How to calculate an electric car's charging time?

There is a formula to calculate an electric car's charging time, and it fits on a receipt. The problem is that it is right in one case and wrong in the other: it works very well at home, and it no longer means much at a fast charger. Here is how to calculate correctly in both situations, and what really happens in the battery when the power drops. Updated on 26 August 2026.

The formula, and what it is worth

Charging time in hours = energy to transfer in kWh ÷ power actually accepted in kW.

Two clarifications make all the difference. We are talking about energy to transfer, not total capacity: nobody charges from 0 to 100%. And we are talking about the power accepted by the car, not the one shown on the charger.

Example. A Citroën ë-C3 with 44.2 kWh that you charge from 20 to 80% has to absorb 26.5 kWh. On a 7.4 kW wallbox: 26.5 ÷ 7.4 = 3.6 hours, i.e. about 3h35. This result is reliable.

The three figures you need

Your battery's usable capacity. That is the one that counts, and it is lower than the quoted gross capacity. A Peugeot e-208 shows 51 kWh gross for 46.3 kWh usable. A Renault Twingo E-Tech, 27.5 kWh usable.

The power your car accepts, which is not the same depending on the current. On alternating current, most models cap at 11 kW, a few at 7.4 kW or even 6.6 kW on city cars, and a rare few go up to 22 kW. On direct current, the gap is much wider: 50 kW for a Twingo E-Tech, 100 kW for a Renault 5, 250 kW for a Tesla Model Y.

The target charge range, in percentage points. Charging from 20 to 80% transfers only 60% of the capacity.

On alternating current, the formula works

At home, on a socket or a wallbox, the power stays constant from the first to the last percent. The theoretical calculation and reality meet, give or take a few minutes.

These values assume the car accepts the available power. A Twingo E-Tech limited to 6.6 kW will not get more from an 11 kW wallbox, and plugging a car capped at 11 kW into a 22 kW charger changes nothing.

Watch how you read this table: these are the amounts drawn from the socket, not the ones reaching the battery. Between 5 and 10% is lost in the onboard charger and thermal management. So an eight-hour night on a 7.4 kW wallbox actually fills the battery with about 54 kWh, not 59.

On direct current, the formula is no longer enough

At a fast charger, the power is never constant. It rises, holds briefly, then drops, and this drop starts well before 80%. Dividing the capacity by the peak power therefore gives a result that is systematically too optimistic.

Instrumented measurements show it unambiguously. A Tesla Model 3 Long Range rated at 250 kW delivers an average of 106 kW over the 10 to 80% range, and takes 33 minutes. A rear-wheel-drive Porsche Taycan rated at 320 kW holds a 265 kW average and finishes in 17 minutes. The ratio between peak and average therefore ranges from 1.2 on the Taycan to nearly 2.4 on the Model 3.

So the right method for fast charging is not to calculate, but to start from the 10 to 80% time provided by the manufacturer, or better, measured in a test. You will find these values model by model in our charging time table.

If you still want a ballpark figure, use the average power, not the peak. Count on about half the quoted power on a 400-volt model, and 70 to 80% on an 800-volt one, whose curve holds up much better over time. You will be closer to reality than with the raw formula.

Why does the power drop?

Because the battery protects itself as it fills, because it is not always at the right temperature, and sometimes because the station splits its supply between several cars. Three mechanisms that add up.

Cell protection first. The more the battery fills, the higher the voltage rises and the more demanding charging becomes. The system gradually reduces the power to avoid overheating and premature ageing. That is why the last percentages are the slowest: on a Renault Scénic E-Tech, the 10 to 80% range takes 38 minutes while the 80 to 100% range takes 60.

Temperature next. A cold battery accepts less power until it reaches its operating range. The effect is real but often exaggerated: across sixteen models tested between cold and warm battery, the average gap over the 10 to 80% range is six or seven minutes. The most penalised models lose thirteen minutes, the best-managed two minutes, and some lose nothing.

Power sharing finally, at poorly sized stations where several charging points share the same supply. Our stations are sized to deliver the stated power at each charging point, even when they are all in use.

Preconditioning, and its forgotten condition

Preconditioning means bringing the battery to its ideal temperature before arriving at the charger. Most manufacturers now offer it, but its effectiveness varies greatly from one model to another. A Touring Club Suisse test on five models measured gains of six to nine minutes on three of them, and no gain on the other two, at the cost of 3 to 8% extra consumption.

The truly useful point is elsewhere, and it is almost always forgotten: on most cars, preconditioning only kicks in if the charger is set as the destination in the vehicle's built-in sat-nav. Navigating to the same charger with Google Maps or Waze on your phone triggers nothing. If you arrive at a fast charger in cold weather after following your favourite app, your battery will be cold and your charge slower.

Should you charge to 100%?

No, except the day before a long trip. Day to day, staying between 20 and 80% preserves the battery and spares you the slowest phase. Most cars let you set this limit in their interface: set it to 80% by default, and raise it occasionally.

On a trip, the calculation is even clearer: leaving at 80% to stop a second time further on often saves time compared with a single charge pushed to 100%.

Frequently asked questions about calculating charging time

What is the formula to calculate a charging time?

Energy to transfer in kWh divided by the power accepted by the car in kW. It is reliable on alternating current, but too optimistic on fast charging.

Why does the actual time exceed the calculated time?

Because part of the energy goes to the onboard charger and thermal management, and because on direct current the power decreases as the charge progresses.

How do you estimate a fast charge without getting it wrong?

Start from the 10 to 80% time quoted by the manufacturer rather than calculating. Failing that, divide the energy to transfer by half the quoted peak power.

Does cold slow charging down a lot?

Less than people say. The average gap measured between cold and warm battery is six to seven minutes over the 10 to 80% range, with big differences between models.

How many kilometres do you recover overnight?

On a 7.4 kW wallbox for eight hours, about 54 kWh reach the battery, i.e. 300 to 350 km depending on your car's consumption. A household socket delivers only a third of that.

Key takeaways

The formula works at home and stops working on the motorway. At home, divide the energy to transfer by your wallbox's power and add 5 to 10%: you will be spot on. On fast charging, do not calculate, take the manufacturer's 10 to 80% time, and know that the advertised peak power is almost never held for long. Finally, if you drive in winter, set the charger in the car's sat-nav and not on your phone: it is that action, not the formula, that will save you minutes.

If you would rather not calculate at all, the Electra app shows the estimated time and cost before you plug in. Our fast-charging stations deliver up to 600 kW per charging point, and Autocharge starts charging automatically, with no badge. The price varies with demand between €0.39 and €0.61 incl. VAT per kilowatt-hour with the app, and starts at €0.64 incl. VAT without it. Two commitment-free subscriptions reduce this amount:

Electra+ Essential: €1.99 per month, no commitment, and €0.10 off per kWh on every charge on the Electra network.

Electra+ Smart: €4.99 per month, no commitment, and €0.20 off per kWh on every charge on the Electra network.

Both plans give access to a preferential rate of €0.49 per kWh at Atlante, Fastned and Ionity. Paying annually reduces the subscription amount by 16%.

Source: Electra pricing and Electra+ page, consulted on 26 August 2026.

To locate the stations near you, download the app on the App Store or Google Play, or check the station map.

The power and charging-time measurements quoted come from the instrumented tests published by Automobile Propre, carried out on the road and at fast chargers.

Written by Nicolas - Electric mobility expert at Electra