kW, kWh and charging time in an electric car: how to interpret them
A guide to telling power and energy apart, understanding AC and DC charging, estimating charging times and comparing specifications without mistaking a power peak for the result of a full charging session.
Key points
kWh indicate how much energy a battery can store or receive, while kW indicate charging power at a given moment. To estimate a charging session, compare the energy still needed with the power actually available, bearing in mind that it changes during the session.
The quick answer: energy, power and charging that is not constant
The difference between kW and kWh in an electric car can be summed up simply: kWh measure energy and kW measure power. The battery is described by the energy it can store or make available to the car; the charging point and the vehicle are described, among other details, by the power at which they can transfer energy.
In the International System of Units, the watt is a unit of power and equals one joule per second, while the joule is a unit of energy. One kilowatt is 1,000 watts; one kilowatt-hour is 1,000 watt-hours. The “h” in kWh does not turn the unit into power: it expresses energy accumulated over time.
For this reason, reading the kW printed on a charger is not enough to know how long a charging session will take. You need to know how many kWh the battery still needs and the power the car can receive at that moment. Power also changes during a session and may fall as the battery approaches a full charge. The result of a simple division is a useful guide, not a promised duration.
Section references: SI Brochure ↗ · NIST Handbook 44, Appendix D: Definitions ↗ · Información de la batería de alta tensión — Model 3 ↗
- kWh: battery energy, energy added during a session, or energy consumption per distance.
- kW: the instant rate at which energy is delivered or accepted.
- Time: the energy to be recovered divided by a power level that, in practice, is not usually constant.
What each specification-sheet figure tells you
Battery capacity is expressed in kWh. It is worth identifying the usable capacity specifically: Renault defines this as the amount of battery energy the vehicle can actually use. When comparing two vehicles, do not simply mix one vehicle’s usable figure with another’s total or gross capacity figure unless the specification sheet makes clear that they are equivalent.
European type-approval regulations include lifetime electric energy consumption in kWh/100 km. This unit answers how much energy is declared to travel a distance, not how quickly the car charges.
Charging power is expressed in kW, but it needs a qualifier: AC or DC, and maximum or sustained. A maximum figure indicates a published limit; by itself, it does not clarify how long it is maintained or under what conditions, so it is worth checking those details in the specification sheet.
| Figure | Usual unit | Question it answers | What to check |
|---|---|---|---|
| Usable capacity | kWh | How much energy can the car actually use? | That the specification sheet states it is usable capacity. |
| Charging power | kW | At what rate can it accept energy in AC or DC? | The current type and whether it is a maximum figure. |
| Charging time | Minutes or hours | How long does a specific interval take? | Starting and ending percentages, power and stated conditions. |
| Electric energy consumption | kWh/100 km | How much energy is used per distance? | The published electric energy consumption figure. |
Section references: Glosario de la tecnología E‑Tech ↗ · Reglamento de Ejecución (UE) 2025/1707 ↗
- To estimate energy still needed: usable capacity and the starting and ending battery percentages.
- To choose a charging point: the power accepted by the car in AC or DC, as applicable.
- To anticipate a stop: the stated charging interval—for example, from 10% to 80%—and its conditions.
- To assess efficiency: consumption in kWh/100 km, without confusing it with charging speed.
The basic calculation: kWh needed ÷ kW available
The approximate relationship is simple: time in hours = energy you want to add, in kWh, divided by charging power, in kW. The units fit: if 11 kWh are transferred at a constant rate of 11 kW, the ideal calculation gives one hour. This formula helps to understand orders of magnitude and spot impossible comparisons.
Illustrative scenario: imagine a battery with 60 kWh of usable capacity, at 30%, with a target of 80%. The increase is 50 percentage points. Fifty percent of 60 kWh is 30 kWh: that is the approximate energy to be recovered. If, for mathematical purposes only, power were constant at 10 kW, 30 ÷ 10 would give three hours.
The actual session may take longer than this ideal result because effective power does not have to remain constant. The US Department of Energy states that charging time depends on how discharged the battery is, its energy capacity, battery type, the vehicle’s onboard charger capacity and the charging equipment. Use the calculation as a first filter, then compare it with the vehicle’s official figure for a comparable percentage interval.
Section references: NIST Handbook 44, Appendix D: Definitions ↗ · Electric Vehicle Charging Stations ↗ · Glosario de la tecnología E‑Tech ↗
- Step 1: subtract the starting percentage from the target percentage.
- Step 2: apply that percentage to usable capacity, not to a figure whose nature is not stated.
- Step 3: divide the resulting kWh by the kW that the car-and-charging-point combination can provide.
- Step 4: treat the result as a theoretical reference; allow planning margin and check stated times for similar conditions.
