When it comes to liquid fuels, a litre is the volume contained in a 10cm x 10cm x 10cm box. It’s a reasonably well understood concept. From there, it’s a matter of understanding how many litres are needed to travel 100kms to get an appreciation of energy use.
With electric cars, based around kilowatts and kilowatt hours, it’s a different matte. As a person with a degree in science, and an electrician who taught electrical apprentices for many years before moving into EVs, I find the kW and kWh easy to use and understand in terms of how I drive an EV.
But I do know that they are not familiar concepts for the vast majority of the public, so I will explain here how to mentally swap the watt for the litre in terms of refuelling, fuel use, and driving range.
What’s with the watt?
First-up, a bit of background: the watt is a unit in the metric system, named after James Watt who invented the Watt steam engine back in the late 1700s. (He also is credited with defining the unit of power called the horsepower).
As such, like any metric unit named after a person – the abbreviated form is capitalised. Therefore you write the unit as ‘watt’, but the abbreviated form uses a capital ‘W’.
When it comes to the watt, it is actually quite a small quantity. I personally would rapidly tire of typing all the zeros involved to say that my Kona recharges at a maximum 70,000W on a DC charger or 7,400W on an AC one!
Instead, we use the metric multiplier ‘kilo’ (abbreviation – lower case ‘k’) to denote 1000. Therefore we write 70,000W and 7,400W as 70kW and 7.4kW. (With the zero key on my computer sending its thanks to those who designed the metric system for saving it from a rapid death!).
Therefore, in terms of an EV, the kilowatt (kW) measures the rate we charge a battery at – or conversely, the rate we pull energy out of the battery.
The only other concept we need to know about when it comes to EVs is how the kilowatt-hour (kWh) relates to the kilowatt (kW).
If we recharge a car battery for one hour at a rate of 70kW, we have pushed 70 kilowatt-hours (abbreviated as kWh) into that battery to use. (70 kilowatts x 1 hour = 70kWh). If you are paying 40 cents per kilowatt hour (a typical public DC charger electricity price), that recharge will cost you 70 x 40c, or $28.
At home, at a typical off-peak price of 20c per kWh, that cost would be $14. (Now you see why most people charge at home if they can – and off their own solar if they are able to).
By the way, if charging at a typical home charger rate of 7kW instead of 70kW on a DC charger, then it takes 7kW x 10h to reach that same 70kWh point. Great for overnight charging, but not on the road!
Why kW charging rates matter
As you can see, charging is now a ‘horses for courses’ choice and it is good to understand why kW charging rates matter.
Next: if you drive pulling an average 20kW for 3.5 hours, you will have used all your 70kWh recharge. (20kW x 3.5h = 70kWh). If the car is more efficient and at the same speed you use an average 15kW instead of 20kW, then you could drive for over four and a half hours before the battery is flat.
This is what is behind the showroom windscreen stickers for fuel/energy use. For a petrol or diesel car, this will be a litres per 100km number (L/100km). For an EV it will be a kilowatt hour per 100km figure, abbreviated as kWh/100km. (Or the watt hours per km version. This is abbreviated as Wh/km).
These fuel/energy use numbers are derived from a government mandated, standardised test cycle. That test cycle was created to simulate a ‘typical’ car use (be it EV or ICE) in order to:
- give an indication of how much electrical energy or fossil fuel the car may reasonably use to indicate its running cost and
- give a standard against which we can compare different car fuel/energy efficiencies.
However, just like the fuel economy figures for petrol or diesel cars, the kWh/100km figure also varies a lot with driving style and conditions. This means what we get from a full battery (or tank) can vary wildly from that sticker figure.
Sample EV consumption label to ADR81/02
Image: Department of Infrastructure, Transport, Regional Development, Communications and the Arts
By the way, to get kWh/100km from a Wh/km figure, I simply divide the Wh/km number by 10. For example, my Kona has a WLTP rating of 131 Wh/km. If you divide that by 10, you get 13.1kWh/100km. (I find it easier to compare battery and/or charge rates that way).
An EV battery stores electrical energy in kilowatt-hours (kWh) rather than litres, and we recharge at rates measured in kilowatts rather than just pour it into the tank at whatever the flow out of the pump is. That means the faster the kW charge rate, the quicker the ‘refill’ is.
We also ‘drain’ the battery in terms of kW. At any one moment, the kW use varies wildly as you accelerate, slow down or drive at different speeds. However, that varying use can be averaged out over time to give the kWh/100km (or Wh/km) figure – which is where the mandated test cycle window sticker number comes from.
That means the lower the number on the sticker, the more energy efficient the EV is and the less it will cost you to run. It also means the more kWh in the battery, the further it will go on a single charge. (Even though in real-life, just like a petrol car, you may never achieve the actual efficiency or driving range numbers shown on the sticker!)
As a final note …
The abbreviation for kilo is k and the abbreviation for hour is h. Therefore the correct abbreviation for kilowatt-hour is kWh.
On the other hand, K is the unit for absolute temperature (named after Lord Kelvin who correctly defined the lowest temperature possible), and H is for Henry, the unit for electromagnetic induction. (Named after Joseph Henry, who co-discovered electromagnetic induction). Therefore, the oft written version of kWh as KWH, or kelvinwatthenry is a rather mind boggling unit of, er, something!
Bryce Gaton is an expert on electric vehicles and contributor for The Driven and Renew Economy. He has been working in the EV sector since 2008 and is currently working as EV electrical safety trainer/supervisor for the University of Melbourne. He also provides support for the EV Transition to business, government and the public through his EV Transition consultancy EVchoice.