The watt, used everywhere today, and the famous "horsepower" of cars both come from the same man: James Watt.
James Watt was born in 1736 in Greenock, Scotland, the son of a merchant and shipbuilder who gave him early access to precision tools and instruments. Watt had fragile health as a child and attended school irregularly, learning much of his mathematics and geometry at home, on his own — an unconventional start for someone who would become one of the most influential engineers in history. As a young man, he moved to London to learn the trade of scientific instrument making, before returning to Scotland and establishing himself as an instrument maker at the University of Glasgow.
It was in Glasgow, while working on maintaining university equipment, that Watt had direct contact with a model of a Newcomen steam engine — the dominant design of the time, but extremely inefficient, wasting most of the heat generated with each operating cycle. Watt did not invent the steam engine from scratch (that credit belongs to Thomas Newcomen, decades earlier), but in 1765 he noticed an elegant solution to the original design's main energy waste: adding a separate condenser, keeping the main cylinder always hot and avoiding the repeated cycle of heating and cooling the same chamber.
This seemingly simple improvement increased the efficiency of steam engines several times over, making them commercially viable for a much broader range of industrial applications — from pumping water out of mines to powering looms in textile factories. In partnership with businessman Matthew Boulton, Watt founded the company Boulton & Watt in 1775, which would dominate the steam engine market in Britain for decades and is widely considered one of the central driving forces of the Industrial Revolution.
To sell his steam engines convincingly, Watt faced a very practical marketing challenge: how to compare the power of an unfamiliar machine to something his customers, often mine and mill owners, already understood intuitively? The answer was to compare it to draft horses, then widely used to operate mill wheels and mining pumps. Watt ran experiments observing how much sustained work a strong horse could perform over a period of time, and from that defined the commercial unit "horsepower."
There are historical accounts, though not fully confirmed, suggesting Watt may have deliberately overestimated the real capacity of an average horse when calculating this reference — a clever marketing decision, since it made his machines seem even more powerful by comparison, "beating" the equivalent of more horses than a real animal could sustain for long. True or not, the "horsepower" unit quickly caught on and became the international commercial standard for power for over a century, used both for steam engines and, later, for early internal combustion engines.
Watt made other notable technical contributions beyond the separate condenser, including the parallel motion mechanism (which he proudly considered one of his most elegant inventions from a purely mathematical standpoint) and the centrifugal governor, one of the first automatic speed-control mechanisms in the history of engineering — a direct ancestor of the automated control systems used today across virtually all modern industry.
In tribute to his fundamental contribution to engineering and applied physics, the International System of Units' power unit was named the watt (W) in 1960, more than 140 years after his death in 1819. One watt equals exactly one joule of energy transferred per second — a mathematically clean, universal definition, quite different from the comparative, commercial logic behind the original horsepower.
Horsepower is still present today, especially in the automotive industry, where it remains widely used in advertisements and vehicle spec sheets for its intuitive appeal to the average consumer. But officially, in scientific and engineering contexts and across virtually the entire industrial world outside automotive marketing, the watt and its multiples (kilowatt, megawatt) are the universal standard — a reminder of how a unit born from practical commercial necessity can coexist, for centuries, alongside the scientific standard that officially replaced it.