What size AC does a 1800 sq ft house in Mount Pleasant, SC need?

Notes

• This is a rule of thumb keyed to Mount Pleasant's climate, not a Manual J. The tonnage above comes from 25.5 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 Mount Pleasant's own 2,653 cooling degree days — two cities in the same zone 800 degree-days apart do not get the same answer here. What Mount Pleasant actually contributes is the two design temperatures — 95°F in summer and 26°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 Mount Pleasant the humidity matters more than the heat. Summer nights fall only 17°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.
• 59 days a year reach 90°F here, and the design temperature is 95°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.
• Bigger is not safer. An oversized compressor reaches setpoint and stops before it has run long enough to dehumidify, then starts again minutes later. Short cycling is the single most common cause of early compressor failure, and it is bought, not suffered.
• Running cost, roughly. 2,653 cooling degree days works out to about 1,194 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 3,916 kWh a season, about $652 at the national average rate. Moving to SEER 18 would cut it to roughly $507 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 Mount Pleasant's own coordinates over 2015–2024, not copied from a table: 1,563 heating and 2,653 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.