Much less than Petrol or Diesel! At home, it's your standard electricity rate, which is probably in the region of around 14p per kW. So, a car with a 20kWh battery will cost less than £3 to fully charge. This cost can be around half the amount for homes with an Economy 7 meter installed, where charging can be done at night on cheaper rates. Out and about, there are public charging facilities where costs vary from being completely free to around £6 for half an hour on the fastest "Rapid" chargers.
However an electric car is being charged, it's easily possible to achieve around 5p per mile. With a bit more consideration around how and where you charge, it can be under 2p per mile. What's more, if you have your own solar panels at home, you can charge from the sun - for