Kilowatt hour - kWh - is a measure of battery capacity. The equivalent in a petrol or diesel car would be the capacity of the fuel tank; usually measured in litres, or gallons in the past.
As a measure of a unit of electricity, it is also used by utility companies when calculating fuel bills, though in the case of electric vehicles, it shows how much charge the battery can store.
At a very basic level, the greater the battery capacity in kWh, the further the car will go. Elements such as battery weight and vehicle efficiency come into play, but broadly speaking a car with an 80 kWh battery will go around twice as far as one with a 40 kWh battery.
What does kWh have to do with charging?
The term kWh does not refer to the charging of electric vehicles itself, but plays a part in how quickly it takes to recharge an EV.
Using the same broad terms as above, an 80 kWh battery may take you roughly twice as far on a single charge, but it will also take twice as long to recharge than a 40 kWh battery pack when plugged in to the same type of charge point.
Rapid chargers, vehicle charging specifications, and charge point power all mean this isn’t a hard and fast rule, but as an approximate guide, the bigger the battery, the longer it takes to charge.
Again, there is a comparison in more familiar models. If a car has an 80 litre fuel tank, it will take twice as long to refill it then someone pumping petrol into a 40 litre tank.
What does kW mean?
For electric vehicles, kWh is relatively simple, as it will always refer to the battery capacity. However, kilowatt - kW - refers to two different elements of electric vehicles and charging.
The first is simpler to understand, since kW is a measure of power, in this case of the electric motor(s). With an EV’s motor output rated in kW, it can also be converted to horsepower, but increasingly engines are having their outputs displayed with kW alongside traditional hp or bhp.
The second use of kW will be less familiar to most, and refers to a charge point’s power. This indicates the maximum amount of power able to be delivered in an hour by the charger, providing the vehicle can take such a rate of charge.
Once again, things aren’t every as simple as the following basic maths, but more as a rule of thumb, a 70 kWh battery, when plugged in to a 7 kW charge point, will take 10 hours for a 0-100% charge; 70 (kWh)/7 (kW) = 10 (hours).
In actual fact, “7 kW” charge points actually tend to be 7.2 kW, and the rate of charge will drop off when the battery gets closer to 100% - and very few drivers will every charge from 0%. But for simple mental maths, dividing the battery capacity in kWh by the charge point’s kW rating will give you the number of hours it will take to charge the electric vehicle… providing the car can take the charge point’s maximum power.
Calculating charging times
There are two factors to consider when charging an electric vehicle; both the charge point and the vehicle itself. The lowest common denominator determines the rate of charge possible for that situation.
For example, if an electric car can take 7.2 kW AC, when plugged into a 7.2 kW charge point, both the car and the charger will be working at maximum potential.
However, if the charge point is an 11 kW or 22 kW unit, and the same car with a 7.2 kW AC on-board charger is plugged in to it, the rate of charge will remain 7.2 kW in an hour - despite the charger theoretically capable of delivering 11 or 22 kW in that time.
Equally, if the car - with its 7.2 kW on-board charger - is plugged in to a 3.6 kW charge point, the maximum charge that can be delivered in an hour is 3.6 kW. The lowest figure is that with which calculations must be made. The car will still charge, and the unit will still put out power, but the charging speed is based on the lowest kW rating found between the car and charger.
So you’re considering an electric car – say, for example, the Nissan Leaf Plus. It has a 160-kW electric motor, a 62-kWh lithium-ion battery, 6.6-kW onboard charger, it fast-charges up to 100 kW, has an estimated 363-km range, and it’s rated at 2.2 Le/100 km in combined city/highway driving.
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And if all that made your eyes glaze over, you’re definitely not alone. There can be a steep learning curve with electric vehicles (EVs), and we’ll try to take some of the mystery out of it.
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A watt is a unit of electric power, and a kilowatt (kW) refers to 1,000 watts. But a kilowatt hour (kWh) is a measurement of energy capacity — power multiplied by hours. It’s equivalent to the energy needed to keep a 1,000-watt (1 kW) electrical appliance (or an electric car) running for one hour.
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In a vehicle, kW is used to measure the power of the electric motor that drives the wheels, while kWh measures the capacity of the battery that supplies the electric motor. Compared to a gasoline car, think of kW as the engine’s horsepower — they translate directly — and kWh as the amount of energy contained in the gas inside the tank.
