For your outlet?
U * I = P
P = Watt
I = Amps
U = Voltage
I = P / U
So if your outlet is 230v (which it is in Europe) and your appliance use 300 watts = 300 / 230 = 0,77 A
Lower voltage (V) = Higher Current (A)
Hope that helps
But if it is between an outlet and something you should always use a switch rated at the same rating as the fuse of the outlet, so always use a switch that has the same rating as the fuse before it