Why Isn't Horsepower an SI Unit, Despite Its Popularity?

• bagasme
In summary: Thanks for reading this thread.In summary, the usage of kilowatts as power unit in automotive engines become more popular thanks to use electric cars.
bagasme
When we talk about engines on automotive industry (cars, motorbikes, trucks, etc.), we often refer the power produced by such engines by horsepower. But when we talk about electricity, the power is measured in kilowatts.

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.

bagasme said:
On the other hand, a watt is simply derived from other SI unit:

$$1 ~ W = 1 \frac {J} {s} = \frac {1 \cdot N \cdot m} {s} = \frac {1 \cdot kg \cdot m^2} {s^3}$$
This is one significant advantage of the SI units - that they are not random with respect to each other. You don't have to go look up what a watt is.

bagasme, anorlunda, vanhees71 and 1 other person
bagasme said:
When we talk about engines on automotive industry (cars, motorbikes, trucks, etc.), we often refer the power produced by such engines by horsepower. But when we talk about electricity, the power is measured in kilowatts.

Despite the former case is highly popular, why isn't horsepower considered SI unit for power?
Because SI units are based on practicality and consistency, not popularity.

hutchphd, bagasme, Vanadium 50 and 1 other person
bagasme said:
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.

That's historically how the definition began. There are several different definitions in use.

One advantage to using the ##\mathrm{SI}## is that there is only one definition of the watt.

Another advantage is that the ##\mathrm{SI}## is a coherent system, meaning that you don't have to insert conversion factors.

Quoting from the Wikipedia article https://en.wikipedia.org/wiki/Coherence_(units_of_measurement):

"In SI, which is a coherent system, the unit of power is the watt, which is defined as one joule per second.[3] In the US customary system of measurement, which is non-coherent, the unit of power is the horsepower, which is defined as 550 foot-pounds per second (the pound in this context being the pound-force); similarly the gallon is not equal to a cubic yard but instead 231 cubic inches."

vanhees71 and bagasme
bagasme said:
Despite the former case is highly popular, why isn't horsepower considered SI unit for power?

You can make the same argument for the foot, the pound, the day, and the hogshead. Maybe not the hogshead.

vanhees71 and FactChecker
You can make the same argument for the foot, the pound... [snip]
[I let this go from the OP...]
While it is true that people can make arguments, I wouldn't make this one. I doubt even the horsepower is actually uniquely popular outside the USA.

bagasme said:
The usage of kilowatts as power unit in automotive engines become more popular thanks to use electric cars.
I don't think this is true either.

bagasme said:
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.
gmax137 said:
I don't think this is true either.
Well, Tesla uses kW.

You can make the same argument for the foot, the pound, the day, and the hogshead. Maybe not the hogshead.
This hogshead seems to be a great example for making things utmost complicated. It's a volume unit depending on the fluid/liquid/liquor you measure. That makes the non-metric units so utmost complicated. I always wondered why the heck in the US one measures distances in miles and heights in feet. Otherwise lengths are also given in inches, and nothing converts with simple numbers into the other... Then I prefer the SI (except in electromagnetism, but that's another story).

bagasme said:
When we talk about engines on automotive industry (cars, motorbikes, trucks, etc.), we often refer the power produced by such engines by horsepower. But when we talk about electricity, the power is measured in kilowatts.

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

Quoting from Automotive News:
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.

Your whole starting premise is faulty, because you appear to have the assumption that the SI units are based on popularity. They aren't!

In vacuum systems, most of us do not use Pascal for units of pressure. We tend to use Torr, mm of Hg, psi, etc. Yet, the SI unit for pressure is Pa! It has nothing to do with popularity.

Secondly, if you pay attention carefully, many of the basic SI units (m, s, kg, ...) are now being defined using fundamental constants, rather than some arbitrary units of measure. This is a more unambiguous way to define such units, and is independent of any existing reference object.

Zz.

vanhees71
vanhees71 said:
I always wondered why the heck in the US one measures distances in miles and heights in feet

And depths in fathoms.

vanhees71
And depths in fathoms.

...and shoe sizes in Barleycorns. :-)

vanhees71
vanhees71 said:
I always wondered why the heck in the US one measures distances in miles and heights in feet.
Except in aviation. They measure distance in nautical miles (about 1.15 normal miles) and speed in knots (nautical miles per hour). I don't know how or when that happened, but older airplanes have airspeed indicators marked in miles per hour, while newer ones are in knots.

jrmichler said:
Except in aviation. They measure distance in nautical miles (about 1.15 normal miles) and speed in knots (nautical miles per hour).
A nautical mile is not an English unit (not SI either I think.) It is one distance minute latitude or longitude at the equator, and thus related to 360 degrees in a circle.

It makes sense for mariners and aviators because they do spherical navigation, not linear navigation. Hence feet and meters don't apply.

Edit: By the way the US congress regularly passes resolutions saying "this is the year we convert to metric." The legend is that they passed this resolution every year since 1789. Obviously, the resolution has no teeth.

https://www.congress.gov/bill/93rd-congress/house-joint-resolution/192

vanhees71
vanhees71 said:
I always wondered why the heck in the US one measures...
It is just part of our charm. Like this European language thing with masculine / feminine nouns, what's up with that?

One of my favorites, in reactor engineering we consider the fuel linear heat rate in kW per foot. Go figure...

hutchphd and vanhees71
vanhees71 said:
I always wondered why the heck in the US one measures distances in miles and heights in feet.
Unless you are driving -- then distances are measured in hours or minutes.

vanhees71 and PhanthomJay
anorlunda said:
A nautical mile is not an English unit (not SI either I think.) It is one distance minute latitude or longitude at the equator, and thus related to 360 degrees in a circle.

