The formula
Time (hours) = capacity × (target − start) ÷ 100 ÷ (average input power × efficiency).
A worked example
A 60 kWh battery from 20% to 80%, at an average 7.2 kW input and 90% efficiency, takes approximately 5.56 hours.
What this estimate includes
Use the average input power the car can actually accept. Peak DC charger ratings are not session averages. Tapering, temperature, battery conditioning and load balancing can extend the session.
Common questions
Can I enter a 150 kW fast charger rating?
Only if that is the average input power over the modeled session. Peak charger capacity alone is not enough to predict DC charging time.
Why is my real session slower?
The vehicle, charger, supply and temperature can limit power. Charging also commonly slows as the battery approaches full charge.
Further reading
US Department of Energy: charging at home. Background reference; these tools are independent and are not endorsed by the source organization.