Charging time is energy divided by power. The energy needed is battery capacity multiplied by the change in state of charge, so 60 kWh from 20% to 80% is 36 kWh. The power is the charger rating multiplied by efficiency, because some of what comes out of the charger becomes heat rather than stored charge. Divide one by the other and you have the hours.
Charger ratings cluster around a few standard figures. A granny lead on a 13 amp socket delivers about 2.3 kW, a single-phase home wallbox 7.4 kW, and a three-phase unit 11 or 22 kW where a three-phase supply exists. Public DC rapids are typically 50 kW, with ultra-rapids at 150 kW and above. The car itself may accept less than the charger offers, and on AC the onboard charger is usually the limit rather than the wallbox.
A DC rapid does not hold its headline power to the end. Above roughly 80 per cent state of charge the battery management system tapers the current sharply to protect the cells, so the last 20 per cent takes disproportionately long — often as long as the first 60. Charging 60 kWh from 20% to 80% on a 50 kW rapid takes about 49 min, but continuing to 100 per cent takes 1h 36m in total. On the 7.4 kW home wallbox the same 20 to 80 per cent charge takes 5h 32m, which is why overnight charging is measured in hours and nobody minds.
The formula
battery- Usable pack capacity in kWh
start- State of charge at the start
target- State of charge you want to reach
kW- Charger power, or the car’s onboard limit if lower
efficiency- Share of delivered power that becomes stored charge
taper- Power retained above 80% on a DC rapid
How it works, step by step
- Enter the usable battery capacity and the start and target percentages.
- Choose the charger power, or the lower of the charger and the car’s onboard limit.
- Adjust efficiency: about 88% on AC, higher on DC.
- Set the taper if you are charging past 80 per cent on a rapid.
Worked examples
60 kWh from 20 to 80 per cent on a 7.4 kW wallbox
36 kWh is needed. At 7.4 kW and 88 per cent efficiency the battery sees 6.51 kW, so the charge takes 5h 32m. On a 2.3 kW granny lead the same charge takes 17h 47m — longer than a night.
Why the last 20 per cent is the slow part
On a 50 kW rapid, 20 to 80 per cent takes about 49 min. Pushing on to 100 per cent adds only 12 kWh but takes 47 min more at a 35 per cent taper, for 1h 36m in total. That final fifth costs 49 per cent of the session time.
How to read your score
Frequently asked questions
Why does my car charge slower than the charger is rated?
Because the limiting factor is often the car. On AC the onboard charger sets the ceiling, and many models accept only 7.4 kW even on a 22 kW post. On DC the battery temperature and state of charge both restrict the current the pack will accept.
What is the rapid charging taper?
It is the battery management system reducing current as the pack fills, to keep cell voltage and temperature within safe limits. Power typically starts falling well before 80 per cent and drops steeply after it, which is why rapid sessions are usually planned as 20 to 80 rather than to full.
Should I charge to 100 per cent?
Occasionally, before a long journey, is fine. Routinely holding a lithium pack at full charge accelerates calendar ageing on most chemistries, and on a rapid it also wastes a lot of time. Lithium iron phosphate packs are the exception and are often specified to be charged fully.
Does cold weather change the time?
Considerably, especially on DC. A cold pack accepts far less current until it warms, so a winter rapid session can take half again as long. Cars with battery preconditioning warm the pack on the way to the charger precisely to avoid this.
Is a three-phase wallbox worth fitting?
Only if you have a three-phase supply and a car that accepts more than 7.4 kW on AC. Most domestic UK supplies are single phase, and for overnight charging 7.4 kW already refills a typical daily commute several times over.
EV charging time reference
| Charger | Power | 40 kWh 20-80% | 60 kWh 20-80% | 77 kWh 20-80% | 60 kWh 20-100% |
|---|---|---|---|---|---|
| Granny lead on a 13 A socket | 2.3 kW | 11h 51m | 17h 47m | 22h 50m | 23h 43m |
| Slow home unit | 3.6 kW | 7h 35m | 11h 22m | 14h 35m | 15h 09m |
| Single-phase home wallbox | 7.4 kW | 3h 41m | 5h 32m | 7h 06m | 7h 22m |
| Three-phase 16 A | 11 kW | 2h 29m | 3h 43m | 4h 46m | 4h 58m |
| Three-phase 32 A | 22 kW | 1h 14m | 1h 52m | 2h 23m | 2h 29m |
| DC rapid | 50 kW | 33 min | 49 min | 1h 03m | 1h 36m |
| DC ultra-rapid | 150 kW | 11 min | 16 min | 21 min | 32 min |
DC units of 43 kW and above taper to 35% of rated power over 80%, which is why the last column grows so much on those rows.