Energy versus power

A kilowatt-hour, abbreviated kWh, measures energy. Battery capacity and electricity bills are commonly expressed in kWh. A kilowatt, abbreviated kW, measures power: the rate at which energy is being delivered.

Think of kWh as the amount to add and kW as how quickly it is added. Adding 20 kWh at an average of 5 kW takes about four hours before allowing for losses.

Volts

Voltage describes electrical potential. In home charging, a common household outlet supplies lower voltage for Level 1 charging, while a dedicated 240-volt circuit supports Level 2 charging.

Voltage is only part of the power calculation. A 240-volt circuit can support different charging outputs depending on the available amperage and equipment configuration.

Amps

Amperage describes electrical current. A charger configured for more amps can deliver more power when voltage is the same, but the circuit and vehicle must support it.

EV charging is a continuous load, so the charger's output is set below the circuit breaker's rating. An electrician determines the correct circuit design for the equipment and site.

The formulas that matter

Approximate charging power in watts equals volts multiplied by amps. Divide watts by 1,000 to get kilowatts. Estimated charging time equals energy to add in kWh divided by usable charging power in kW, then adjusted for losses.

Electricity cost is energy used in kWh multiplied by the utility rate per kWh. These relationships stay the same regardless of charger brand.

A simple example

A charger delivering 240 volts at 32 amps supplies about 7,680 watts, or 7.68 kW, before losses. If the battery needs 30 kWh, the idealized time is about 3.9 hours.

Real charging takes longer because conversion is not perfectly efficient and the vehicle may vary the rate. Use the math for planning, not as a promise of an exact finish time.

Remember this: kW is speed, kWh is amount, volts × amps is power, and kWh × your electricity rate is energy cost.

Frequently asked questions

Not necessarily. A larger battery stores more energy, while charging speed depends on usable power and how much energy must be added.

The utility charges for the total energy consumed over time, measured in kWh. Some rate plans may also include other charges.

They describe a maximum or nominal power. Actual power can be lower because of circuit settings, vehicle limits, and normal losses.