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You can either go for a 1 ton AC or a 1.5 ton AC depending upon the amount of light that enters the room, area and direction of windows, the floor on which your room is situated, number of people occupying the room and the number of lights.
Let's find what generator you need for a 5-ton air conditioner. At nominal voltage such an a/c would initially draw around 145A. But at 70% voltage it would need only 145*0.7=101.5 A. From the genset chart above we see that to provide such an inrush current you need a system rated 14kW or greater.