Electricity

U = Voltage (V) 
  battery  1..12
  "from wall" 220
I = Current (A)
  small robotics 0.02 .. 0.5
  small lights ~1
  coffe machine ~10 
  car battery/starter ~500
P = Power (W)
  simple led 0.1..1
  mobile charger 5..20
  bigger lamp 100
  coffe heater 2000
P=U*I
R = Resistance (Ohm)
R=U/I 
  Mobile charger 20W
  U=220V
  I=? =P/U = 20/220 = 1/11 = 0.091 A
  R=? = 220/0.091 = 2420 Ohm
 
  Mobile charger 20W
  U=5V
  I=?= 20/5 = 4A
  R=5/4 ~1.25 Ohm
                    2   2   2
Series connection   R - R - R = 6 Ohm

Parallel connection   
   | - R - |          
 - - - R - - - 
   | - R - |
   
   each 330 ohm - result: 110