insulation, or high ceilings. Kitchens usually need extra cooling because of ovens and refrigerators. People also add heat: include roughly 500–600 BTU/h per additional occupant beyond two. Electronics matter too—TVs, computers, and lighting can noticeably increase demand. Don’t ignore the building envelope. Newer windows, good attic insulation, and shading from trees can reduce the required BTU significantly. Conversely, older homes common in parts of LA may leak air, raising the load and causing a small unit to run nonstop. Avoid oversizing. An AC that is too powerful cools the air quickly but may short-cycle, leaving humidity and comfort issues and wasting energy. Undersizing leads to long runtimes and high bills. For best results, use a detailed load calculation (Manual J) and choose efficient equipment with the right airflow and duct design.