The vocabulary

Volts, amps, kW and kWh, in plain English

Four words and one formula. Learn them once and every charger spec sheet on the internet becomes readable.

By Sawyer V. · Published September 8, 2026

An electric car display showing energy and charging information

Every confusing thing about EV charging is confusing because of four words. Here they are, with the analogy everyone uses because it genuinely works.

Think of it as water in a pipe

The four terms
Term	Unit	The water analogy	What it means for you
Voltage	volts (V)	Water pressure	120 V is a normal outlet. 240 V is a dryer circuit. Higher pressure moves more.
Current	amps (A)	How wide the pipe is	More amps means more flow, and it needs thicker wire.
Power	kilowatts (kW)	How fast the bucket fills	Pressure x flow. This is charging speed.
Energy	kilowatt-hours (kWh)	How much water ended up in the bucket	Power over time. Your battery's size, and what your bill charges for.

The one formula

Everything follows from this

watts = volts x amps

kilowatts = volts x amps / 1000

So a 40 A charger on 240 V: 240 x 40 = 9,600 W = 9.6 kW

And a Level 1 cable at 12 A on 120 V: 120 x 12 = 1,440 W = 1.44 kW

That is the entire explanation of why Level 2 is roughly seven times faster than Level 1. Double the voltage, triple the current.

kW versus kWh: the one people mix up

kW is a rate. kWh is an amount. A 9.6 kW charger running for one hour delivers 9.6 kWh. Running for two hours, 19.2 kWh.

Your battery is measured in kWh because it is a quantity of stored energy. Your charger is measured in kW because it is a speed. Your electricity bill charges per kWh, because you pay for the amount, not the rate.

Putting it together

Battery: 75 kWh

Charger: 9.6 kW

Time from empty: 75 / 9.6 = about 7.8 hours

Cost at $0.16/kWh: 75 x 0.16 = $12.00

Two more terms you will meet

mi/kWh -- your car's efficiency, how far it goes on a kilowatt-hour. Roughly 2 to 4 for most EVs. Multiply it by kW to get miles of range added per hour.
Continuous load -- a device that draws its full current for three hours or more. EV chargers are one, which is why the circuit must be rated at 125% of the charger's draw. That single rule generates every breaker size in the sizing table.

Put your own numbers into the charging speed calculator and watch the formula do the work.

FAQ

Questions people actually ask

What is the difference between kW and kWh?

kW is a rate -- how fast energy moves. kWh is an amount -- how much moved. A 9.6 kW charger running one hour delivers 9.6 kWh.

How do I convert amps to kilowatts?

Multiply amps by volts and divide by 1,000. A 40-amp charger on a 240-volt circuit is 40 x 240 / 1000 = 9.6 kW.

What does mi/kWh mean?

How far your car travels on one kilowatt-hour, typically 2 to 4 miles. Multiply it by your charger's kW to get miles of range added per hour.

Why does an EV charger need a bigger breaker than its rating?

Because it is a continuous load -- it draws full current for hours. The code requires the circuit to be rated at 125% of that draw.

Sources

Where these numbers come from

NEC Article 625: EV charging equipment — WireRefContinuous-load definition and requirement.