EV charging time:
formula and worked example

What actually sets charging time

Charging time comes down to two quantities: how much energy has to go into the battery, and how quickly it can flow. The first depends on battery size and on the gap between starting and ending state of charge. The second depends on the charger's rating, the car's onboard limits and the losses along the way.

Because energy lost as heat during charging still has to be drawn from the supply, the effective rate into the battery is lower than the charger's label. That is why the formula includes an efficiency term.

A garden hose analogy fits well enough: the battery is the bucket, state of charge is how full it is, and the charger is the tap. A bigger bucket or a gentler tap both mean a longer wait, and a leak in the hose, which is the efficiency loss, means the tap has to run for longer than the bucket alone suggests.

The formula and the five inputs

• Battery kWh: usable capacity, which may be lower than the nominal figure on the brochure.
• Start and end SoC: written as fractions, so 20% is 0.2 and 80% is 0.8.
• Charger kW: the power the car actually accepts, which is the lower of the charger's output and the car's onboard limit.
• Efficiency: the fraction of supplied energy that reaches the battery, written as a decimal.

The state-of-charge gain is a difference, so it is the window that matters rather than the starting point. Charging from 10% to 70% needs the same energy as 30% to 90%, but the second window falls in a region where fast chargers usually taper, which shows up in real sessions.

Worked example: 48 kWh battery, 20% to 80%, 7 kW

A driver plugs a 48 kWh car into a 7 kW home charger at 20% and wants to stop at 80%, assuming 90% efficiency.

1. 1Energy into the battery: 48 × (0.8 − 0.2) = 28.8 kWh.
2. 2Effective power: 7 × 0.9 = 6.3 kW.
3. 3Divide: 28.8 ÷ 6.3 = 4.571 hours.
4. 4That is about 4 hours 34 minutes.

The energy drawn from the wall in this case is 28.8 ÷ 0.9 = 32 kWh, which is what your meter and bill will show.

If the same car is plugged into an 11 kW supply and the car accepts it, the effective rate is 9.9 kW and the session drops to about 2.9 hours. If the onboard charger caps at 7.4 kW, though, the extra capacity is wasted, which is why checking the car's limit first matters.

Reading the result for real sessions

The result is a good guide for AC home or workplace charging, where power is steady. For DC fast charging the car's battery management lowers the rate as the battery fills, so a straight division understates the time, especially above 80%.

• Overnight home charging: a figure of four to five hours is comfortable within a night.
• Fast charging: use the stated 10–80% time rather than the formula for a long trip.
• Top-up sessions: the formula works better for small gaps than for large ones.

A useful habit is to add a margin. Real sessions are rarely faster than the formula and are often slower, so planning for an extra half hour keeps the schedule honest.

How battery size changes the session

A 10% bigger battery moves the session from 4.571 to 5.029 hours, because there is 10% more energy to deliver across the same window. A stronger charger works the other way: doubling the effective power halves the time, until the car's onboard charger becomes the limit.

The takeaway for planning is to estimate charging in terms of energy per hour: a 7 kW home unit gives roughly 6.3 kWh of battery per hour here, which on a 48 kWh pack is about 13 percentage points of charge per hour.

Mistakes that make estimates optimistic

• Entering percentages as 20 and 80 instead of 0.2 and 0.8.
• Using the charger's nameplate power when the car's onboard charger accepts less.
• Setting efficiency to 1, which assumes there are no losses.
• Applying the formula through to 100% on a fast charger, where the rate falls sharply near full.

Each of these makes the estimate optimistic, never pessimistic, which is why a small margin is wise.

Planning a charge window

1. 1Find usable battery capacity from the manual.
2. 2Choose the SoC window you really need.
3. 3Check the lower of charger and car limits.
4. 4Calculate time, then add buffer for cold weather.

Most people find that a single number, percentage points gained per hour, is the easiest way to plan. Work it out once for each charger you use and keep it in your notes.