Horsing Around With Energy

By The Metric Maven

Mini-Bulldog Edition

It is quite possible, that the best living author of popular science is Sam Keen. His book The Disappearing Spoon is on par, or perhaps, in my weaker moments, slightly better than Isaac Asimov’s Building Blocks of The Universe. I never thought that would happen. His book The Violinist’s Thumb was very engaging, after a somewhat slow beginning. I did not have high expectations for his new book, Caesar’s Last Breath, as I have read extensively about gasses and the atmosphere; I expected much to be a repeat. While some was, the majority of the book was unfamiliar, offered rich details concerning what I did know, and is quite interesting. His asides are as engaging as his intended narrative. When Keen arrived at James Watt (1736-1819), he offered a perspective that had passed me by:

The expansion into new markets got Watt thinking about steam engines in more grandiose terms as well. To most people, the engines were just tools built to accomplish a specific task—pump water, drive a lathe, whatever. Watt envisioned the engines more as universal sources of energy—machines capable of powering any mechanical process. As an (anachronistic) analogy, most people saw steam engines as something like calculators: proficient at one task but useless beyond that. Watt dreamed of building the steam equivalent of computers, machines versatile enough to work in any industry.

Rather than calculate every factory’s case separately, Watt invented a universal standard of comparison, the horse power. He defined this rather literally by watching several horses push a mill wheel around and then calculating how far they moved the weight in a certain amount of time (550 foot-pounds per second, he found). This unit was shrewd in several ways. By invoking horses, Watt slyly reminded factory owners what they could give up—all the oats and broken legs and
vet bills. Customers also understood the unit intuitively. If ten horses had run their mill wheel before, well, they needed a ten-horsepower engine.

Scientifically, the idea proved prescient as well. Over the next century chemistry and physics would be dominated by thermodynamics, the study of heat and energy. Energy is a vast topic in science, popping up in all sorts of different contexts, and scientists needed a standard unit of comparison to understand how quickly different processes absorbed and released energy. The horsepower fit the bill perfectly. Little did those scientists know that the whole idea started as a marketing scheme by James Watt.

(As thermodynamics branched out into new phenomena, however, like light and magnetic fields, the absurdity of the name “horsepower” became obvious—as if you could still hitch old Bessie to the apparatus. In 1882 physicists finally voted to establish a new universal unit of power, which applies just as readily to light bulbs and refrigerators as to engines for raising water by fire. They called it the watt.) pp 173-174

The phrase horsepower is such a powerful meme that if you look up the specifications for a 2017 Dodge Charger, its power output is given only in horsepower as:

SAE Net Horsepower @ RPM : 485 @ 6100

The marketing power of horsepower caused misguided people to define a metric horsepower. A mechanical horsepower is about 745.7 watts, whereas a metric horsepower is approximately 735.5 watts. Electric motors in Europe have both metric and mechanical horsepower ratings. There is also a boiler horsepower, used to rate steam boilers, which is equal to 9809.5 watts.

The output of a horse is not constant, there is a peak and sustained value. Data gathered at the 1926 Iowa State Fair indicates that over a few seconds a horse can achieve a power output of 14.9 horsepower (11.1 Kilowatts). The data indicated that indeed, the horses measured achieved about a 1 horsepower sustained output.

The continued use of horsepower throughout the world demonstrates the power of romantic metaphor over a carefully designed quantity, even when the resulting unit is named after the horse-trading marketer who came up with horsepower.


The Metric Maven has published a book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.

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4 thoughts on “Horsing Around With Energy

  1. So a Bugatti Veyron develops 736 kW.. not even a MW…

    There are electric cars that claim 1MW – but not for very long –

    I think it is possible to have a ICE powered car with 1MW output…

  2. With respect to James Watt and watt (W), we should keep this in mind:

    Stigler’s law of eponymy is a process proposed by University of Chicago statistics professor Stephen Stigler in his 1980 publication “Stigler’s law of eponymy”. It states that no scientific discovery is named after its original discoverer.

  3. I grew up seeing engine power described in horsepower and power for everything else (heaters, stereos, light bulbs,) described in watts. This made me think there was something fundamentally different about the power measurement of engines compared with that of other machines. (Also, rocket motors and jet engines had thrust measured in pounds making them different again.)

    It was only when Australia converted to the metric system and engine power was expressed in kilowatts that I realised they were the same measure. It’s amazing how customary usage moulds your thinking on a subject.

    I remember seeing vehicle power expressed in kilowatts in a science fiction story as a kid. I also remember thinking that science fiction was the only place I would see the metric system in common use.

  4. Until recently in the UK, gas central heating boilers, fires and water heaters were rated in BTU per hour, whereas electric space and water heaters were rated properly (i.e., in kilowatts).

    1 BTU = the amount of energy required to make 0.454kg. of water 5/9 of a degree hotter. Yes, really. A transparent parody of the nutritionist’s Calorie.

    This had the effect of preventing easy, direct comparisons between running costs of gas and electric appliances. Of course, on the Continent, where everything was rated in kilowatts, it was easier to decide whether to install a gas central heating system with a 12kW boiler and six radiators, or six separate 2kW electric heaters. (Electricity costs more than gas, but central heating becomes inefficient when not running flat-out; this may be an important consideration if some rooms are often unoccupied.)

    At least gas has been billed in kilowatts-hours for the 20 or so years I’ve been paying my own bills. They helpfully show you the calculation: start with cubic metres of gas (really old gas meters used to read cubic feet; until mine was replaced with a modern one, there was a preceding step from cubic feet to cubic metres. Oddly, though, despite a foot being smaller than a metre, I seem to be using more cubic metres of gas than I used to use cubic feet), multiply this by the energy value to in MJ/m3 to get megajoules, multiply by a “volume correction” (a seasonal adjustment factor, to account for the fact that the gas is not being delivered at exactly 100 kPa and 273K) and then lastly divide by 3.6 to get kilowatts-hours.

    The thing is, energy is the great unifying concept in all of Physics, and it is less than useless to have disparate, unconnected units for expressing it. (Unfortunately, we’re kind of stuck with legacy time units; which means occasionally encountering oddities like the kilowatt-hour, the watt-minute or the megajoule per day. And I’m not optimistic about this changing before the development of interplanetary travel renders Earth-based units useless. Anyone who has to work with Unix timestamps, soon gets pretty conversant with the 3600 and 86400-times tables.) When you have the power used by an electric motor measured in kilowatts, the output of any other non-electric type of engine rpresented in horsepower, the energy produced by metabolising food given in Calories and heat energy measured in BTUs (unless it’s coming from an electrical source, in which case it will be given in kilowatts-hours) then it’s much harder to see the connection.

    By the way, just for a spot of light relief, here’s an interesting equivalency:

    Consider the amount of kinetic energy imparted by a ten-ton truck decelerating suddenly from 60 mph to a dead stop. That’s the same as the energy it would have taken to get it up to that speed in the first place; and remember KE = .5 * m * v**2, so all we need do is put everything into the proper units.

    m = 10 * 2240 * 0.45359237 = 10160.47 kg. (I’m assuming an Old English ton, which is 2240lb, not a US ton which is only 2000lb.)
    v = 60 miles in 3600 seconds = 60 * 1609.344 metres in 3600 seconds = 26.8224 m/s (I’m assuming an Old English mile of 1760 yards, each 0.9144m., which may or may not be the same as a US mile).
    So E = .5 * m * v ** 2
    = .5 * 10160.47 * 26.8224 * 26.8224
    = 3 654 928.741J
    ….. or about a kilowatt-hour.

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