Quick answer
The right AC size depends on how much heat your home gains, not only how big it is. A Manual J calculation, published by the Air Conditioning Contractors of America (ACCA), adds up heat gain from windows, insulation, air leakage, orientation, occupants, and local climate to find the cooling load in BTUs per hour. Equipment is then picked to match that number.
What Manual J looks at
Windows and sun: west-facing glass in the San Fernando Valley can add a lot of afternoon heat. Shading, window type, and overhangs matter.
Insulation and air leakage: many older Los Angeles homes have little attic insulation and leaky ducts in hot attics, which raises the load.
Climate data: design temperatures for coastal Santa Monica are very different from Woodland Hills or Pasadena, so the same house needs different equipment in different places.
People and appliances: occupants, kitchens, and electronics all add heat.
Why bigger isn't better
An oversized system reaches the thermostat setting quickly and shuts off before it removes much moisture. Rooms feel cool but damp, temperatures swing, and the frequent starts add wear. Variable-speed and inverter systems handle this better because they can run at low capacity for longer, but they still need to be sized correctly.
Sizing and California Title 24
When central equipment is replaced in California, the Title 24 energy code generally requires a permit, and in many cases duct leakage testing and verification by a HERS rater. A proper load calculation is part of doing the job right. Ask any contractor to show you how they sized your system.
