Kilowatts vs Torque: What’s the Difference?

Ever found yourself nodding along as your mechanic or friend who’s into cars goes on about your car’s performance and describes it in jargon like kilowatts and torque, with you pretending to follow, but secretly feeling clueless? If so, you’re not alone.

Kilowatts and torque are two essential concepts used to evaluate the performance and efficiency of various systems, from car and jet engines to electric motors and industrial equipment. 

Yet, very few can tell what they really mean or the difference between them. We’ve put together this article to help clear things up and to help you make sense of these concepts. We’ll delve into their definitions and differences, and highlight what each is best suited for. So, buckle up, and let’s get started!

Kilowatts vs Torque: Side-by-Side Comparison

Kilowatts vs Torque: What’s the Difference?

Definition

A watt is defined as a unit of power used to measure the rate at which energy is transferred or converted. A kilowatt (kW) is equal to 1000 watts, or 1 kilojoule, per second. Torque, on the other hand, is a measure of rotational force or twisting action and is measured in Newton meters (Nm). 

In mechanics, kilowatts and torque are more or less two sides of the same coin. Kilowatts measure the speed at which energy is transferred into or from mechanical energy, while torque measures the twisting force produced by a rotating body, such as an engine’s crankshaft. 

In electricity, kilowatts measure the rate at which electrical energy is transferred from one point to another, whereas torque is often used to measure the twisting force exerted by an electric motor.

It should be noted that besides electricity and mechanics, kilowatts are a crucial unit of measurement in other fields such as thermodynamics, where they measure the rate of heat transfer, as well as optics, where they measure the power of a laser.

Calculation

In Linear Mechanics

In the context of linear mechanics, kilowatts are calculated by multiplying the force applied to an object by its linear velocity. Mathematically, this can be written as follows:

Power (kW) = force (kN) x velocity (m/s)

We can better understand kilowatts here by thinking of them as indicators of how fast an object can move with a certain amount of force. 

As an example, let’s consider the hypothetical case of a small car, such as a Mini Cooper, whose engine produces 5000 N or 5kN (1125 lb) of force and propels the car at a speed of 45 mph (almost 20 m/s).

By applying the formula above, we find that the engine’s power output is approximately 100 kW. That’s like having about 134 horses pull the car (1hp is about 0.746kW)!

Torque, on the other hand, can be viewed as the measure of the twisting force that an engine produces to turn the wheels. Mathematically, this is:

Torque (Nm) = force (N) x distance from the pivot point (m)

If you’ve ever used a wrench to tighten a bolt, you know how important torque is. Let’s say you’re tightening a bolt with a wrench and applying a force of 50 N (11.24 lb), and the bolt is located 30 cm (11.81 in) away from the center of the hole, the pivot point. Then the torque applied to the bolt is 15 Nm (11.06 lb-ft), which means you’re tightening it pretty securely.

In Rotating Bodies (Rotordynamics)

Kilowatts and torque can be related directly when it comes to rotary machines like engines and electric motors. In general, the relationship between the two can be expressed using the following equation:

Power (kW) = Torque (kNm) x Angular Velocity (rad/s) or simply P = (T ω)

The equation above shows that power and torque are proportional to each other, with the angular velocity acting as the proportionality constant. This means that an increase in torque will result in an increase in power, assuming the angular velocity remains constant.

The relationship between kilowatt and torque can also be simply expressed as kW = Nm x 2π/60, where Nm stands for Newton meter and 2π/60 stands for angular velocity in radians per second. 

Using our previous Mini Cooper example, let’s assume that it puts out 100kW of power at 5000 rpm. We can obtain the torque generated by the engine by rearranging the equation P = Tω to solve for torque: 

T = P/ω

We can convert the 5000 rpm into radians per second:

 ω = (5000 rpm) x (2π rad/rev) / (60 s/min) = 523.6 rad/s

Substituting this value and the given power into the equation, we get:

 T = (100 kW) / (523.6 rad/s) = 191 Nm

Electricity Context

In the context of electricity, kilowatts can be understood better as a measure of the rate at which electricity is consumed (or generated) by an appliance. Kilowatts are calculated by multiplying the voltage and current of an electrical system. The mathematical formula is:

Power (kW) = voltage (V) x current (A)/1000

So, let’s say you have a toaster oven that uses 120 volts and draws a current of 5 amps. That means its power consumption is 0.6 kW, which is like using 6 100-watt light bulbs at the same time.

