What the inputs mean
- Usable battery capacity: the energy the car can actually use, in kWh. Some makers quote a larger gross figure that includes a reserve; use the usable one if you have it.
- Start and target charge: the state of charge, in percent, where the session starts and where you stop it.
- Charger power: the power of the wall box or charging station, in kW. The car can take less. On AC, its onboard charger limits the power; on a DC fast charger, the car's own DC limit does. The car takes whichever is lower, so enter the car's limit if you know it.
- Charging loss: an assumption, default 10%. Some energy turns to heat in the charger, cables and battery. Set it to what you measure, or to 0 to see the ideal case.
- Electricity price: what you pay per kWh, in your own currency.
The formulas
energy into battery = usable kWh × (target % − start %) ÷ 100energy from grid = energy into battery ÷ (1 − loss %)hours = energy from grid ÷ power used cost = energy from grid × pricePower used is the lower of the charger power and the car's limit. The reverse: in N minutes, the battery gains power × N ÷ 60 × (1 − loss %) kWh, up to full.
Worked example
A 60 kWh usable battery, from 20% to 80%, on an 11 kW charger with a 10% loss and electricity at 0.16 per kWh. The battery takes 60 × 0.60 = 36 kWh. The grid supplies 36 ÷ 0.90 = 40 kWh. At 11 kW that is 40 ÷ 11 = 3.64 hours, or 3 h 38 min, and it costs 40 × 0.16 = 6.40. With no loss, the time would be 3 h 16 min.
The reverse: 30 minutes on the same charger draws 5.5 kWh, puts 4.95 kWh in the battery and lifts the charge from 20% to 28.3%.
On a 150 kW fast charger with a car limited to 50 kW, the power used is 50 kW and the same 20% to 80% takes 48 minutes at best.
Why fast charging takes longer than this
The calculator assumes the power stays constant for the whole session. On AC charging that is close to true. On DC fast charging, the car lowers the power as the battery fills, and often much more above about 80%. The battery's temperature matters too: a cold battery can accept less power. So the constant-power time on a fast charger is a best case. For a real figure, time a session on your own car and compare.
Installing a Level 2 charger at home? See Wire size for a home charger on nailsandnumbers.com.
Questions
How do I calculate EV charging time?
Take the energy you need, add the charging loss, and divide by the power. A 60 kWh battery from 20% to 80% needs 36 kWh; with a 10% loss that is 40 kWh from the grid, which takes 3 h 38 min at 11 kW.
How long does it take to charge an EV at 7.2 kW?
It depends on the battery and how much you add. From 10% to 100% of a 60 kWh usable battery is 54 kWh; with a 10% loss that is 60 kWh from the grid, or 8 h 20 min at 7.2 kW, if the car accepts 7.2 kW.
Why does my car charge slower than the charger's rating?
The car takes the lower of the charger's power and its own limit, and on DC fast chargers it lowers the power as the battery fills. A 150 kW charger on a car limited to 50 kW charges at 50 kW at most.
How much does it cost to charge an EV from 20% to 80%?
Multiply the energy from the grid by your price. For a 60 kWh usable battery with a 10% loss, 20% to 80% draws 40 kWh, which costs 6.40 at 0.16 per kWh.