BTU.
The unit heat is measured in. Everything else about capacity is arithmetic on top of it.
A BTU, or British Thermal Unit, is the amount of heat needed to raise one pound of water by one degree Fahrenheit. It is a quantity of energy, and on its own it says nothing about time.
What matters in air conditioning is BTU per hour, often written BTUh, which is a rate. When a spec sheet says 36,000 BTU it almost always means per hour.
BTU and tons
One ton of cooling is 12,000 BTU per hour. So:
- 2 ton: 24,000 BTUh
- 3 ton: 36,000 BTUh
- 4 ton: 48,000 BTUh
- 5 ton: 60,000 BTUh
Residential central equipment is usually talked about in tons and window or portable units in BTU, which is the only real reason both words survive.
Why the number on the box is optimistic
Rated capacity is measured under standard test conditions. A Phoenix afternoon is not standard test conditions.
Capacity falls as outdoor temperature rises, because the condenser has to reject heat into hotter air. A system rated at 36,000 BTUh delivers meaningfully less at 115 than it does at the rating point, which is one of several reasons Valley systems run so much longer than the same equipment elsewhere.
Bigger is not better
An oversized system satisfies the thermostat quickly, shuts off, and leaves the house cool and clammy because it never ran long enough to remove moisture. It is also hard on the compressor.
Correct capacity comes from a load calculation on the actual house. Square footage rules of thumb are how houses end up with the wrong equipment.