Calculate energy added to the battery
Multiply usable battery capacity by the percentage-point increase in state of charge. A 60 kWh usable battery charged from 20% to 80% gains about 36 kWh of stored energy.
Allow for charging losses
The charger must usually supply more energy than the battery stores. If overall charging efficiency were 90%, storing 36 kWh would require about 40 kWh from the wall.
Apply the tariff that matches the charging time
Multiply wall energy by your electricity price. If you use a time-of-use tariff, apply the price for the hours in which the charging session actually occurs.
Do not confuse charger power with session energy
A 7.4 kW home charger describes the approximate charging power available, not the energy used by every session. Energy depends on both power and charging time.
Public charging can have extra fees
Some public networks may add session, time, parking or other charges. If pricing is not purely per kWh, include those separately from the energy calculation.