How many BTU per square foot?
ENERGY STAR's sizing chart starts at 5,000 BTU for 100–150 sq ft and works out to roughly 20 BTU per square foot for typical rooms. A 12' x 15' bedroom (180 sq ft) needs about 6,000 BTU; a 450–550 sq ft living room needs about 12,000 BTU. The full chart is printed under the planner.
Adjustments
- Sun: reduce the capacity by 10% for a heavily shaded room, and add 10% for a very sunny one.
- People: add 600 BTU for each person beyond two who regularly uses the room.
- Kitchen: add 4,000 BTU if the unit will cool a kitchen.
- Ceiling: the chart assumes 8 ft ceilings. For higher ceilings this planner scales the load by room volume — a 10 ft ceiling adds 25%.
Why bigger isn't better
An oversized air conditioner cools the room quickly and then shuts off before it has run long enough to pull moisture out of the air. The room ends up cold and damp, and the short on-off cycles waste energy and wear out the compressor. Pick the stock size closest to the load; if the next size up is much bigger, a variable-speed (inverter) unit handles the extra capacity better.
Window, portable or mini-split?
Window units cover most single rooms up to about 12,000 BTU and are the most efficient for the money. Portable units are easiest to install but lose some cooling through their exhaust hose, so many people size them up. For larger rooms, or to cool and heat quietly, a ductless mini-split is the better choice. Check the electrical, too: units over about 14,000 BTU often need a 230-volt outlet.
BTU and tons
One ton of cooling is 12,000 BTU per hour. A 24,000 BTU mini-split is a 2-ton system. For a whole house or a central system, have an HVAC contractor run an ACCA Manual J load calculation — it accounts for windows, insulation and climate, which a single-room chart can't.
Sources: ENERGY STAR — room air conditioners.