EV Charging Speed Calculator (Free, No Signup)

An electric car display showing battery and charging information

Most charging calculators hand you a number and hide the method. This one shows every step, because the arithmetic is simple enough to check and you should not have to trust anybody's black box -- including ours.

Work out your charging speed

Charger ampsVoltsYour car, mi per kWhBattery, kWhCharge from %Charge to %

Power

11.5 kW

Range added per hour

40 mi

20% to 80%

3h 54m

The working

48 A × 240 V ÷ 1000 = 11.5 kW

11.5 kW × 3.5 mi/kWh = 40 miles per hour

75 kWh × 60% ÷ 11.5 kW = 3h 54m

Real-world charging is a few percent slower than this: some energy becomes heat in the cable and the car’s onboard charger, and most cars taper the last stretch to 100%. Treat the result as the ceiling, not a promise.

Where to find each number

Input	Where to get it
Charger amps	On the charger, or in its manual. If you have not bought one yet, try 32, 40 and 48 to compare.
Volts	240 V for a home Level 2 circuit. 120 V for a normal wall outlet. 208 V in some apartment buildings.
Your car's mi/kWh	The trip computer. Use your own long-term average, not the EPA figure -- yours reflects your driving.
Battery kWh	The owner's manual, under specifications. Use usable capacity if both are listed.

The formulas

What the calculator does

Power: kW = amps x volts / 1000

Range added: miles per hour = kW x mi/kWh

Time: hours = (battery kWh x charge window) / kW

That is all of it. No hidden factors.

Two limits the calculator cannot know

Your car's onboard charger. It caps AC charging speed. If your car accepts 7.7 kW, entering 48 amps gives an answer your car cannot achieve -- enter the amps your car can actually use. Check the manual.
Charging losses and tapering. Some energy becomes heat, and most cars slow near 100%. Real charging runs a few percent behind the calculation. We do not apply a fudge factor because we cannot source one for your car -- treat the result as the ceiling.

FAQ

Questions people actually ask

How do I calculate EV charging time?

Divide the energy you need to add by your charging power. Power is amps times volts divided by 1,000; energy is battery capacity times the percentage you are adding.

How many miles per hour does my charger add?

Multiply the charger's kW by your car's efficiency in miles per kWh. A 9.6 kW charger on a car doing 3.5 mi/kWh adds about 34 miles per hour.

Why is my real charging slower than the calculator says?

Some energy becomes heat in the cable and onboard charger, most cars taper near 100%, and cold slows the battery's acceptance rate. Treat the result as the ceiling.

What efficiency figure should I use?

Your car's own long-term average from the trip computer. It reflects how and where you actually drive, which the EPA figure does not.

Sources

Where these numbers come from

NEC Article 625: EV charging equipment — WireRefReference for Level 1 and Level 2 power levels.