Cooling capacity of an air conditioner is measured in tonnes. The tonnage of an air conditioner depends on its BTU/hr (British Thermal Units per hour) value. Choosing the correct cooling capacity helps reduce your electricity bills. Divide the square feet area of your room (length multiplied by width) by 600 to arrive at the basic tonnage capacity required. Add 0.5 tonnes for every 10 people occupying the room at the same time. Similarly, add 0.5 tonnes for every 1500 watts of appliances or lighting present in the room (A computer would consume about 300 watts and a regular bulb, 40 to 60 watts). Always round off on the higher side.