What is a Kilowatt? Why Not Horsepower? 

What is Horsepower Anyway?

The first thing to understand is what horsepower meant in the first place. Most of us are comfortable understanding that a car with 150 hp is likely slower both in acceleration and top speed than a car with 400 hp, but we're not so sure what exactly a 'hp' is. Horsepower is a way of comparing how quickly (or slowly) work is being done. In the case of vehicles that 'work' is moving the car. In more technical terms horsepower is a unit of measurement of power, and power is the rate at which work is done. James Watt defined term horsepower to compare the power of a workhorse (literally a horse for work, like the Clydesdale in the picture above) to machines.

Ok, But Why Kilowatts?

Like horsepower, the kilowatt is also a unit of measurement of power. Unlike the horsepower, however, it is used across many industries and in most of the world as part of the International System of Units (SI) or metric system.

While horsepower remained an excellent way of measuring and comparing power for well over a century, it has lost relevance today. There are very few people in the western world using horses for work, and since we're not actually debating if we should buy a horse or machine, the utility of this comparison is lost.

The watt on the other hand is connected to all the other aspects of our lives. When you pay for electricity the units are kilowatt hours (kWh), light bulbs are purchased based on their watt (W) rating, microwaves are rated in kilowatts (kW) and so on.

Map of countries that have adopted standard international units.

Adoption of metric system by country over time.

Since most electric vehicles today are powered by battery there is the additional convenience of comparing the power of the motor to the storage capacity of the battery. When buying a battery electric vehicle you will notice that the battery capacity is rated in kWh and the motor is rated in kW. This provides you with a rough idea of how long (in hours) you can drive the vehicle at full power. Full power could be in terms of racing, or it could be a low power motor and you would like to drive at 110 km/h. Say you have a 80 kW motor and a 20 kWh battery. At full power this will last 15 minutes. Luckily for the electric vehicle movement, this senario is unlikely as you really only need full power when accelerating onto a freeway.  Once you reach freeway speeds, the power demand is much lower.

Tesla Roadster and Lotus Elise

Tesla Roadster (orange) Lotus Elise (yellow). Image taken from Road and Track.

How To Compare Electric Cars to Internal Combustion Cars

You might be thinking that it's all fine and convenient that kilowatts work well for electric vehicles, but how do you compare the power output of a regular car to an electric car? This is a little bit of a complicated question because we generally don't actually care about the power. What we care about is feel, or more quanitfiably; acceleration and top speed. In terms of the horsepower though, if you are a motor head, a kW is roughly 3/4 of a hp. In other words, multiply your horsepower rating by .75 and you will have converted to a number in kW.

For the Kia Soul which has both a gas version and electric version here's how it looks. This comparison is great because the only difference in the Soul Base vs Soul EV is the powertrain and energy storage.

Kia Soul Base model has 130 hp at 6,300 rpm. The "+" and "!" versions have 164 hp at 6,200 rpm.

The Kia Soul EV has 81.4 kW.

Here's the math:

Kia Soul Base 130 hp x .75 = 97.5 kW.

Kia Soul +/! 164 hp x .75 = 123 kW.

Kia Soul EV 81.4 kW ÷ .75 = 108 hp.

As you can see the EV is provided with fewer horses (or kW) than it's gasoline brothers. While I haven't driven the Kia Soul + or ! versions of the vehicle, I have driven both the Soul Base and Soul EV. Despite the lower hp, increased weight, and slower acceleration than the Soul Base, the Soul EV is noticably more fun to drive than the Base.

This comparison will help you understand some of the differences between gas and electric vehicles, but ultimately a test drive will tell you far more than any numbers so don't be deterred by some of the low power ratings on the electric vehiles and take the fun option - go out and test drive everything that catches your eye.

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