Torque in the electrical context can be considered no different from what it stands for in the context of rotary mechanics since it’s still used to measure the twisting force that is, in most cases, exerted by an electric motor. As we saw earlier with rotating bodies, torque is calculated by dividing the motor’s power output by its angular velocity:

Torque (Nm) = power (W) / angular velocity (rad/s)

As an example, let’s say you have an electric motor that produces 1 kW of power and rotates at a speed of 1000 revolutions per minute (rpm). To calculate the torque produced by the motor, we need to convert the angular velocity from rpm to radians per second. 

One revolution is equivalent to 2π radians, so 1000 rpm is equal to (1000/60) x 2π = 104.72 rad/s.

Using the torque formula, we get:

Torque = 1000 W / 104.72 rad/s = 9.55 Nm (7.04 lb-ft)

So, this motor produces a torque of 9.55 Nm, which means it can generate a twisting force of 9.55 N at a distance of 1 meter from the motor’s pivot point. This torque is essential for the motor to perform tasks like driving a machine, turning a shaft, or rotating a wheel.

Kilowatts and Torque in Vehicles

Kilowatts and torque are both important for getting any type of vehicle moving, but they have different roles to play. In an automobile, for instance, kilowatts measure how quickly motive power is availed by an engine.

Pretty much like in a sprinter, they’re a measure of speed and agility, expressing how quick bursts of power can get a vehicle moving rapidly. Power is an important factor in sports cars and high-performance vehicles that need to hit top speeds quickly. 

Torque, on the other hand, measures the twisting force an engine produces. It’s like the muscle that gets things moving and continues to provide steady and reliable power that can keep a vehicle moving. It’s important for all kinds of vehicles but mostly in heavy-duty vehicles like trucks and SUVs that need to tow heavy loads.

Torque is also crucial for acceleration as it provides the necessary force to overcome the inertia of a vehicle and get it moving quickly. A higher torque allows a car to accelerate faster from a standstill. In racing cars, torque is especially important for quick acceleration out of corners, as high torque enables the driver to gain speed much faster.

In order to get the best performance from a vehicle, it needs to have the right balance of kilowatts and torque. Too many kilowatts and not enough torque, and the vehicle will struggle to climb inclines or tow heavy loads. Too much torque and not enough kilowatts, and the vehicle will be slow and sluggish. Finding the perfect balance is key to getting the most out of a vehicle.

Kilowatts vs Torque: 5 Must-Know Facts

1. The term “torque” comes from the Latin word “torquere,” which means “to twist.”
2. A single horsepower (1 hp) equals 746 watts and one Newton-meter (Nm) equals approximately 0.7376 horsepower.
3. The first automobile to use an internal combustion engine was the Benz Patent-Motorwagen, which had an engine that produced about 0.55 kilowatts of power.
4. The Bugatti Chiron, one of the world’s fastest cars, has an engine that produces 1480 horsepower (1.10 kW) and 1600 Nm of torque.
5. The world’s largest wind turbine, the Vestas V236, has a rotor diameter of 236 meters. It produces up to 15,000 kilowatts (15MW) of power. Assuming its blades rotate at 12 RPM and that all mechanical power gets converted into electrical power, the turbine generates a mind-boggling 11,937 kNm (kilo Newton-meters) of torque!

Kilowatts vs Torque: Which One Is Better? Which One Should You Choose?

The answer to these questions is simply contextual. Kilowatts are a measure of power, while torque measures rotational force. So, which one should you choose? Well, it depends on what you need.

If you want sheer speed in your next car, then kilowatts (and weight) are what you should consider. The higher the kilowatt rating of the engine, the more power it can produce and the faster the car can go. 

On the other hand, if you’re looking for an SUV to go off-roading or haul heavy goods, then torque is the more important measurement to consider in a vehicle. High torque means the engine can apply more turning force to the wheels, allowing you to move heavy loads easily.

It’s important to note that kilowatts and torque are not interchangeable measurements. A high kilowatt rating does not necessarily mean high torque and vice versa. Rather, they serve different purposes and are important to consider depending on what you need from your engine or motor.

The image featured at the top of this post is ©yanik88/Shutterstock.com.