What size mini split does a 500 sq ft bedroom in East Chattanooga, TN need?

Notes

• The "20 BTU per square foot" rule is a Midwest number. East Chattanooga runs 2,114 cooling degree days, which works out to 24.3 BTU/ft² — so for this bedroom the rule is 21% short. A single national figure has to be wrong somewhere, and this is where.
• Bedroom: the room where oversizing hurts most — quiet, low load, long runtimes wanted.
• 15,000 BTU/hr is 24% above the load, and the gap between head sizes is wide — the next size down is 12,000. In a single room that matters more than it would on a central system: there are no other rooms to absorb the excess capacity. If the room is well insulated and shaded, the smaller head is usually the better buy, and modern inverter compressors modulate down far better than they modulate up.
• East Chattanooga is fairly dry — summer nights stay within 20°F of the day's high, the signature of humid air. A meaningful share of this head's work is removing moisture, and it only removes moisture while it is running. This is the climate where an oversized head produces a room that reads cold and feels clammy.
• Do not forget the winter side. East Chattanooga designs to 20°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 20°F in writing.
• Where these numbers come from. Lovell Field, 6 km from downtown East Chattanooga, daily records 2015–2024. Design temperatures are calibrated against published ASHRAE values at eight airports and land within about 2°F of them.