As with petrol cars, the running costs will vary according to the size of the vehicle and how it is being driven. As a rule of thumb however, the electricity costs for an EV will be around one third to one quarter the petrol costs for a similar vehicle.
The formula to work out the cost of charging a car is simply to multiply the size of the battery by the cost of electricity. Eg if your electricity is $0.25/kW and you drive a Tesla Model 3 with a 50kWh battery, the cost to charge will be 0.25 x 50 = $12.50.
Read more on EV charging costs.