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, officially 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 (kW) 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 since the late 2010s, many new EVs have had 7.6 kW charging capacity. Some high-end models go 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.

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 for instance, go as high as 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.”

15 thoughts on “PEV Charging: Amps, Volts and Watts”

  1. Hi John
    We are looking into charging options for the office- both for the fleet and for staff . we have the capacity for 80 amps but It seems that an 80 amp charger is over kill based on most batteries out there today. However, based on most manufactures increasing the charging times rates, in 3-4 years out it might make sense – so should we just go with 80 now?
    Many thanks

    • Hi, If you can go with 80 amps you should. EV Level 2 charging speeds are getting faster with each new iteration and it is no problem now to have more amperage than needed. Going with 80 now will save you the cost of upgrading later.

  2. Hi I just had a 50 amp 14-50p outlet setup in my garage and I have a 2015 bmw i3 . Would it make any difference if I buy a level 2 charger that is 16 amps vs one that is 30+ amps? Does having more amps make the charging fo faster? Possibly a dumb question but have to ask as just getting into this space. Thank you!

    • Thanks for asking – yes, the lower the amperage the slower the charge. It is always better to get the highest amp charging station – also called an EVSE – that you can. That way so you don’t have to keep replacing it every time you get a new/faster-charging EV!!! A 30-amp EVSE will deliver juice to the car approximately 2x as fast as will a 16-amp. As long as the car’s charge can accept the higher flow (and an i3 will – the 2015 is rated at 7.4 kW on a 32 amp level-2 charger), you’ll be able to charge faster. 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 kW of electricity to a plug-in vehicle, so it will be fine to service cars with on-board chargers rated at 7.4 kW or less. A Level 2 charger rated at 16 amps will only deliver 3.8 kilowatts per hour, even if the car is able to accept 7.4 kW each hour. The 2015 i3 has 18.8 kWh of usable capacity. It will charge in about 3 hours with a 30 amp Level 2 EVSE, vs 6 hours with a 16-amp station. Hope this helps.

  3. Any idea about the efficiency of charging a clarity plug-in hybrid? I’m sure that it is not 100% so that it would take more than 15 kWh of house current to increase the battery charge by 15 kWhs. I also suspect that the efficiency could be different for level 1 and 2 chargers. Perhaps your sources could shed some light on this often neglected topic.

    • I don’t have figures for the Clarity specifically, but from my own 3 years of metering charging for our 2011 Leaf, plus experience of others I’ve talked to about this, it appears that a decent rule of thumb for line losses and other parasitic consumption of power on 240 volt charging systems is about 15-20%, for approx. 80-85% efficiency; for 120-volt (household) current in the U.S.that could be down to 70 percent efficiency. I’m not an electrical engineer or even a decent electrician, but as I understand things, losses can vary depending on amperage of charging system, quality of EVSE charging cord and numerous other variables.

  4. Hi, I strolled into my local Honda car showroom today and became suddenly enamored with the hybrid Clarity car after seeing the mpg statistics. I’ve been reading this entire mass of info with diligence for a couple hours now and trying to understand it as I go along. I know quite a bit more now but have a very basic question. How can I calculate the actual cost of power from my home outlet per charge? That car gets about 47 miles per charge before switching to gas power…(I think that’s correct). So in actual money per charge, how much am I paying for those 47 miles. I want to calculate the cost efficiency of this car verses a traditional all gas car. My salesman knows less than me at this point and I am an elderly lady trying to stretch my pennies. I would love to help the environment as well. Thank you for all the helpful info thus far…fascinating!

    • Diane – The Clarity plug-in hybrid has a 17 kilowatt-hour battery. You likely won’t ever drain it completely, but for computation purposes we’ll use that figure. Electricity is typically billed by your utility company on a price per kilowatt-hour (kwh) basis. Some utilities have a flat rate, other charge in usage tiers (the more you use, the more you pay) and others use time of day (cheapest rate, usually, is for overnight, most expensive is during daytime hours of 8 or 9 am to 5 pm or so) and some use combinations of these methods. Some have special rate systems for electric vehicle charging, most don’t. You can ask your electricity provider for an explanation of exactly how it charges you, or you might be able to find out simply by looking at your itemized monthly bill – providing you get a monthly bill that is itemized.
      You don’t say where you live, so I don’t know what you pay for electricity, but let’s use fairly high numbers and for purposes of illustration say your utility uses time of day charging, with electricity billed at 30 cents/kwh at peak demand periods, 20 cents per hour off-peak (early mornings, evenings) and 12 cents for overnight (midnight t 6 am).
      If you set your Clarity to charge from midnight to 6, you will pay 12 cents per kwh under this example. With a 17 kwh battery, that will cost you $2.04.
      If you are getting just 40 miles of range – it is difficult to get the exact 47 miles of range that the EPA estimates and Honda advertises (speed, weather, hills, cargo loads, heater and air conditioner use all affect electric range and usually diminish it somewhat) – that’s 5 cents a mile.
      $2.04 for 40 miles is probably less than you’ll pay for each gallon of gas it takes to get the 42 mpg combined fuel efficiency the EPA rates the Clarity PHEV at when it is running in standard hybrid mode after the battery charge has been used up.
      The national average for electricity is just under 11 cents per kwh, and in many areas overnight charging is far less than the 12 cents/kwh I used in the example. When you find your rate(s), just multiply 17 by the price you’ll be paying per kwh at the time you’ll be charging and you’ll have a cost. If you drain the battery, you’ll be using close to 17 kwh to recharge it; if you only drive 23 miles a day, you’ll be using about 8 kwh to recharge…etc.
      Additionally, on days when you drive more than 40 miles (per the example), you will also be using gasoline, but remember than the first 40 miles is all-electric and cost just $2.04. If you get 40 mpg average in regular hybrid mode, you’d have to drive 80 miles to use one gallon (80 mpg), 120 miles to use 2 gallons (60 mpg),160 miles for 3 gallons (53.3 mpg),etc. So the electric miles also effectively reduce the cost of gasoline by improving your overall fuel efficiency.
      If you’d like more info, send me your contact info at [email protected].

    • Diane. Thought i would tell you about our 2018 Clarity phev and my calculations…I live in northeastern Washington state. My Clarity phev was delivered at the start of June. Until October, we were getting 57 miles per charge consistently. It started to drop as the weather got colder. After my 7500 mile service the range has dropped to 39 miles. There are a number of reasons for that- snow tires [mean]more loss to friction, my wife runs the heater at 78 degrees and uses the heated seats, batteries are just less efficient with cold. That all being said- the EPA estimate seems right. Six months at 57mi/charge and 6 months at 39/charge comes out to an average of 48. On long trips when we have to use the hybrid, we do the highway driving on gas and in town/last few miles on battery. We get better than EPA at 47mpg. (Calculation is done taking out the 57 miles of electric…Electricity in our area is very cheap and clean and I pay 8.75 cents per kw. We have driven 8172 miles. About 83% [of it on] electricity. Used about 36 gallons gas. Total electricity cost is about $175. Total gas plus electricity cost is $292. Compared to driving our former Camry, we have saved $700 in 5 months…My above figures are based on what people online report as usable battery capacity. If my recent charging on a metered charger is correct, usable battery capacity is much lower [than the online reports] and the electricity cost for us is about 60% of what I reported 9 [about $105]. If you want a better estimate…follow the link to the epas website: You can personalize the calculations and it looks like they are fairly accurate. (Note ..this comment was edited for length and clarity – no pun intended)

  5. 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.

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

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