What decides how long a charge takes
The time to charge an electric vehicle depends on three things: how much energy the battery needs, the power available, and the losses along the way. The calculator divides the energy needed by the effective charging power:
Time = battery capacity × (target % − start %) ÷ (charging power × efficiency)
The charging power is the lower of what the charger delivers and what the car accepts. On AC charging, the car’s onboard charger is usually the limit; on DC fast charging, it is the battery’s maximum rate.
Charging levels in the US
- Level 1, a standard 120 V outlet: about 1.4 kW, or 3 to 5 miles of range per hour.
- Level 2, 240 V home or public charger: typically 7 to 11.5 kW, or 20 to 40 miles per hour.
- DC fast charging: 50 to 350 kW, often adding 150 miles or more in 20 to 30 minutes, if the car can take it.
Example
A car with a 75 kWh usable battery, charged from 20% to 80% on a 7.7 kW Level 2 charger, with an 11 kW onboard charger and 90% efficiency:
- Energy added to the battery: 75 × 60% = 45 kWh
- Energy drawn from the grid: 45 ÷ 0.9 = 50 kWh
- Charging time: 50 ÷ 7.7 = 6 hours 30 minutes
- Range added at 30 kWh per 100 miles: 150 miles, about 23 miles per hour of charging
The same charge on a Level 1 outlet at 1.44 kW would take about 35 hours.
Why real charging times differ
- Tapering: above about 80%, the car reduces the power to protect the battery, so the last part of a charge is slower, especially on DC fast chargers.
- Temperature: a cold battery charges more slowly until it warms up, and preconditioning helps.
- Shared power: some public stations split power between two cars.
- Efficiency: losses are higher at low power, so Level 1 charging is the least efficient.
Tips
- For daily driving, a Level 2 charger overnight covers most needs.
- Charging to 80% is enough for most days and kind to the battery; charge to 100% before long trips, or as your manufacturer recommends.
- Check the maximum AC charging rate of your car before buying a more powerful home charger.