When Boulton and Watt introduced their rotative steam engines to manufacturers around 1784, they needed to guarantee a certain level of power output for their machines. Boulton & Watt aimed to communicate this power in terms that potential customers could easily grasp. At that time, the machinery in London's major breweries and distilleries was powered by horses, and the proprietors—Watt's first customers—often asked how many horses an engine would be equivalent to.

As a result, Mr. Watt conducted experiments on the strong horses employed by London brewers. He discovered that a horse could walk at a rate of 2½ miles per hour while pulling 150 pounds avoirdupois using a rope over a pulley. At that time in England, at least five different definitions of a pound were in use. However, thanks to Watt's specification that 33,000 pounds equated to 528 cubic feet of water, we can determine that he calculated with one pound equal to 0.4531 kg. [1] Watt thus defined one horsepower as the power needed to lift 150 pounds avoirdupois vertically at a rate of 220 feet per minute. Early steam engines typically raised water from mines, so horsepower is often expressed as the ability to raise 33,000 pounds of water one foot high per minute.[2]

According to legend, this high standard resulted from a shrewd London brewer, a Scotsman, who showcased his strongest horse to Watt, pushing it to its limits over an eight-hour period. [3] In reality, Watt likely chose a high definition for horsepower to avoid accusations of underperformance, allowing for performance degradation over time.

Smeaton and other engineers conducted observations on horses working in mills and found that a realistic measure of horsepower was 22,000 pounds raised one foot per minute, a workload a good horse could maintain for an eight-hour day. [4] Traditionally, horses worked in short shifts of 2-3 hours with rest periods, enhancing their performance beyond the 22,000-pound benchmark.

In Germany, likely rounded for simplicity, DIN 66036 (German industrial standard) defines the metric horsepower, or PS for "Pferde stärke" as 75.0 kgm/s, being slightly smaller than the imperial horsepower, which is 76.04 kgm/s. In SI units, this translates to approximately 735.5 Watts for metric horsepower and 745.7 Watts for imperial horsepower. For the purposes of this case study, I will refer to imperial horsepower, as most historical documents utilize this standard, facilitating easy comparison. Despite the shift toward electric vehicles, horsepower remains a familiar measurement for most.