A steel flywheel turns, and a glint runs along its rim, the kind you only get on well-polished metal; from it a rod moves back and forth, driving the piston into the cylinder. At the lower right a horse walks, picking up its feet in its own rhythm and flicking its tail — the very one that was once harnessed not to a cart but to metrology. Along the scale at the bottom a brass bar creeps up and each time reaches the red mark: that is the thirty-three thousand foot-pounds per minute appointed by Watt.
Unit passport
Non-SI · quantity: power sheet 1/1 · rev. 2026-09Horsepower hp · PS
Non-SI unit · quantity: power · 745.6999 W or 735.49875 W
The first unit in history invented by a sales department
James Watt had a splendid machine and a wretched task: to explain to the owner of a brewery why he should buy an iron box when there was already a horse in the shed. Talking about joules per second was pointless — the joule had not been invented yet, and the brewer had no need of it. Watt took another route: he measured how much work a mill horse does in a minute, called it a horsepower, and began writing in his contracts that his machine replaced twenty of them.
The trick worked so well that it outlived the steam engines, the inventor himself, the metric system and even an outright ban: in Europe power has had to be stated in kilowatts since 1992, yet horses stubbornly stand beside it in brackets, because the buyer wants to know not the work per second but how many horses he has under the bonnet.
A separate charm of it is that a real horse is not capable of such power. Watt rounded his measurements up, and generously: a living horse in long work delivers about seven tenths of a kilowatt, noticeably less than one Watt horse, while in a short burst it gives some fifteen times more. So the unit is named after an animal that matches it neither over the long haul nor over the short one.
In English the mechanical horse is hp, the German and metric one PS, the French ch, the Dutch pk, and all of them except the first mean one and the same number. The Italians write CV, which muddles matters completely, since in France those same letters denote the tax horse, which has almost nothing to do with power.
The mechanical horse is a good ten watts stronger than the metric one, and at three hundred horses the difference already comes to four horses — exactly enough to start an argument on the internet. The boiler horsepower is thirteen times larger than either, because it measures not a shaft but the evaporation of water. The tax horse measures nothing at all: it is a formula based on cylinder bore, invented by a treasury.
01 · Definition
torque on the shaft · revs · a lifted loadOne mechanical horsepower is the power at which a load weighing thirty-three thousand pounds rises one foot in a minute, which in SI units comes to seven hundred and forty-five point seven watts. The metric one is built the same way, only it counts in kilograms-force: seventy-five kilograms over one metre per second, that is seven hundred and thirty-five and a half watts.
Where those thirty-three thousand came from is known exactly. Watt watched horses walking in a circle at a brewery capstan and worked out that such a horse pulls with a force of about one hundred and eighty pounds while covering about one hundred and eighty feet a minute. The product gave thirty-two and a half thousand, but a round number looked more solid in a contract, and the figure was raised to thirty-three.
On the shaft of a machine the same power is found from torque and revs, and that pair holds the whole difference of character. A motorcycle engine and a locomotive diesel may deliver the same kilowatts, but the first takes them in revs at a modest torque, while the second takes them with a monstrous torque at revs where the motorcycle has not even started. The horse, incidentally, is built along diesel lines: its torque is excellent and its revs are four a minute.
Finally, every data-sheet horse carries a note saying by which rule it was measured. The gross power of the old American protocols was measured without the fan, the generator and the silencer, which is why the horses in the brochures of the sixties were noticeably friskier than the ones the buyer got; since 1972 the net figure is used — with all the accessories in place — and the numbers promptly became about twenty per cent more modest.
Who lifts a tonne faster
On the left stands a winch with a one-tonne load, and to it are harnessed in turn a horse, Watt's steam engine, a car engine and a motorcycle engine: the drum turns, the rope winds up, and the load creeps upwards the faster the greater the power — which is exactly the picture with which Watt explained power to a buyer without uttering the word «watt». On the right the same engine is shown as a pair of curves: the brass one is torque, the steel one power, and a red dot runs along them following the rev slider. Note that torque and power reach their peaks in different places, and it is precisely because of this that conversations about engines contain so much mutual misunderstanding.
02 · Conversion
kilowatts · what it means · data-sheet rulesOne power in three conversations
The first tab spreads the power you enter over every horse that exists in nature and in legislation. The second turns it into something tangible: a lifted load, boiled water, a car accelerating. The third shows how the same piece of iron becomes different data-sheet figures under different test rules.
A rule to remember: a kilowatt is about one and four tenths of a horse, and a horse is three quarters of a kilowatt, and both estimates will agree until the matter reaches a tax return.
When comparing two engines by their data sheets, first find out by which rule the figures were taken. The gross American power of the sixties, the net figure under today's rules and the European metric horse give three different numbers for one and the same engine, and an argument about whose engine is more powerful often turns out to be an argument about whose protocol is more lenient.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
Prony brake · water brake · rollers · indicatorFour ways to catch power by the shaft
A shoe, a rope and a steelyard
The earliest and the most transparent of meters: the shaft is gripped by wooden shoes, the shoes are joined to a long lever, and the end of the lever rests on scales. The machine drags the shoes along with it, the scales show the force, force times the arm gives torque, and torque together with revs gives power. The whole contraption heats up as it works, smokes and smells of burnt wood, but it does not lie, and that is why it gave us the expression «brake power», which lives on in data sheets to this day.
