if devices are connected in series how will we calculate its wattage if the power of each appliance is given 

When appliances of power P₁, P₂, P₃…P_n are connected  in series with a voltage source, the effective power consumed P_effective is given by: 

 

For example:

Consider the following electric circuit in which the electric bulbs of power rating 60W each are connected in series with the voltage source 220 V.

 

Here the bulbs are connected in series. The effective power consumed is:

 

 

In series connections bulbs share the power.Therefore each bulb will light up with power = 20 W.

Total power used = 20×3 =60 W