EV charging speed: kW, kWh, AC and DC

kW measures charging power now. kWh measures energy added over time. Divide battery energy by average battery-side power. When starting with wall-side input power, apply a stated efficiency because the charger’s largest kW number is only a ceiling.

What is the difference between kW and kWh?

A kilowatt is a unit of power. It describes the rate at which energy is being transferred at one moment. A kilowatt-hour is a unit of energy.

If power stayed at 10 kW for one hour, the charger would supply 10 kWh. If it stayed at 10 kW for two hours, it would supply 20 kWh. Real sessions may change power, so the charger or car integrates that changing rate over time.

Think of kW as flow and kWh as quantity. A battery capacity might be stated in kWh. A wallbox or rapid charger is rated in kW.

Do not write “a 60 kW battery” when you mean 60 kWh. Do not describe a charger as “11 kWh” when you mean its 11 kW power rating.

How is AC charging power calculated?

For a simple single-phase estimate, multiply voltage by current. IEC 60038 gives standard voltage values used as reference values. With nominal 230 V supply, the familiar calculations are:

These are nominal input powers. The charging system can also select a lower current.

For AC charging, the wallbox supplies controlled AC to the car. The onboard charger converts it to DC for the battery. Its current and phase capability can be lower than the wallbox rating.

Why can an 11 kW car use only 3.7 kW?

The labels may describe different phase arrangements. An 11 kW AC rating normally refers to 16 A on three phases at nominal European voltage. A 16 A single-phase point can provide only about 3.7 kW.

The reverse mismatch also matters. A single-phase onboard charger connected to a three-phase point uses only the phase arrangement it supports. A cable can add another limit if its current or phase rating is lower.

Charging control keeps the session within the available limits. IEC 61851-1 covers the connection and control requirements. You cannot combine unused phases with an adapter to exceed the car’s design.

How is DC charging different?

A DC charging station converts electricity outside the vehicle and supplies controlled DC to the battery system. This avoids the vehicle’s AC onboard-charger path for that session. IEC 62196-3 covers standardized DC and combined AC/DC coupler configurations.

The car still does not accept arbitrary power. It communicates limits to the station and controls the session through its battery system. The station must remain within both its own capability and the vehicle request.

DC equipment is often built for higher power than home AC equipment. Yet “DC” alone does not tell you the speed. Compare the station rating with the vehicle’s documented DC capability and expected charging curve.

How do you estimate charging time?

First calculate the energy you want to add, not the full battery capacity unless you truly plan a full charge. For a 60 kWh usable battery moving from 20% to 80%, the nominal battery-energy change is:

60 kWh × (0.80 − 0.20) = 36 kWh

An 11 kW wallbox rating describes wall-side input power. If e is the verified wall-to-battery efficiency for the estimate, use:

time = 36 kWh ÷ (11 kW × e)

With a theoretical efficiency of 1, the lower bound is 3.27 hours, or about 3 hours 16 minutes. Real efficiency is below 1, and input power can change. Use a documented or clearly stated efficiency assumption rather than presenting the lower bound as an expected time.

Use usable battery capacity when the manufacturer provides it. Gross capacity includes energy the vehicle may reserve outside the displayed range. Mixing gross and usable figures creates a false level of accuracy.

Why does charging power change during a session?

The battery system adjusts power to operate within its limits. State of charge, cell temperature, battery voltage and the vehicle’s thermal system can all influence the request. Site power management can limit the supply too.

DC charging often starts below the station maximum, rises or settles, then reduces as the battery approaches its target. The exact curve belongs to the vehicle, software version and conditions. Do not publish one curve as a rule for all cars.

AC charging can appear flatter because the onboard charger has a fixed power ceiling. It may still reduce power for thermal control, scheduled charging, site load management or a current setting in the car.

Why is the battery gaining fewer kWh than the meter shows?

The energy delivered at the charging point and energy stored in the battery are not identical. Conversion, cabling, electronics and thermal management use energy. The difference changes with equipment, conditions and charging power.

Avoid applying one fixed efficiency percentage to every session. For a planning estimate, show the assumption you use. For a bill, use the meter or invoice value because that is the measured energy supplied under the tariff.

Short sessions can also make average overhead look larger. Preconditioning a cold or hot battery uses energy but may help the battery reach a suitable charging condition. The dashboard may report battery gain differently from the charger invoice.

What limits the final speed?

For AC charging, check these ceilings:

• charging point current and phase capability;
• cable current and phase rating;
• car onboard-charger current, power and phase capability;
• site load-management setting;
• user-selected charging limit.

For DC charging, check the connector and the car’s supported system. Actual charging is set by connector compatibility and active power, current and voltage limits across the site, station and vehicle. Any one of those limits can reduce the result.

Does a larger charger save time at home?

Only if the car and installation can use it. A 22 kW wallbox cannot make an 11 kW onboard charger accept 22 kW. It may also require more connection capacity and a different approval process.

Start from energy need. If the car is parked for ten hours and needs 20 kWh, a modest average power can restore it. Faster equipment may still help with several vehicles or short parking windows, but the use case should justify it.

Dynamic load management does not raise the building’s connection limit. It allocates available current to charging while respecting other measured loads. This can make a constrained supply easier to use without promising maximum power all night.

How should you compare charging claims?

Ask whether the number is kW or kWh. Then ask whether it describes the station, cable, vehicle maximum, battery peak or session average. Those are not interchangeable.

For a useful time estimate, write down the starting and target percentages and usable battery capacity. Use average battery-side power, or convert wall-side input with a stated efficiency.

Energy divided by average power gives time. The hard part is choosing an average that reflects the real session.