How EV charging works & what you need to know

How electricity is measured

• Kilowatts: Electrical power is measured in watts, with a kilowatt (abbreviated kW) being equal to 1,000 watts. EV charging speeds are measured in kW. The more kW, the faster the charger.
• Kilowatt-hours: A kilowatt-hour (abbreviated kWh) is equal to 1 kilowatt of power delivered for 1 hour. You've likely seen kWh metered out on your electric bill. EV chargers use kWh the same way gas pumps use gallons to measure gas. 
• Miles per kilowatt-hour: Where gas cars use miles per gallon (MPG) to describe efficiency, EVs use a similar metric called miles per kilowatt-hour (mi/kWh). Sometimes this number is converted into miles per gallon equivalent (MPGe) to allow direct comparison between gas and electric vehicle fuel efficiency.

Understanding EV charging speeds

Charging speeds are measured in kW, and the quantity of charge that is delivered is measured in kWh. For example, a 50 kW fast charger can deliver 25 kWh of energy in a half-hour, whereas a slow 2 kW charger would only deliver 1 kWh in the same period.

To simplify things, chargers are divided into 3 broad categories:

Level 1

Charge rate: 1 to 2 kW

Typical range added per hour: 2 to 5 miles

Level 1 chargers often come with an EV when you buy it. They plug into standard 120-volt household outlets and are meant to be easily portable, requiring no installation.

For those who drive less than 50 miles daily, Level 1 charging at home every night may be enough to provide a full charge each morning. Adding more range can take a very long time on Level 1, though, with a full charge from empty potentially taking days. Those who consistently drive more may be better served by a Level 2 home charger.

Level 2

Charge rate: 3.3 to 19.2 kW

Typical range added per hour: 10 to 35 miles

Level 2 chargers operate on a 240-volt source, either hardwired or on a NEMA outlet like household appliances use. Level 2 chargers are available publicly and can also be installed at your home.

While Level 2 chargers can charge anywhere from 3.3 to 19.2 kW, most modern ones are rated between 6 and 12 kW. That's enough to refill most EV batteries overnight, though slower Level 2 chargers may not be able to refill larger batteries completely.

During the day they're best used as destination chargers at places where you'll be spending at least a few hours, such as a restaurant or mall.

Level 3 (DC fast charging)

Charge rate: 50 kW to 350 kW

Typical range added per hour: 180 to 240 miles

Level 3 chargers, also known as DC fast chargers, run at 480 volts or higher and are currently only available at public charging stations. Capable of adding up to 10 miles of range per minute, they’re the chargers that make longer-distance EV trips practical. All Tesla Superchargers are Level 3. 

Not all EVs can use DC fast chargers, and those that can aren’t always able to accept the charger’s highest rate of charge.

Though Level 3 chargers have high outputs, EVs typically only charge at their fastest on Level 3 when their batteries are mostly empty. At higher charge levels, Level 3 charging slows down to avoid excessive battery heat.

This is known as the "charging curve." The specifics of your vehicle's charging curve will depend on its programming and a few other factors, such as ambient temperature and whether your battery is already warm when fast charging begins. 

It may take your EV as long to charge from 80% battery to 100% battery as it does to charge from 10% to 80%. Thus, on an EV road trip, it saves time to charge at DC fast chargers only when your battery is running low, and then depart after reaching 80% charge.

Charging rate vs. acceptance rate

Just as chargers differ in how quickly they can deliver charge to an EV, EVs differ in how quickly they can accept charge. The highest rate at which an EV can charge is known as its “acceptance rate.”

When a charger’s maximum speed and an EV’s acceptance rate differ, the EV will charge at the lower of the 2 speeds.

Understanding EV charging connectors

In North America, there are 3 main types of EV plugs:

Which connector can you expect on which vehicles?
 

Non-Tesla EVs:

• All use J1772 for Level 1 and 2 charging. 
• The vast majority will use CCS for Level 3 charging. A few (most notably the Nissan Leaf) use CHAdeMO.
• In addition, there are aftermarket converters that allow J1772/CCS-standard EVs to use Level 1 and 2 Tesla NACS chargers. 

As of 2023, non-Tesla EVs cannot use the vast majority of the Level 3 Tesla Supercharger network, even with a converter. The exception is a small number of Supercharger locations that have an integrated NACS-to-CCS converter called Magic Dock, mostly in New York and California. These can be identified in the Tesla app.
 

Tesla EVs:

• All use the NACS connector for Level 1, 2, and 3 charging.
• Aftermarket converters allow Teslas to use J1772 and CCS chargers as well.

It's easy determine a Level 2 home charger's kW speed by multiplying its amperage by 240 volts. For example, a 40-amp charger will have a maximum charging speed of 9.6 kW (240 volts x 40 amps = 9,600 watts). 

RELATED: A how-to guide to buying a home EV charger

Public chargers

Public charging stations advertise their available chargers by plug type and kW charging speed. This makes it easy to find a compatible charger as well as estimate how long you need to charge, or how much range you can expect to see for a given charging session.

There are a few different tools for finding public chargers:

• Charging network apps: Most public chargers are part of a major charging station network such as EVgo or ChargePoint. Each of these networks has a mobile app so you can find their stations, and see if they're out of service or currently in use. 

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