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The watt is a clean, physics-derived unit: one watt equals one joule of energy transferred per second, and it's named after James Watt, the Scottish engineer whose improvements to the steam engine drove much of the Industrial Revolution. Horsepower, ironically, was Watt's own invention — but for a very different reason: he needed a way to market his steam engines to buyers who understood horses, not joules. He calculated that a strong dray horse could move 33,000 pounds one foot in one minute, then defined “one horsepower” as that rate of work — a figure that's actually about 50% higher than what an average horse can sustain over a full working day, since Watt (reasonably, for sales purposes) benchmarked against strong horses working in short bursts, not average sustained output.
To make matters more confusing, “horsepower” isn't even one single unit — mechanical (imperial) horsepower is defined as 550 foot-pounds per second (≈745.7 watts), while metric horsepower (used in much of Europe and Asia, often labeled PS or CV) is defined slightly differently at 75 kilogram-force-metres per second (≈735.5 watts), a roughly 1.4% difference that matters when comparing a car's advertised power figure across regions. Despite not being part of the International System of Units, horsepower remains deeply entrenched in the automotive and engine industry worldwide precisely because Watt's 18th-century marketing decision worked too well — buyers still relate more intuitively to a familiar animal-based unit than to an abstract joule-per-second figure, even 250 years later.
James Watt calculated that a strong dray horse could move 33,000 pounds a distance of one foot in one minute, and defined that rate of work as "one horsepower" to help market his steam engines to buyers who understood horses rather than the physics units of his era. It's roughly 50% higher than what an average horse can sustain over a full working day, since he benchmarked against a strong horse's short-burst output.
No — mechanical (imperial) horsepower equals approximately 745.7 watts, while metric horsepower (labeled PS or CV in parts of Europe and Asia) equals approximately 735.5 watts. The roughly 1.4% difference means the same physical engine can show a slightly different horsepower figure depending on which regional convention is used to advertise it.
Watt's engineering work advancing the steam engine was significant enough that the SI unit of power was later named in his honor — separate from his earlier, commercially-motivated invention of the horsepower unit. It's a slightly ironic historical footnote that both a rigorous physics unit and a marketing-driven comparative unit trace back to the same person.
Mostly cultural momentum — horsepower has been the standard way to communicate engine output to consumers since the 18th century, and it remains more intuitively relatable to most car buyers than an abstract joule-per-second figure, even though watts (or kilowatts) is the internationally standardized SI unit of power used in physics and most other engineering contexts.