PEV Charging: Amps, Volts and Watts

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Understanding PEV charging isn’t that difficult, but it does involve a language – electrical – that is foreign to most people. If you’re the type who just isn’t satisfied with the overview, here’s a deeper dive into the topic.

[Estimated Reading Time: 4 minutes, 20 seconds.]

This article can help you understand the conversation when plug-in vehicle (PEV) enthusiasts are going on and on about their fuel efficiency and charging tricks. Better yet, it will help you learn to accurately compute the home electricity consumption and charging speed should you decide to buy that PEV you’ve been thinking about.

The important elements of a home system are the PEV charging unit, called an EVSE,  its amperage, and the capacity of the plug-in vehicle’s on-board charger, usually given in kilowatts, or kW.

What’s a Watt?

The first battery-electric vehicles, back in the dark ages of 2010 and 2011, were equipped with chargers rated at 3.3 kW. That means they could only accept a maximum of 3.3 kilowatts of electricity each hour they were connected to a 240-volt, or Level 2, power supply.

A watt is the basic unit of electricity, and a kilowatt is 1,000 watts. A PEV charging station that could send only 1 kilowatt of electricity to the battery pack in an hour would be a 1 kilowatt-hour (kWh) device. It wouldn’t be very efficient.

Those 3.3 kW chargers in the early EVs weren’t all that quick, so manufacturers soon started using 6.6 kW chargers and for a few high-end models went even higher. The basic charging capacity of a Tesla, for instance, is 10 kWh, and buyers can order a second on-board charger that double the charging speed to 20 kWh.

While battery-electric vehicles (BEVs) have stepped up their charging game, most plug-in hybrids still use 3.3 kW chargers because they have much smaller batteries, often less than a third the size of a BEV’s. They don’t need the faster chargers and buyers don’t need the extra cost they’d add to the car.

But for BEVs, greater charging speed means less time tethered to a plug and more time on the road.

Amping It Up

While the charger on the car is rated by the speed at which it can push electricity from the home device’s connector nozzle through to the battery pack, the EVSE itself is rated for the maximum flow of current it can deliver. That volume is measured in amperage, or amps.

Think of the EVSE and its charging cord as a hose and the charger on the car as the opening in the neck of a bottle. While a wider bottle neck can take in more water, it is limited by the volume of water being delivered through the hose. And the hose is limited by the pressure of water. In plug-in vehicle charging, the “pressure” that pushes the electrical current through the EVSE “hose” is measured in volts.

In the U.S., normal household current is nominally 120 volts and it can vary a bit depending on all sorts of factors.  A home EVSE that uses a 110-volt supply is called a Level 1 device. Big appliances, like electric ovens, clothes dryers and PEV charging devices, need more current. So they are hooked to 240-volt circuits. In PEV charging, that’s a Level 2 system and that’s typical for home EVSEs.

Basic Level 2 EVSEs start at 16 amps, though most professional installers will recommend a larger 30- or 40-amp system. Some, for those Teslas with twin chargers, are 80 amps.

Building codes require that the wiring circuit that supplies the electricity be rated higher than the EVSE, to account for losses that occur as the power flows through it and into the vehicle.

The professional installing your 30-amp system typically will wire the circuit from the home power service to handle 40 amps and will install a 40-amp circuit breaker. A 40 amp EVSE will need a 50 -amp breaker and an 80-amp unit will require a 100-amp breaker. Some homes with very small or heavily used electric service capacity might need to have the service upgraded to handle an EVSE.

How Much is Enough?

To compute how much juice will be delivered to your plug-in vehicle through any properly installed EVSE, multiply the amps by the volts and divide by 1,000.

A 240-volt Level 2 EVSE rated at 30 amps will deliver 7.2 kilowatts (240 x 30/1000). In one hour, that will send 7.2 kWh of electricity to a plug-in vehicle, so it will be fine to service cars with on-board chargers rated at 7.2 kw or less.

EVSEs will work with chargers that have less capacity than the EVSE is rated for – part of the EVSE’s job is to determine how much current to actually send through. But if your EVSE’s capacity is less than that of the car’s charger, the car won’t charge as fast as the manufacturer says it should.

The moral? Buy the largest capacity EVSE you can afford. That will allow you to keep the same home charging station if (or when) you later step up to a new PEV with a larger capacity on-board charger than the model it replaces. (You won’t need an 80-amp model unless you are going to get a twin-charger Tesla one of these days.)


The basics of home charging are explained in “Plug-In Vehicle Charging – What You Need to Know.”

6 thoughts on “PEV Charging: Amps, Volts and Watts

  1. In the last paragraph the sentence “But the largest capacity EVSE you can afford.” should say “Buy the largest capacity EVSE you can afford.”

  2. A quick question please
    When my Tesla s is charging it reads 12W 16/16A 240 V (3 phase)
    Base melbourne Australia
    When the Tesla charger was installed I was told that was appropriate.
    I have seen on videos that 40/40 is on the Tesla’s screen.
    I’m confused.
    Also how does the formula P=I x V apply with my numbers ?

    • Hi. I wish I were an electrical engineer, but I’m not – not even an electrician, just a journalist. So I can’t help with some of this. If I am understanding the numbers in the first part of your query, then I believe that the Tesla video screen reading of 40/40 you saw likely refers to the best amperage for a 240-volt Tesla home charger: 40 amps.
      The 16 amp circuit you apparently have for the charger you use will fill the battery, so in that sense it is “appropriate,” but it takes much longer to do so than with a 40-amp circuit. Think of it this way – charging at 16 amps vs 40 amps is like trying to fill your pool from a 1-inch hose verses a 2.5-inch hose.
      As for the power equation P=IV, that’s one I can’t help with.

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