Why a charger with many kW does not always charge at that power
The power stated by a charging point is a capability of the equipment, not a guarantee of power that any car will accept. The vehicle has its own limits. Tesla Spain, for example, states that maximum AC power depends on the vehicle’s capabilities and the Wall Connector current settings. When reviewing a specification sheet, always compare the charging point’s power with the power accepted by that specific version of the car.
State of charge also matters. Tesla explains that charging speed decreases as battery level rises and that reaching 100% usually takes considerably longer than reaching 80%. This is why a “10–80%” figure should not be mechanically converted into a “0–100%” time. They are different intervals, with power evolving differently.
Battery temperature and management matter as well. The Model 3 manual states that speed may be reduced if the battery is too cold, nearly full, or if its condition changes through use and over the years. When planning cautiously, published times should be understood as applying to specific conditions, not as a commitment valid for every starting point or day.
Section references: Cargador integrado ↗ · Información de la batería de alta tensión — Model 3 ↗ · Supercarga ↗
- Common mistake: choosing a charger by its maximum power without checking the car’s charging acceptance.
- Common mistake: multiplying a 10–80% time to calculate a 0–100% time.
- Common mistake: treating maximum kW as power sustained throughout the whole session.
- Practical check: confirm the version, current type, percentage interval and conditions of the published time.
AC and DC: what changes between home, destination and fast charging
Alternating current (AC) and direct current (DC) are not just abbreviations on a specification sheet. With AC, the vehicle’s onboard charger converts incoming alternating current into direct current to charge the traction battery. The US Department of Energy explicitly describes this, and Tesla Spain notes that Wall Connectors and mobile connectors supply AC while batteries store energy as DC.
Therefore, during AC charging, both the charging point’s capacity and the car’s onboard charger capacity matter. A higher-power point will not make the battery receive more than the vehicle can accept in that mode. With DC, compare the charging interval, power and time stated by the manufacturer for that charge.
A Hyundai specification sheet illustrates why these cases must be kept separate: for a specific KONA, it states 41 minutes from 10% to 80% at a 102.3 kW DC charger and, separately, 4 hours and 55 minutes for a full charge of the 48.4 kWh battery on 11 kW three-phase AC. These are not contradictory figures: they describe different currents, power levels and intervals.
Section references: How Do All-Electric Cars Work? ↗ · Cargador integrado ↗ · Nuevo KONA: especificaciones de carga y consumo ↗
- AC: check the charging point’s power and the vehicle’s onboard charger capacity.
- DC: check the percentage interval for the stated time and the power indicated for that charge.
- Before comparing: make sure both cars are measured with the same current mode and the same battery interval.
- To prepare a home installation, see this guide on what to check before installing a charger.
A method for comparing two cars and planning your first charge
Start with a like-for-like comparison. Note the usable capacity, consumption in kWh/100 km, power accepted in AC and DC, and official times with their starting and ending percentages. If a brand publishes power as “up to” a certain level, do not compare it directly with another car’s time without checking the conditions behind both figures.
Next, estimate the energy needed for your situation. If you know the usable capacity and the percentage you want to recover, calculate the corresponding kWh. Use power compatible with the car and the charging-point type for an initial estimate, but allow extra time when arranging an appointment, journey or stop: power can fall as battery level rises.
Finally, separate the figures according to your intended use. If routine charging will be in AC, check the power accepted by the onboard charger. If you will make stops on journeys, compare DC times for a comparable interval, usually the one each manufacturer states. The charging time and cost calculator can help organise these figures, provided you enter data for the exact version.
Section references: Electric Vehicle Charging Stations ↗ · Cargador integrado ↗ · Supercarga ↗ · Nuevo KONA: especificaciones de carga y consumo ↗
- Checklist before deciding:
- Confirmed usable capacity, rather than only an advertised capacity figure without clarification.
- AC and DC power accepted for the version, battery and market you intend to buy.
- An official time linked to clearly stated starting and ending percentages.
- Available charging-point power and applicable settings.
- A reasonable time margin, especially if you intend to charge to a high battery level.
Do battery kWh equal the car’s range?
Not directly. kWh express available energy; to compare energy consumption per distance, check the published kWh/100 km figure, as explained in the section “What each specification-sheet figure tells you”.
What does it mean when a car charges at 11 kW?
It indicates charging power of 11 kilowatts in the stated mode and conditions. As explained in “The quick answer” and “AC and DC”, effective power can vary and, with AC, the car’s onboard charger capacity also plays a part.
Why do manufacturers usually state the time from 10% to 80%?
Because charging speed decreases as battery level rises. That time describes a specific interval and should not automatically be extrapolated to a full charge to 100%.
Which figure should I prioritise if I will charge at home?
Check the AC charging power accepted by the exact version of the car and the capacity of the charging point to be installed or used. Higher charging-point power does not guarantee that the vehicle can use it.
