What size AC does a 2400 sq ft house in San Diego, CA need?

Notes

• This is a rule of thumb keyed to San Diego's climate, not a Manual J. The tonnage above comes from 20.5 BTU per square foot. That figure is not a lookup: the industry's zone table turns out to be a degree-day table in disguise, so it is interpolated continuously against San Diego's own 1,047 cooling degree days — two cities in the same zone 800 degree-days apart do not get the same answer here. What San Diego actually contributes is the two design temperatures — 85°F in summer and 41°F in winter — and those are the numbers a real load calculation begins from. A house here with new windows and a sealed envelope can come in a full ton under this; a 1960s house with an uninsulated attic can come in a ton over.
• In San Diego the humidity matters more than the heat. Summer nights fall only 10°F below the afternoon high, and that narrow swing is the signature of humid air — water vapour holds the night temperature up. It means a large share of the machine's work is pulling water out of the air, not lowering temperature — and that work only happens while the compressor is actually running. An oversized unit here cools the room fast, shuts off, and leaves the moisture behind: the house reads 75°F on the thermostat and still feels clammy. This is the climate where buying one size up "to be safe" backfires hardest, and where variable-speed equipment earns its price difference.
• 4 days a year reach 90°F here, and the design temperature is 85°F. Equipment is sized for the 1% condition, not the hottest hour on record — which means on the hottest afternoons of the year this system is expected to run continuously and still drift a degree or two above setpoint. That is the intended behaviour. Sizing for the record high instead would leave the unit oversized for the other 361 days, short-cycling through all of them.
• Rounding up to 5 tons puts this 22% above the calculated load. Nominal sizes jump in half-ton steps and there is nothing between them, so ask whether two-stage or variable-capacity equipment at this size makes sense — that hardware can run at part load and sidestep the oversizing, which single-stage equipment cannot.
• Running cost, roughly. 1,047 cooling degree days works out to about 471 equivalent full-load hours here — the number of hours the compressor would run at full output if it ran flat out instead of cycling. At SEER 14 that is 1,657 kWh a season, about $276 at the national average rate. Moving to SEER 18 would cut it to roughly $215 a year — worth less here than the brochures suggest, because the system simply does not run enough hours to earn the difference back quickly.
• About the climate figures. Everything above is computed from ERA5 reanalysis at San Diego's own coordinates over 2015–2024, not copied from a table: 945 heating and 1,047 cooling degree days, base 65°F. The zone shown alongside is derived from those degree days against the ASHRAE 169 boundaries — your building department assigns zones by county from older climate normals and may list a different one, so go by the county table when you are pulling a permit. City-centre readings also run a degree or two warmer than the airport station most published tables use.