What Size AC Do I Need?

What a load calculation looks at

• Floor area, ceiling heights and volume
• Insulation levels in walls and attic
• Window area, glazing type and which direction each face points
• Air leakage — how tight or leaky the house is
• Shading, roof colour and orientation
• Whether the house is coastal or inland, which materially changes the load in San Diego County

Why oversizing is a real problem

Dehumidification only happens while the system is running. A system that satisfies the thermostat in short bursts never runs long enough to pull moisture out, so the house feels clammy at a temperature that should feel comfortable. It also short cycles, and compressors wear out on starts.

Why undersizing is less common than people fear

A slightly smaller system that runs longer, steadier cycles usually delivers better comfort and better humidity control than an oversized one. Long run times are a feature, not a fault.

The San Diego County wrinkle

Coastal and inland homes in this county have genuinely different cooling loads. A house in Encinitas under a regular marine layer and a house in Escondido in an inland valley are not the same problem, and sizing them the same way gets one of them wrong.