It makes sense for mariners and aviators because they do spherical navigation, not linear navigation. Hence feet and meters don't apply.
The meter was also based on the Earth, but as a decimal fraction, not a 360/60 one:
https://en.wikipedia.org/wiki/Metre#Meridional_definition

bagasme said:
Despite the former case is highly popular, why isn't horsepower considered SI unit for power?
The problem is deciding which horsepower unit should be selected as a new SI standard. There are at least half a dozen different horsepower definitions to choose from.
On the other hand, we have only one definition of the watt.

bagasme
Baluncore said:
The problem is deciding which horsepower unit should be selected as a new SI standard. There are at least half a dozen different horsepower definitions to choose from.
On the other hand, we have only one definition of the watt.

People refer HP as mechanical horsepower (the original definition by Watt himself). But in automotive engines (especially trucks), metric horsepower (aka PS) is sometimes used.

bagasme said:
People refer HP as mechanical horsepower (the original definition by Watt himself). But in automotive engines (especially trucks), metric horsepower (aka PS) is sometimes used.

How are you able to ignore all the direct responses to your original post and focused just on THIS?

Things on PF can go in so many directions, but it is nice once in a while if we can answer or settle the original issue. Is your original premise in your first post still stand? Do you still think that SI units, or any accepted standard units, is based on "popularity"? Or can we put this issue to rest now and that you've moved on to one of the side discussion from this thread?

Zz.

ZapperZ said:
How are you able to ignore all the direct responses to your original post and focused just on THIS?

Things on PF can go in so many directions, but it is nice once in a while if we can answer or settle the original issue. Is your original premise in your first post still stand? Do you still think that SI units, or any accepted standard units, is based on "popularity"? Or can we put this issue to rest now and that you've moved on to one of the side discussion from this thread?

Zz.

Popularity of an unit doesn't necessarily make it SI unit.

I'm happy with the explanation from this thread.

Popularity is not the issue. Coherency and a world-wide standard are the issues. Every country that originally signed the "Treaty of the Meter" and all the other countries that have come on board since adhere to the same standards. If you're one of those nations and you use a non-SI unit such as horsepower in commerce, you must define it in terms of the corresponding SI unit. For horsepower, that unit is the watt.

bagasme
Mister T said:
Popularity is not the issue. Coherency and a world-wide standard are the issues. Every country that originally signed the "Treaty of the Meter" and all the other countries that have come on board since adhere to the same standards. If you're one of those nations and you use a non-SI unit such as horsepower in commerce, you must define it in terms of the corresponding SI unit. For horsepower, that unit is the watt.

For engines, kilowatts are used instead.

Cue another thread about why the US uses Degrees Farenheit. They will use it forever until the majority of temperature information is only available in Celsius - at which point, the experience of 0°C will actually mean something to them. Likewise for 10, 21, 60 and 100.

No votes for Plank units? ##c=\hbar=G=1## (Just kidding ;-)
sophiecentaur said:
Cue another thread about why the US uses Degrees Farenheit. They will use it forever until the majority of temperature information is only available in Celsius - at which point, the experience of 0°C will actually mean something to them. Likewise for 10, 21, 60 and 100.
That's very true. Personal calibration for "how hot is hot" is based on personal memories of what it felt like at the temperature in question. I grew up with F, then I spent 10 years in Sweden and built up memories in C, then moved back to the US and had to re-learn F.

It's going to be worse in the future because national TV almost never gives temperature any more. They show "wind chill temperature" when its cold and "heat index" when its hot. They argue that's more real than the thermometer because it relates to what it feels like and thus memory-based-calibration. Only old-fashioned people like me want to hear what the thermometer says F or C.

It is interesting how threatened species such as the English mile, the inch, the British thermal unit, and Fahrenheit live on in the protective custody of the USA. There must be several reasons why they handicap themselves with such artefacts.

Baluncore said:
It is interesting how threatened species such as the English mile, the inch, the British thermal unit, and Fahrenheit live on in the protective custody of the USA. There must be several reasons why they handicap themselves with such artefacts.
Half of us are embarrassed by the insular arrogance of the other half.

vanhees71 and DaveC426913
jbriggs444 said:
Half of us are embarrassed by the insular arrogance of the other half.

On that we can all agree!

Bystander, vanhees71 and jbriggs444
Thread has run its course. Thanks everyone.

anorlunda

1. Why is horsepower not considered an SI unit?

Horsepower is not considered an SI unit because it is not a fundamental unit of measurement. The International System of Units (SI) is based on seven base units, including the meter, kilogram, and second. Horsepower, on the other hand, is a derived unit that was originally used to measure the power of steam engines in the 18th and 19th centuries.

2. What is the SI unit for power?

The SI unit for power is the watt (W), which is equivalent to one joule per second. This unit was established in the 19th century and is used to measure the rate at which work is done or energy is transferred.

3. Why is horsepower still commonly used in the automotive industry?

Horsepower is still commonly used in the automotive industry because it is a familiar unit of measurement for many people and has been used for over a century to describe the power of engines. It is also still used in some countries as a legal unit of measurement for vehicle engine power.

4. Can horsepower be converted to watts?

Yes, horsepower can be converted to watts. One horsepower is equivalent to 745.7 watts. This conversion factor was established in the 19th century when the watt was first defined.

5. Are there any efforts to make horsepower an SI unit?

There have been some efforts to make horsepower an SI unit, but they have not been successful. In 1937, the International Electrotechnical Commission (IEC) proposed a new unit of power called the "metric horsepower" (hpM), which was equivalent to 75 kgf-m/s. However, this unit was not widely adopted and is no longer used today.

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