A brake that does not burn out
When powers passed hundreds of kilowatts, wooden shoes stopped coping, and the brake was made watery: the shaft spins an impeller inside a casing filled with water, the water resists and heats up, and the casing tries to turn with it and pushes against a force sensor. Such a device swallows thousands of kilowatts as long as there is running water enough, and it can count power twice over — from the torque and from the heating of the flow, which makes an excellent mutual check.
A car that drives without going anywhere
Taking the engine out of a car for a measurement is a long business, so the car is driven onto rollers and accelerated right there in the workshop. The device sees the power at the wheels, and that is less than the engine's by everything the gearbox, the axle and the bearings have eaten — usually about fifteen per cent with rear-wheel drive and rather more with all-wheel drive. Hence the eternal disappointment of an owner who sees less on the printout than the data sheet promises, and the eternal consolation of the mechanic: the sheet was measured at the shaft, and you drive on wheels.
An invention that was kept secret
A pencil linked to the piston along one axis and to a pressure spring along the other draws a closed figure on paper whose area equals the work of one cycle; multiply it by the number of cycles a minute and you get the power inside the cylinder, before any friction losses at all. The device was invented at Watt's works and was shown to nobody for some twenty years, quite rightly treated as a competitive advantage: while rivals argued about whose machine was better, here they simply took a diagram and looked at the area.
05 · Writing rules
PS with a space · kilowatts first · the test ruleKilowatts first, horses in brackets
The unit is non-SI, and the rules for handling it are simple: the main value goes in watts, the horse stands beside it for reference, and it is always said which horse exactly and under which protocol.
The first mistake is purely a matter of spelling, but it occurs more often than all the others put together. The second robs the number of meaning, because the same iron under different protocols spreads by a fifth. The third equates the mechanical horse with the metric one, and on a powerful engine that already adds up to several horses of difference. The fourth is a misunderstanding that lives in the very name: force is measured in newtons, while horsepower is power, that is work over time. The fifth confuses the shaft with the wheels. The sixth, finally, drags out the tax horse, which is counted from cylinder bore and has no relation to real power — something advertisers duly made use of in their day.
06 · Neighbouring units
watt · newton-metre · kilowatt-hourThe horse is surrounded by relatives it is often confused with: the watt is the same quantity in SI units, the newton-metre is torque, from which power comes by multiplying by revs, and the kilowatt-hour is no longer power but an amount of work, the one you get a bill for.
The SI unit, named after the very man who invented the horse. Amusingly, the inventor was passed over by his own invention: watts measure everything, horses only engines.
Power multiplied by time: not how strong a machine is, but how much work it has done. The meter in the cupboard counts exactly this quantity, not horses.
What an engine pulls with, whereas power is how fast it works while doing so. A horse takes it with torque, a motorcycle with revs, and people argue about the two as if they were the same thing.
Among the Simetrium data sheets, standing nearby are kilowatt-hour and transmitter power: in the first case power turns into an electricity bill, in the second into the right to occupy a frequency, and both sheets are about how one and the same quantity means entirely different things in different hands.
07 · Historical section
archive · from the brewery to the treasuryHow the horse outlived all of its machines
{title}
{text}
In three centuries the ordinary machine has grown a hundredfold stronger while the unit has stayed the same, and therein lies its chief virtue: a measuring rod should not grow along with what it measures.
A unit that has no standard and cannot have one
The metre has a definition through the speed of light, the kilogram through the Planck constant, and horsepower has nothing and never had anything beyond an agreement going back to one brewery and a few tired horses. Strictly speaking that is no misfortune: the unit is exactly expressed through the watt, and any bench measures it no worse than it measures kilowatts. The misfortune is elsewhere — the name promises a comparison with a living animal, and nobody survives that comparison, including the animal itself.
Measurements on horses were made later too, by then without commercial interest. It turned out that over a working day a horse confidently holds about seven tenths of a kilowatt, roughly a third weaker than Watt's unit, while in a short effort lasting a few seconds it develops more than ten. Had Watt taken the burst as his model, his machines would have had to be sold at three horses apiece, and the story of the industrial revolution would sound far more modest.
Hence the chief lesson of this sheet, applicable well beyond engines: a successful unit is one the buyer can feel, not one a committee derives strictly. The watt is more correct in every respect, but selling twenty horses proved easier than selling fifteen thousand watts, and the market settled the choice once and for all. Committees have since tried twice to withdraw the horse from circulation and yielded both times: in documents it counts as a reference value, and in conversation as the only one.
Catalogue of quantities
power, work and motionSI base units
the kilogram, the metre and the second are highlighted — the watt is built from themThe herd by machine
kilowatts and horses are counted from one number| Machine | Kilowatt | Metric PS | Mechanical hp | What is notable about it |
|---|---|---|---|---|
| {name} | {kw} | {ps} | {hp} | {note} |
Compare the two right-hand columns: the difference between the metric and the mechanical horse is a little over one per cent and therefore invisible on a two-wheel tractor, yet on a ship diesel it turns into a good thousand head. Hence the custom of serious protocols to write kilowatts first: they are the same for everybody, while each has a horse of his own.