Horsepower and Energy
who originated the idea of a horespower?
What is the relation of the horsepower to a watt?
How is a BTU related to the joule?
Horsepower goes back to the days when horses ruled the world, literally!
The Scottish Engineer and inventor, James Watt (1736-1819), introduced the
term "horsepower". The term was and is used to indicate the rate at which
an engine can deliver work. As such, it is a measure of power, that is,
energy produced or worked done by an engine per unit time.
Mr. Watt made considerable improvements to steam engines (which were
invented about 70 years earlier, in 1698, by Thomas Savery). His
improvements led to engines that were four times more efficient (used less
coal) than others, much smaller than before, and much more powerful (from
earlier 6 horsepower ones to about 200 horsepower). Oh, you could NOT fit
one under the hood, any hood).
Naturally, Mr. Watt wanted to tell how powerful his engines were. So,
after some tests (not with engines but with horses) he established that on
the average, a horse could haul coal at the rate of 22,000 lb-ft per
min. For some reason, unknown to me, he decided the raise this number by
50% to arrive at 33,000 lb-ft per minutes (No, horses those days were not
on drugs; steroids were not known at that time, but I am sure Mr. