The transition to EVs brings with it a whole new set of terminology.
From the inception of the automobile, we’ve become accustomed to defining the performance of its powertrain with a standard unit of measure: horsepower.
While not everybody knows precisely what that measure means mathematically, they do understand its importance to the way a car performs. And the visual image of a horse — or a big herd of horses — reinforces the connection.
It’s a lot harder to visualize a kilowatt! Which means, as electric vehicles (EVs) become a bigger part of the marketplace, we need to educate customers in the interpretation of a new set of specifications applicable to these vehicles.
The logical place to start is with the kilowatt (kW), which is the unit of power used to define an electric motor’s output. Visually, it is easiest explained within our experience as the power required to illuminate ten 100-Watt light bulbs.
As both horsepower (hp) and kilowatts are units of power, they have a fixed relationship with each other: 1 kW = 1.34 hp. To reinforce that relationship, some automakers list both kW and hp numbers in their electric motor specifications.
Easy, then! The power outputs of electric motors and internal-combustion engines (ICEs) should be directly comparable. They are — and they are not!
The published numbers for both represent the peak output of the motor or engine and they are directly comparable in that context, subject to differences in test procedures. But what their power-versus-speed curves look like, and where on that curve those peak numbers are generated, is typically very different between the two. And that difference can be apparent and important to drivers.
The important part is that the battery is an energy storage device serving the same function in an EV as a gasoline tank for an ICE. The energy provided by the battery or fuel tank enables power to be generated by the motor or engine respectively. And the battery’s kWh of energy are analogous to litres of gasoline.
That’s where torque comes in, because power is a function of torque times speed. In an ICE, torque typically builds gradually until it reaches its peak and levels off — common with turbocharged engines — or declines. With an electric motor, peak torque is achieved immediately at startup and remains at that peak level until some mid-speed range before declining.
That instant low-end torque builds power quickly as speed increases, giving electric motors an acceleration advantage from stopping, as well as eliminating the ICE’s inherent need for a multiple-speed transmission for acceleration. It’s why EVs typically feel more responsive than similar ICE-powered vehicles at low speeds. And it is why an EV with a lower peak power rating than a similar ICE-vehicle might still feel more responsive — depending on how the latter is geared.
Kilowatts play a role at the other end of the EV powertrain as well, where battery output is typically rated in terms of kilowatt-hours (kWh). A kWh is a unit of energy (1 kWh = 1kW of power used or available for one hour).
A battery is an energy storage device. So a 100-kWh battery stores enough energy to provide 100 kW of power for one hour. Or 1 kW for 100 hours. Or some other combination of the two that multiplies to 100.
The important part is that the battery is an energy storage device serving the same function in an EV as a gasoline tank for an ICE. The energy provided by the battery or fuel tank enables power to be generated by the motor or engine respectively. And the battery’s kWh of energy are analogous to litres of gasoline. For that reason, the battery’s kWh rating can be somewhat predictive of driving range between charges, just as the capacity of a fuel tank determines range between fill-ups.
If an EV can travel 5 kilometres on 1 kWh of energy, which is within the typical range of current offerings, then our 100-kWh battery should enable 500 km of travel between recharges. Except that, unlike gasoline tanks, only about 80 per cent of a battery’s stored energy is typically available.
So a realistic range expectation would be 400 km — subject to temperature, and hills, and stops and starts, and A/C use, and all the other multiple factors that affect energy use/fuel consumption.
It is also possible to determine an EV’s “fuel efficiency” in the same way as for an ICE vehicle, expressed as kWh/100 km.
A Hyundai Ioniq EV, with a 38.4 kWh battery, for example, has a combined rating of 15.8 kWh/100 km, according to the EnerGuide from Natural Resources Canada. And a Nissan Leaf S Plus with a 62.8 kWh battery is rated at 19.5 kWh/100 km.
To give consumers some relativity to gasoline-fuelled vehicles, the EnerGuide also includes a Le/100 km rating — “Litres equivalent per 100 km” — which may equally serve to confuse the issue.
There is one more important kilowatt-related specification to consider — that of charging stations. But that’s a topic for another column.
