Air Conditioner Sizing
An undersized unit costs more to run than an oversized one.
Reading your result
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How it is calculated
Undersizing costs more in running cost than oversizing. It reads backwards, but a unit that cannot reach the setpoint runs at full output indefinitely, while a larger one throttles back once it gets there. On inverter units the gap is substantial.
Identical floor areas can need very different capacity. Top floors take heat through the roof, west-facing rooms take sun all afternoon, and large glazing admits more of it. All three together push the requirement above 20%.
Marketing size classes are not comparable between manufacturers. Two units sold under the same class can differ in actual capacity, so the kW or BTU figure on the spec sheet is the number to compare — which is why both appear here.
Frequently asked questions
Should I combine adjoining rooms?
Combine them if the doors stay open, calculate separately if they close. Cooling a bedroom from a living-room unit means sizing for the combined area.
Class size or BTU?
BTU or kW is the real specification; the class is closer to a product name. Compare the numbers.
Any harm in going too large?
Little in running cost, but purchase cost rises and a badly oversized unit short-cycles, which leaves humidity behind. One step above the calculated figure is a comfortable margin.
An estimate from area and typical conditions. Real requirements depend on insulation, glazing, floor level, occupancy and equipment heat. Check the manufacturer's rated coverage before buying.