It's the moderate-US-climate average for whole homes with 8 ft ceilings and average insulation — roughly DOE/Energy Star's 12,000 BTU per 500–600 sq ft guidance restated. As a whole-home central-AC baseline in Zone 3, it's genuinely useful.
Hot climates: Zone 1–2 homes run 25–28 BTU/sq ft. Single rooms vs whole homes: one room with exterior walls and direct sun carries far more load per square foot than a whole-home average — a hot Indian bedroom lands near 100 BTU/sq ft once climate, ceiling and roof factors stack. Tall ceilings: add ~10% per foot above 8 ft. Glass: sunrooms and west walls of glass can double a room's load. The fix is always the same: compute the load from the actual factors — the BTU calculator applies them all.
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About 20,000–22,000 BTU as a whole-home moderate-climate figure (a 2-ton system); 25,000–28,000 in hot US climates.
Because they size single rooms in severe heat — climate, solar gain and tall ceilings push per-room loads to ~100 BTU/sq ft, ten times the whole-home US baseline divided across a building.
Put the numbers to work: AC tonnage calculator · bill calculator · model number decoder.