Horsepower is a unit of the measurement of power and it was first coined by the Scottish engineer James Watt (19 January 1736 to 25 August 1819). He is the engineer/scientist who invented the concept of horsepower (h.p.). At the outset the measurement was simple and straightforward although Watt introduced a slight twist to the formula (see below). Today, there are many different standards and types of horsepower which arguably makes the concept confusing to the layperson.

You may be forgiven for believing that if a car has an 8 hp engine that it has the equivalent power of eight horses but this is incorrect rather strangely which is more confusing!

Back in the day of James Watt, the 1700s, it was decided that people needed a way of making the new steam engine more understandable in terms of its power output.

Also, back in that era, people related to the work rate of horses in terms of power output. They were the benchmark and therefore James Watt used the power output of a brewery dray horse to come to his standard.

James Watt went to London breweries where there are strong dray horses toiling away and made some measurements. He came to a figure which he thought was a fair average.

The figure he arrived at was the true horsepower namely the strength of a dray horse to move an object a certain distance over a certain time. But he multiplied that conclusive figure by 1.5 to produce his official figure for the power of one horse which was 33,000 foot-pounds per minute.

This, in other words, means that one horsepower is the amount of force needed to move an object weighing 33,000 pounds a distance of 1 foot in one minute. We can visualise a dray horse moving 33,000 pounds one foot in distance in one minute. For comparison a large car today weighs around 4,400 pounds. 33,000 pounds is around 7.5 large cars in modern terms.

Why did he multiply the true average by 1.5? Desmond Morris PhD believes that he did it to make the steam engine more impressive in its power output.

At the beginning of this system of horsepower it came under attack as it was described as a “new and shockingly unscientific unit”. Critics said that it shouldn’t have come into use. Despite this unpopular start it remained in use for many years because it allowed people to have an understanding of the power output of engines in motorcars as they were able to compare the motorcar with the power of a team of horses pulling a cart.

It seems to have given the early motorcar an inflated profile of more power than it truly had because of the 1.5 multiplying factor that he introduced at the inception of the concept of horsepower.

Below are some more articles on horses.