How James Watt Created Horsepower for the Steam Engine

To understand why horses came to be used as a measure of power, we need to go back to the 18th century and the beginning of the Industrial Revolution.

James Watt and the steam engine

This was the era of James Watt, the renowned Scottish inventor, chemist and engineer. Among his many achievements, perhaps the best known and most significant was his substantial improvement of Thomas Newcomen’s steam engine. Created in 1712, Newcomen’s machine had remained largely unchanged for decades.

After repairing one of these engines, Watt spent time analysing it and carrying out experiments. During the 1760s, he found solutions that greatly improved the steam engine’s efficiency.

The efficiency gains were enormous: consumption fell by 75%, power increased and the machine offered far better value for money.

However, it was not until 1781 that Watt had a practical steam engine built with the precision needed for commercial sale. This raised the first questions about how to advertise and promote its advantages over Newcomen’s machine.

How Watt promoted his steam engine

Watt initially devised a royalty system under which customers would pay him one-third of the savings they made by using his engine instead of others.

But how could he show prospective customers the benefits of his machine if they had never encountered this new technology?

At the time, there was no unit for measuring the gains of one technology over another. A solution had to be invented…

Inventing horsepower

Once an inventor, always an inventor. Watt decided to create a new unit of measurement that would allow his engine to be compared quickly with the “machine” most commonly used for all kinds of work at the time: the horse.

This made it much easier to explain and compare his engine’s performance. In theory, an engine producing 1 horsepower would be equivalent to one horse in terms of productivity.

The unit known as horsepower (hp) was born, eventually giving rise to metric horsepower (cv).

What remains uncertain is which experiments James Watt carried out to establish the equivalent power of a horse. There are many versions of these experiments, and nobody knows exactly how he arrived at the final figures.

There is, however, no doubt about the type of horse used as his reference: the draught horse. This sturdy horse was typically used in mines or for agricultural work, precisely the settings in which his engine could be employed.

How is horsepower calculated?

Ultimately, it was determined that one horsepower was equivalent to 33,000 pound-feet per minute, since power is the amount of work performed per unit of time.

With the creation of the metric system, those figures were converted to 75 kgf·m/s (kilogram-force metres per second). The image below shows the calculation and the experiment carried out to arrive at this result:

Whatever calculations were used to establish its value, horsepower became the principal unit for measuring engine power and remains the most widely used today.

Those different methods of calculation also explain the discrepancies between cv, hp and bhp.

Ironically, James Watt’s contribution to the development of steam engines led to his surname being adopted as the standard unit of power under the SI (International System of Units) in 1972. Yet we still want to know engine power in horsepower rather than watts (W), or rather kilowatts (kW).

And, for reference, one metric horsepower is 735.5 W or 0.7355 kW, while one horsepower is 745.6 W.