EV Charging Time Calculator
Enter the battery size, where you are starting and finishing, and what the charger and the car can each manage. The slower of the two sets the pace.
Charging time
5h 24m
- Energy added
- 36 kWh
- Drawn from the supply
- 40 kWh
- Charging at
- 7.4 kW
- In hours
- 5.41
- The car is the limit here, not the charger: it accepts 7.4 kW from a 11 kW supply, so the rest of the post sits idle.
- A steady rate is an approximation, and a generous one on rapid charging. Charging slows sharply above about 80% to protect the cells, so the last fifth can take as long as the first three.
- Cold cells charge more slowly. A car that has been standing overnight in winter may take half the rated rate until the battery warms up.
About charging time
Charging time is the energy you need divided by the rate it arrives at. The energy is straightforward — the battery size times the percentage you are adding. The rate is where most estimates go wrong.
A charger has a maximum output and the car has a maximum intake, and the charge happens at whichever is lower. A 22kW post charges a car limited to 7.4kW at 7.4kW, and the rest of the post simply sits idle. This catches people out most often with home and destination AC charging, where many cars are limited by a single-phase on-board charger regardless of what the wall can supply.
Losses stretch the time as well as the bill. Energy that never reaches the battery still has to be pushed through the cable, so a charge at 90% efficiency takes about eleven per cent longer than the battery arithmetic alone suggests.
One thing this cannot model is the charging curve. Rapid DC charging is fast up to roughly 80% and then slows sharply to protect the cells, so a 10 to 80% rapid charge is far quicker than the flat-rate figure implies for the last stretch. Treat rapid charging results as an optimistic floor and AC results as close to right.
What it works out
- Time to charge between any two states of charge
- The car's own limit against the charger's
- Energy added and energy drawn from the supply
- Home, destination and rapid charging
The formula
Time = (Battery × Percentage added ÷ Efficiency) ÷ min(Charger, Car)
Going from 20% to 80% of a 60 kWh battery means adding 36 kWh. At 90% charging efficiency the supply has to deliver 40 kWh to get those 36 into the cells.
The rate is the lower of the two limits. The charger offers 11kW but the car will only take 7.4kW, so the charge runs at 7.4kW and takes 5.41 hours — 5h 24m. Plugging the same car into a 22kW post would change nothing at all.
If the car could take the full 11kW, the same charge would finish in 3.64 hours. That is the whole argument for a three-phase on-board charger, and it is a property of the car rather than of the wall.
Rapid DC charging does not follow a flat rate. It holds a high rate to around 80% and then tapers hard, so the last fifth of a rapid charge can take as long as the first three. This calculation assumes a steady rate, which is accurate for AC charging and optimistic above 80% on DC.
- Battery
- Usable capacity in kWh. The figure quoted for a car is sometimes gross, which is slightly larger.
- Percentage
- The gap you are closing, not the state you are ending at.
- Charger
- What the charge point can supply.
- Car
- What the on-board charger will accept. Frequently the real limit on AC.
A worked example
A 60 kWh battery from 20% to 80% on an 11kW charger, in a car that accepts 7.4kW.
That works out to 5h 24m .
- Energy added
- 36 kWh
- Drawn from the supply
- 40 kWh
- Charging at
- 7.4 kW
- In hours
- 5.41
Questions
How long does it take to charge an electric car?
Divide the energy you are adding by the slower of the charger and the car. Adding 36 kWh to a 60 kWh battery at 7.4kW takes about 5h 24m once charging losses are counted.
Why is my car charging slower than the charger allows?
Almost always because the car's on-board charger is the limit. Many cars accept only 7.4kW on AC, so a 22kW post gives them 7.4kW and the rest goes unused. It is a property of the car and cannot be changed by finding a bigger charger.
How long on a normal domestic socket?
A long time. A three-pin socket supplies around 2.3kW, so adding 36 kWh takes about sixteen hours before losses and over seventeen once they are counted — and losses are proportionally worse on a slow charge. It is a top-up method rather than a charging method.
Why do people charge to 80% rather than 100%?
Two reasons. Charging slows sharply above 80% to protect the cells, so the last fifth takes disproportionately long on a rapid charger; and lithium-ion packs last longer if they are not routinely held at full charge.
Does cold weather slow charging?
Considerably. A cold battery will not accept a high rate until it warms, and on a rapid charger the car may spend the first stretch heating the pack. A car left out overnight in winter can charge at half its rated rate at first.
Is this accurate for rapid charging?
It is a reasonable floor rather than a precise figure. Rapid charging follows a curve that holds high to around 80% and then tapers, so a 10 to 80% charge often beats this estimate and a charge above 80% takes much longer than it.
What does kW mean on a charger?
The rate energy is delivered at — kilowatts are kilowatt hours per hour. A 7kW charger running for one hour delivers about 7 kWh, less losses.
Should I include charging losses?
Yes, for both time and cost. Energy lost as heat still has to pass through the cable, so a 90% efficient charge takes about eleven per cent longer than the battery figure alone would suggest.