EV charging
EV charging time from charger power and efficiency
- where
- energy into the battery (kWh)
- usable battery capacity of the vehicle (kWh)
- target state of charge (%)
- starting state of charge (%)
- energy drawn from the supply (kWh)
- charging efficiency (%)
- charging time (h)
- charger power (kW)
Explain this equation
Estimates how long an electric car takes to charge from one state of charge to another, and the energy drawn from the supply. Useful when talking customers through charger options.
The change in state of charge as a fraction of the battery capacity gives the energy that goes into the battery. Some is lost in the charger and cabling, so the energy drawn is that divided by the efficiency. Dividing by the charger power gives the hours.
Time equals energy divided by power, with a loss factor.
Sense check: 20 to 80% of a 60 kWh battery is 36 kWh, about 40 kWh from the supply at 90% efficiency, about 5.4 hours on a 7.4 kW charger.
Watch out: the car's on-board charger may limit the power below the wallbox rating, and charging slows near full.
How long a home or workplace charger takes to add a given amount of charge, and how much energy it draws, for example from surplus solar.
Enter your values to see the result.
Good to know
Use the lower of the charger rating and the car's on-board AC charger limit (many cars accept 7.4 kW single-phase, some only 3.7 kW). ADAC tests measured losses of about 5–7% on an 11 kW wallbox and 13–24% from a domestic socket; 90% is a fair default for a 7 kW home charger. Solar-only charging modes will be slower and depend on surplus.
Common mistake
Ignoring the car's on-board charger limit, or quoting time from battery kWh without charging losses.
Limits
Constant power. Charging slows near full and in cold weather. Install to BS 7671 Section 722 and the IET Code of Practice for EV charging.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.