Aircondition of which capacity, e.g. 1.5 ton or 2 ton, would be sufficient for a 20 ft X12ft top floor room?

Answer 1

The capacity is given in Kilowatts. And you should have the height of the room. Then you multiply 20 by 12 by (height of room in feet). This will give you the total cubic feet of the room. I think 1.5 Kw would be enough but it is much better to ask the seller.

Answer 2

Most air-conditioners are usually measured in hp (horsepower) and not tons. A Half hp will do a large bedroom (about 135sq feet). A 1 - 1 1/2 Hp will do the living room/s of a normal house, or most of a small unit (about 600 sq feet). A 2 - 2 1/2 Hp will cool up to 12 - 14 square (about 1200 sq feet) or a moderately sized home.
Of course the efficiency does depend on the insulation qualities of the place to be cooled, air circulation, and placement of the unit.