As much as 15 HP
It would be reasonable for one to assume that the horsepower of 1 horse would be somewhere around 1. While a horse may produce 1 HP of power, it will often produce much more or much less.
What does 1 horsepower mean?
Horsepower is a unit of power. Power is how fast energy is used or transferred and is often expressed as energy per unit of time. A watt is defined as 1 joule per second. For example, An incandescent lightbulb that is 60 watts consumes 60 joules of energy every second.
Horsepower is officially defined as roughly 746 W. Just to be confusing there is also a metric horsepower which is a little bit less – around 735 W.
So, the power of a horse should be 746 W then, right?
Well, no. A horse that is asleep or resting is expending very little energy over time and their power will be quite low. A horse running in the Belmont Stakes is going to be using energy at a very fast pace for a short period of time and that would equate to high horsepower.
So why do we horsepower as a measurement?
In short – marketing. Before the steam engine, one or more horses were often used by farmers and manufacturers to drag plows and operate machines. In order to sell steam engines, it was important to describe them in terms that the customers would understand.
Some potential customers … were still using draft horses to move weight and power machinery. [James Watt] needed a way to explain his engine in terms that these people would understand.
James Watt made some observations of horses doing work and calculated what he thought would be a reasonable correlation to relate these new machines to how many horses those machines might replace.
This unit of measurement stuck and has become the common standard for expressing the power of a machine or engine. Generally, when horsepower is used to describe an engine, it is describing the maximum power output that the engine is capable of producing.
So, what is the horsepower of a horse?
Every horse is different, of course, of course.
According to the website Energy Education, citing a publication called Delayed Obsolescence: The Horse in European and American Warfare from the Crimean War to the Second World War, measurements have been done which have concluded that horses can generate up to 15 HP.
I looked for a more recent source for a measurement of the power output of a racehorse and couldn’t find one. I have no doubt that high-end breeders and racehorse trainers are using advanced analytics to measure racing performance given the purses at stake, but I guess they keep that information to themselves.
What about humans?
At rest, a human is expending about 100W of power. Ordinary people might be able to generate a couple of hundred watts while for short periods during heavy exercise.
Elite track sprinters are able to attain an instantaneous maximum output of around 2,000 watts, or in excess of 25 watts/kg.
Elite road cyclists may produce 1,600 to 1,700 watts as an instantaneous maximum in their burst to the finish line at the end of a five-hour long road race.
A group of researchers in Mexico calculated the maximum power that Usain Bolt reached during his 9.58s world record 100m run in the World Championships in Berlin in 2009. According to their calculations, Bolt peaked at 2619.5 watts which is equal to 3.5 HP.