How to Calculate EV Charging Time by Hand

EV charging time equals the energy you need from the wall divided by the charger’s power. Take a 75 kWh battery from 20% to 80%. That adds 45 kWh to the battery. At 90% charging efficiency you need 45 / 0.9 = 50 kWh from the wall. A 32 amp, 240 volt Level 2 charger supplies 7.7 kW, so 50 / 7.68 = 6.51 hours, or 6 hours 31 minutes.

The EV charging time calculator does this math for you. Here is how it works by hand, so you can check any answer.

The EV charging time formula

Energy into battery (kWh) = Battery kWh x (End % - Start %)
Energy from wall (kWh)    = Energy into battery / Efficiency
Charging time (hours)     = Energy from wall / Charger kW

Or in one line:

Hours = Battery kWh x (End % - Start %) / (Charger kW x Efficiency)

The kilowatt-hour is the amount of energy. The kilowatt is how fast it flows. Divide one by the other and you get hours.

Step by step: the worked example

1. Find the battery size. Here it is 75 kWh.
2. Find how much you are adding. Going from 20% to 80% is 60 points, so 75 x 0.60 = 45 kWh.
3. Allow for losses. Some energy turns into heat in the charger, cables and battery. At 90% efficiency, 45 / 0.9 = 50 kWh from the wall.
4. Find the charger power. Volts times amps, divided by 1,000. A 32 amp charger at 240 volts is 32 x 240 / 1,000 = 7.68 kW.
5. Divide. 50 / 7.68 = 6.51 hours. The 0.51 of an hour is about 31 minutes.

Swap in a 120 volt outlet at 12 amps and the charger power drops to 1.44 kW. The same 50 kWh then takes 50 / 1.44 = 34.7 hours.

How to find charger kW from amps and volts

Most home chargers are sold by amps, not kW. The conversion is simple:

kW = Volts x Amps / 1,000

Charger	Volts	Amps	Power
Level 1 outlet	120	12	1.44 kW
Level 2, 32 A	240	32	7.68 kW

At 208 volts, the voltage at many commercial sites, the same amps give less power. The Department of Energy’s Alternative Fuels Data Center lists Level 2 equipment from 2.9 kW to 19.2 kW.

The onboard charger limit

On Level 1 and Level 2, the car decides the top speed. The onboard charger turns AC power from the wall into DC for the battery, and it has a maximum rating. If your wall unit can deliver more than that, the car takes only what its onboard charger can handle.

So the number to use in the formula is the smaller of two figures: the charger’s kW and the car’s onboard charger kW. If your car’s onboard charger were rated at 7.7 kW, a larger wall unit would still charge at 7.7 kW. Check your car’s spec sheet, or see the guide to home EV charger sizes.

DC fast charging bypasses the onboard charger, so this limit does not apply there. A different limit takes over: the battery’s own charging curve.

Where charging efficiency comes from

Charging is never 100% efficient. Some power becomes heat in the electronics, wiring and battery. The examples here use 90%. Lithium-ion cells themselves are very efficient: Battery University puts their coulombic efficiency above 99%. Most of the loss happens around the battery, not inside it.

The EPA’s ratings on fueleconomy.gov are measured at the wall, so charging losses are already included. Each car’s label lists its electricity use in kWh per 100 miles and a full charge time on 240 volt service.

Mistakes that throw the answer off

• Using the full battery size when you are only adding part of a charge. A 20% to 80% session uses 60% of the battery, not all of it.
• Forgetting losses. Skipping efficiency makes the time look shorter than it is.
• Using the charger’s rating when the car can’t accept that much.
• Treating DC fast charging as a steady rate. Power falls as the battery fills, which the guide to the EV charging curve explains.

From time to cost

The same wall energy sets your bill. At 18 cents per kWh, 50 kWh costs $9.00. The EV charging cost calculator runs that for your rate, and the FAQ on what it costs to charge an electric car uses the latest national average. For charge times on specific cars, see the EV charging time chart.