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The battery’s kWh rating measures how many hours it can deliver its power, but that’s based on standard tests, rather than what the car may actually use in real-world driving. When cruising, an electric car generally uses 1 kWh of energy to travel approximately 4.8 to 6.4 kilometres (three to four miles). With a Tesla Model S, its 100-kWh battery can continuously deliver 100 kW for one hour, and the car uses 1 kWh of energy to go 6.4 kilometres. Multiply 100 kWh by 6.4, and you get the car’s estimated range of about 640 kilometres on a charge.
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Our hypothetical Leaf Plus has a 62-kWh battery, giving it an estimated range of 363 kilometres. Just as with a gasoline car, real-world mileage may differ; drive more aggressively and just as you use more gas than the estimated fuel consumption, you’ll use more electricity than the estimated range. Cold weather will also cut into the battery’s range. On any EV, the higher the battery’s kWh rating, the farther it goes before it needs recharging — but that battery will be physically larger and heavier, and correspondingly more expensive.
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When measuring engine power, we’re used to horsepower rather than kW. Translate kW into horsepower by multiplying kilowatts by 1.34 — so our Leaf Plus’ 160-kW motor makes 214 horsepower. Torque is rated using the same pound-feet units for both gasoline engines and electric motors.
Battery charging comes in three levels. Level 1 is a regular 120-volt household outlet; it takes the longest and is mostly viable for plug-in hybrids, which have a much smaller battery (generally 8.8 to 18.1 kWh) or for slowly topping up an EV when there’s no other charging option available. Level 2 is 240 volts, using a specially designed home charging unit or at most public charging stations. All EVs contain an onboard charger to accept Level 1 and 2, and most are rated at 6.6 or 7.2 kW. The higher the number, the faster the battery can charge, and both Level 1 and 2 are alternating current (AC).
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Fast-charging, often called Level 3, is direct current (DC) at 480 volts or more, and is only available at public stations. These are still relatively rare (but growing fast) due to the high cost of installation. There are different types — one of the issues automakers and charging providers face is a lack of standardization — including CCS for most vehicles, CHAdeMO for the Leaf and a few others, and Tesla’s proprietary Superchargers.
Not all vehicles can accept fast charging, and those that do have an extra charging port for it. Most fast-charging sites provide power at 50 kW, but newer ones are 100 to 150 kW, and a few can provide more than 250 kW. Most EVs capable of fast charging accept 50 to 150 kW, depending on the model. If the charging station can deliver higher power than the vehicle can take, it reduces energy flow to match the car’s capacity.
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Rapid fill-ups for minimal downtime are key differentiators at the top level. Tesla’s Superchargers are 150 kW (and its upcoming V3 network is 250 kW). The Porsche Taycan can charge at up to 270 kW — if you can find an appropriate station. At that level, a Taycan can pull in enough energy in four minutes to go 100 kilometres.
But on any EV, the quoted power rates are maximum and the car will fast-charge up to that rate. Numerous factors, including temperature and how charged the battery already is, will affect the recharging rate. The time it takes to fast-charge is usually advertised to 80 per cent of battery capacity, rather than to full. That’s because all charging slows down as a battery cell nears its full capacity, to avoid potential damage — your phone does this, too. The last 20 per cent can take as long as the first 80 per cent did.
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Finally, let’s look at the Leaf Plus’ rating of 2.2 Le/100 kilometers. EVs are more accurately rated in kWh/100 km — how many kilowatt-hours of battery it takes to travel 100 kilometres. For that, the Leaf Plus gets 19.5 kWh/100 km, but consumers aren’t used to that. One litre of gas has the energy equivalent of 8.9 kWh of electricity, and so Natural Resources Canada uses Le/100 km for litre equivalent — how much gas you’d burn to get the same energy the car is using from its battery. An EV uses much less equivalent energy than a gasoline car to go that 100 kilometres, which is why the fuel efficiency rating is so low.
Use either rating to compare EVs, along with all the kW and kWh information you’ve now learned to decipher, to know if you’re getting the electric car that’s right for you.
Author’s note: My thanks to John Voelcker, a reporter and analyst who has covered electric cars for 15 years, for his assistance with this story.
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Jil McIntosh
Jil McIntosh specializes in new-car reviews, auto technology and antique cars, including the two 1940s vehicles in her garage.
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