If the AC tonnage is wrong, either the room does not cool well or it uses too much electricity. A rough guide comes from the room area:
1. About 100 to 120 sq ft: 1 ton
2. About 120 to 180 sq ft: 1.5 ton
3. About 180 to 240 sq ft: 2 ton
This is only a rough guide. These things can change your need:
1. The room is on the top floor or gets sun from the roof.
2. There are many windows facing the sun.
3. There are more people and heat-producing appliances in the room.
In such cases think of one size higher. Also take advice from the dealer or the company before you buy.