The BTU is the heat unit US equipment datasheets, gas bills, and older thermal literature are written in; NIST SP 811 Appendix B fixes the international-table BTU at 1,055.056 J. Two anchors do most of the cross-fuel arithmetic: 1 kWh ≈ 3,412 BTU (the number that links electric heaters to gas furnaces — 3.6×10⁶ J ÷ 1,055.056 J/BTU = 3,412.14) and the US therm ≈ 100,000 BTU that natural-gas bills are metered in. The unit’s trap is quantity versus rate: a BTU is an amount of energy, while equipment is rated in BTU per hour — heat flow, not heat.
Cooling equipment shows the rate unit at work. The published ENERGY STAR room-air-conditioner table maps floor area straight to capacity in BTU/h — 5,000 BTU/h for 100–150 ft² up to 34,000 BTU/h for 2,000–2,500 ft² — then adjusts in a published order: ±10% for a very sunny or heavily shaded room, +600 BTU/h per person beyond two, +4,000 BTU/h for a kitchen. A 340 ft² living room reads 8,000 BTU/h from the table; make it very sunny and it needs 8,800.
More BTU/h is not automatically better: an oversized air conditioner cools the air quickly and shuts off before it has wrung the moisture out, leaving a room cold but clammy — ENERGY STAR is explicit that when in doubt in a humid climate, favor the smaller size. At larger scales the industry changes vocabulary rather than units: dividing BTU/h by 12,000 gives tons of refrigeration, and whole-building capacity comes from a Manual-J-style load calculation rather than any per-square-foot rule.