On AC the car's onboard charger caps the power (often 7–11 kW); check that it can take the charger's full rating.
Adding 45 kWh (20% → 80%) draws 50 kWh from the grid: about 6 h 56 min on a 7.2 kW charger for $8.00, good for about 281 km. Per 100 km the EV costs $2.84 against $12.25 for the petrol car.
Energy added is the battery's usable capacity times the change in charge level, and dividing by the charging efficiency gives the energy drawn from the grid. Charging time is that grid energy divided by the charger's power, and the cost is grid energy times the price per kWh. The car's energy use turns the same figures into range added and a running cost per 100 km or 100 miles, set against a petrol car.
The defaults take a 75 kWh battery from 20% to 80%: 45 kWh into the battery, 50 kWh from the grid at 90% efficiency, just under 7 hours on a 7.2 kW charger (the output of most home Level 2 units, per the US Department of Energy) and 8.00 at 0.16 per kWh. That adds about 281 km at 16 kWh/100 km.
The time assumes constant power. DC fast chargers slow as the battery fills, and on AC the car's onboard charger can cap the rate below the charger's rating.
How much does it cost to charge an electric car?
Multiply the usable battery capacity by the share you add, divide by the charging efficiency to get the energy from the grid, and multiply by your price per kWh. Taking a 75 kWh battery from 20% to 80% adds 45 kWh, draws 50 kWh at 90% efficiency and costs 8.00 at 0.16 per kWh.
The same energy sets the charging time and the range added, so one set of numbers answers all three questions.
Worked example: 75 kWh battery, 20% to 80%
Energy into the battery: 75 kWh × (80% − 20%) = 45 kWh.
Energy from the grid: 45 ÷ 0.90 = 50 kWh. The 5 kWh difference is lost in charging.
Cost: 50 kWh × 0.16 = 8.00.
Time on a 7.2 kW charger: 50 ÷ 7.2 = 6.94 hours, or 6 h 56 min 40 s.
Range added at 16 kWh/100 km: 45 ÷ 0.16 per km = 281.25 km.
Charging levels work on the change, not the end point: 20% to 80% is 60 percentage points of the battery, not 80% of it.
How long does it take to charge an electric car?
Divide the grid energy by the charger's power. 7.2 kW is the output of most home Level 2 units, according to the US Department of Energy, so the 50 kWh session above takes just under 7 hours.
The Alternative Fuels Data Center puts Level 1 (120 V) at about 5 miles of range per hour, Level 2 at about 25 miles per hour, and DC fast charging at 100 to 200+ miles per 30 minutes.
Power is rarely constant. DC fast chargers slow sharply above about 80%, and on AC the car's onboard charger, often 7 to 11 kW, can cap the rate below the charger's rating. Treat the result as a minimum time.
Charging losses and efficiency
The EPA names the charging cable and the car's onboard charger as where the energy is lost. fueleconomy.gov, citing Idaho National Laboratory data, says charging efficiency is often 84% to 93%, so a 45 kWh top-up draws roughly 48 to 54 kWh.
A lossless session is the lower bound. At 100% the default top-up draws 45 kWh, costs 7.20 and takes 6 h 15 min, against 8.00 and 6 h 57 min at 90%.
EPA ratings vs battery-side energy use
Range and running cost need the car's energy use measured at the battery, such as the trip computer's figure. A mid-size EV uses about 15–20 kWh/100 km, or 3–4 mi/kWh.
EPA ratings include charging losses measured at the wall outlet. Pairing one with a 90% charging efficiency counts the loss twice.
To approximate battery-side energy use, multiply a wall rating in kWh per distance by the efficiency: 30 wall kWh/100 mi at 90% becomes 27 kWh/100 mi. For a distance-per-kWh rating, divide by the efficiency instead.
