Horsepower vs. Kilowatts: What Engineers Need to Know About Measuring Power 

Power – and the measurement and calculation of it – will always be of central importance across a broad variety of engineering applications.  

When it comes to the measurement of the power output of engines, motors, and other mechanical devices, both horsepower (hp) and kilowatts (kW) are routinely used.  

So, in this article, we will set out an explanation of what these two common units of power are, and the range of systems and situations in which engineers use them.   

An Introduction to Horsepower and Kilowatts  

Below are some of the fundamentals of kilowatts versus horsepower that engineers and other professionals engaged with measurements of power need to be aware of:  

• Kilowatts 

Kilowatts are, for virtually every form of engineering that doesn’t involve internal combustion, the standard measurement of power. 

This unit, which constitutes a measurement of energy transfer over time, is an “SI unit” – in other words, it is an official metric system, in accordance with the International System of Units (SI). A kilowatt is equivalent to 1,000 watts, with this unit being named in honor of the physicist James Watt (1736-1819). 

Between horsepower and kilowatts, the latter is the preferred unit for electrical systems. This applies particularly strongly for applications that deal with large power generation or consumption.  

• Horsepower  

Horsepower, too, is a unit for measuring power, albeit one that is especially prominently associated with car engines and the automotive industry.  

It was the aforementioned James Watt who coined the term “horsepower” in the 18th century. This related to his theory that one horse was capable of undertaking 33,000 foot-pounds of work in a minute – or to put it a simpler way, that it could move 1,000 pounds of coal (or another object) 33 feet in 60 seconds.  

As explained by TechTarget, the horsepower metric can be “defined as the work done by a force of 550 pounds acting through one foot in a second, or foot-pounds of work, the unit of power needed to raise 550 pounds one foot in a second.”  

It says something about the strength of Watt’s legacy that to this day, when it comes to internal combustion engines, many of us tend to refer to “hp” rather than “kW”.  

With the advent of electric cars, however, there appears to have been a shift towards kW as the usually stated unit of power measurement, even if some motoring journalists still insist on converting it back to horsepower.   

So, How Do Horsepower and Kilowatts Tend to Differ in How They Are Applied?  

As engineers quickly come to know, there are different contexts in which these respective scales of power measurement are customarily used:  

• Although horsepower continues to be frequently used to specify the output of car engines, it is also used for evaluating the power requirements of industrial machinery such as pumps, generators, and compressors, as well as construction equipment like excavators and loaders. You might have also seen it used in reference to the power output of marine (boat) engines.  

• Where, on the other hand, there are mentions of the power that power plants generate, this is likely to be expressed in kilowatts. As we touched on above, this unit is routinely used to indicate the power of electric vehicle (EV) motors, too. It is used, as well, to specify the power output of electric motors across other industrial applications.  

If you are an engineer drawing upon either or both these units for purposes like those detailed above, it shouldn’t take you long to find a good online horsepower to kilowatt converter for energy calculations, and related tools that will help you work more efficiently and accurately.  

Once you have found the right tools and resources, you will be well-placed to achieve optimal results from even particularly specialized and demanding engineering assignments.