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bagasme

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Despite the former case is highly popular, why isn't horsepower considered SI unit for power?

Quoting from Automotive News:

Volkswagen Group's initial reason for developing the Bugatti Veyron first launched in 2005 was to build a car with 1,000 PS.But that goal flies out the window when converted to 987 bhp in the imperial system. At the very least that is a notable difference. When it comes to cars that mere mortals can afford, the difference is just a rounding error.Somewhat frustratingly this means that the gap between metric and imperial horsepower -- or PS and bhp -- is just enough to be measurable, but not enough to be meaningful.The problem arose because Scottish inventor James Watt wanted to come up with a term to compare the effectiveness of his steam engine against the horses it was designed to replace. One unit of horsepower derived from his crude estimate that a horse can lift 330 pounds of coal out of a mine shaft at 100 feet per minute. Compare this antiquated definition with the simple elegance of a watt, which is the energy transferred when applying one Newton of force over one meter of distance in one second.

By definition, a horsepower is an arbitrary unit: it is an estimate of average power of a horse that do the work which would be replaced by steam engine, at the time of invention of steam engine. Its derivation are based on non-SI (imperial) units. On the other hand, a watt is simply derived from other SI unit:

$$ 1 ~ W = 1 \frac {J} {s} = \frac {1 \cdot N \cdot m} {s} = \frac {1 \cdot kg \cdot m^2} {s^3} $$

The usage of kilowatts as power unit in automotive engines become more popular thanks to use electric cars. Since such cars use battery as source of electricity, and kilowatts are common in electricity domain, it is natural for those cars to express its powers in terms of kilowatts.

Thanks for reading this thread.