How the calculation works
Charging time is the energy you need divided by the power your source can deliver, adjusted for the losses between the wall and the cells.
Charging efficiency is typically around 85 to 90% for AC home charging, since the onboard charger converts AC to DC and both it and the battery generate heat. DC fast charging bypasses the onboard charger and runs a little more efficiently, though thermal management then consumes some of that advantage.
Why the last 20% takes so long
AC charging is essentially flat: 7.4 kW at 30% state of charge and 7.4 kW at 90%. DC fast charging is not. Above roughly 80%, the battery management system tapers power sharply to protect the cells, so a charger capable of 150 kW may be delivering 40 kW or less near the top. That is why road trip advice universally recommends charging 20 to 80% and driving on rather than waiting for a full battery.
At home the taper barely matters, because the charger is slower than the taper threshold anyway and the car is parked all night regardless.
Worked example
A 60 kWh battery going from 20% to 80% on a 7.4 kW home charger:
Comfortably overnight. The same job on a household outlet at 1.4 kW would take about 29 hours, which is why Level 1 charging suits plug-in hybrids and low-mileage drivers but frustrates full EV owners.
Common mistakes
- Assuming the charger dictates speed. The car has its own onboard AC limit. A vehicle capped at 7.4 kW gains nothing from a 22 kW wallbox; check the vehicle spec before buying the charger.
- Expecting rated DC speed throughout. Peak power lasts a narrow window. Real average power on a 150 kW charger over a 20 to 80% session is often half the headline figure.
- Ignoring cold weather. A cold battery accepts charge slowly until it warms. In winter, DC sessions can start at a fraction of rated power; preconditioning while driving to the charger helps significantly.
- Charging to 100% daily. Most manufacturers recommend an 80 to 90% daily ceiling for lithium longevity, reserving 100% for trips.
Frequently asked questions
How long to charge an EV at home overnight?
A 7.4 kW home charger adds roughly 6.5 kWh per hour after losses, or about 40 kilometres of range per hour for a typical car. An eight hour night therefore adds around 50 kWh, which covers all but the largest batteries from near empty.
Can I charge an EV from solar?
Yes, and the arithmetic is the same: your array output in kW replaces the charger figure. A 6 kW array producing 30 kWh on a good day can add roughly half a typical battery, though only while the sun is up unless a home battery buffers it. Solar diverters that match charging current to surplus production are the usual solution.
Does charging speed damage the battery?
Frequent DC fast charging accelerates degradation slightly compared with slow AC charging, mainly through heat. Occasional fast charging on trips is not a concern; relying on it exclusively for daily charging is the pattern manufacturers advise against.