EV vs petrol cost per 100 km
Cost per 100 km is battery kWh/100 km ÷ efficiency × price: 16 ÷ 0.9 × 0.16 = 2.84. A petrol car using 7 L/100 km at 1.75 a liter costs 7 × 1.75 = 12.25 for the same distance.
In US units, a car doing 3.5 mi/kWh costs 100 ÷ 3.5 ÷ 0.9 × 0.16 = $5.08 per 100 miles at the same tariff. A 30 mpg car with gas at $3.50 a gallon costs 100 ÷ 30 × 3.50 = $11.67.
These are energy costs only. Purchase price, depreciation, insurance and servicing sit in the car ownership cost calculator.
Charging session examples
Lossless 60 kWh charges a 60 kWh battery from 10% to 90% at 11 kW with no losses and power at 0.30 per kWh. DC high charge takes the default 75 kWh battery from 10% to 100% at 150 kW.
Miles per kWh is the default session for a car rated 4 mi/kWh, and converted wall rating is the same session at 27 battery kWh/100 mi. Times are in seconds.
Scenario
Grid energy, kWh
Time, seconds
Range added, km
Default
50.000000
25000.000000
281.250000
Lossless 60 kWh
48.000000
15709.090909
300.000000
DC high charge
75.000000
1800.000000
421.875000
Miles per kWh
50.000000
25000.000000
289.681920
Converted wall rating
50.000000
25000.000000
268.224000
The lossless session takes 4 h 22 min and the DC session 30 minutes at constant power; in practice the DC charge would slow well before 100%.
Common EV charging mistakes
Using gross battery capacity. Advertised gross capacity can include a buffer the driver cannot use, which overstates the energy in each percentage point.
Counting losses twice. An EPA rating already includes them; use battery-side consumption with the session's efficiency.
Trusting the charger's label for time. Taper near full and the car's AC limit both stretch the session.
Ignoring winter. fueleconomy.gov, citing AAA's 2019 tests, reports range about 41% lower at 20 °F than at 75 °F with the cabin heater on, so raise the energy-use figure in cold weather.
Divide the energy needed by the charger's power. Taking a 75 kWh battery from 20% to 80% draws 50 kWh from the grid, which is 6.9 hours at 7.2 kW. The US Department of Energy's Alternative Fuels Data Center puts Level 1 (120 V) at about 5 miles of range per hour, Level 2 at about 25 miles per hour, and DC fast charging at 100 to 200+ miles per 30 minutes.
How much does it cost to charge an electric car?
Multiply the energy drawn from the grid, including charging losses, by your price per kWh. The default session puts 45 kWh into the battery, draws 50 kWh at 90% efficiency and costs 8.00 at 0.16 per kWh. At 16 kWh/100 km that is 2.84 per 100 km, against 12.25 for a petrol car using 7 L/100 km at 1.75 a litre.
How much energy is lost when charging an EV?
Typically 7% to 16%. fueleconomy.gov, citing Idaho National Laboratory data, says battery charging efficiency is often 84% to 93%, so a 45 kWh top-up draws roughly 48 to 54 kWh from the meter. The 90% default sits in that range. The EPA names the charging cable and the car's onboard charger as the places where the energy is lost.
Should I enter the EPA rating or my car's own energy figure?
Use battery-side consumption with the charging efficiency for the session. EPA ratings include charging losses measured at the wall. To approximate battery-side kWh per distance from a wall rating, multiply by the efficiency; for distance per kWh, divide by it. For example, 30 wall kWh/100 mi at 90% becomes 27 battery kWh/100 mi, retaining 90% efficiency. Setting efficiency to 100% makes the whole charging session lossless and changes its energy, cost and time.
Does cold weather reduce EV range?
Yes, mostly through cabin heating. fueleconomy.gov, citing AAA's 2019 tests, reports that at 20 °F (−7 °C), compared with 75 °F (24 °C), EV range fell about 41% with the cabin heater in use and about 12% without it. Raise the energy-use figure in winter to see the effect on range and cost.
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