kW, kWh, amps: finally understand EV charging

"7.4 kW charger", "60 kWh battery", "32 A circuit breaker", "meter in kVA": the technical specs of EV charging seem to be written for electricians. As a result, many drivers choose their charger without really knowing what these figures mean — and then discover that charging is slower than expected, or that the meter trips when the oven and the charger run at the same time. The good news: three concepts are enough to understand everything, and a simple division lets you estimate any charging time. Let's take ten minutes to set the record straight.

kW and kWh: power is not energy

This is the most common confusion, and it disappears with a simple image: the tap and the bathtub. The kilowatt (kW) measures power — it's the flow rate of the tap, the speed at which electricity flows at a given moment. The kilowatt-hour (kWh) measures energy — it's the volume of water in the bathtub, the total amount accumulated.

The link between the two is time: a power of 1 kW maintained for one hour transfers 1 kWh of energy. A battery's capacity is therefore expressed in kWh (the volume of the bathtub), while your charger's speed is expressed in kW (the flow rate of the tap). Once you've grasped that, half the jargon becomes clear: your electricity bill counts the kWh consumed, your charger displays the kW delivered.

Volts and amps: the two ingredients of power

So where does power come from? From the combination of two quantities. Voltage, in volts (V), is comparable to the water pressure in the pipe; in France, the domestic grid delivers 230 V single-phase and 400 V three-phase. Current, in amps (A), corresponds to the flow of electricity that actually circulates in the conductor.

Power is simply the product of the two: power = voltage × current. It's this formula that links together all the figures on the technical specs, and that explains why cable cross-sections and protection ratings are sized in amps: it's the current that heats the conductors, and it's the current that the NF C 15-100 standard regulates.

Why a 7.4 kW charger corresponds to 32 A single-phase

Let's apply the formula. Single-phase, the voltage is 230 V. A current of 32 A gives: 230 V × 32 A ≈ 7.4 kW. That's the origin of this figure you see everywhere — the 7.4 kW charger is nothing more than a single-phase 32 A circuit. In the same way, 16 A at 230 V gives about 3.7 kW.

Three-phase, the current flows over three phases simultaneously: the power is the sum of the three. With 16 A per phase, you get about 11 kW; with 32 A per phase, about 22 kW.

Charger power	Grid	Current
2.3 kW (domestic socket)	Single-phase 230 V	about 10 A
3.7 kW	Single-phase 230 V	16 A
7.4 kW	Single-phase 230 V	32 A
11 kW	Three-phase 400 V	16 A per phase
22 kW	Three-phase 400 V	32 A per phase

These current levels also explain why a charger requires a dedicated circuit with its own protections, and properly sized conductors — as a rough guide, cross-sections of around 3G6 mm² single-phase and 5G6 mm² three-phase depending on the length of the run. An ordinary domestic socket, designed for intermittent use, is not made to draw a high current for hours on end.

Estimating a charging time: a simple division

The calculation fits in one line: charging time ≈ energy to recharge ÷ charging power. The bathtub divided by the flow rate of the tap.

Take a textbook assumption: 50 kWh to recharge. At 7.4 kW, count on a little under seven hours — one night is more than enough. At 11 kW, about four and a half hours. At 2.3 kW on a domestic socket, more than twenty hours: that's the whole gap between a dedicated charger and a standard socket. And at 22 kW, provided the vehicle's onboard charger accepts it, a little over two hours.

Three caveats to stay honest about this calculation. First, it gives an order of magnitude: conversion losses and thermal management slightly lengthen the real result. Next, the power isn't constant: at the end of charging, the vehicle reduces it to preserve the battery. Finally, and above all, in day-to-day use you almost never recharge an empty battery: you top up the day's trips, which represents a fraction of the total capacity. That's why overnight charging at home comfortably covers the vast majority of uses.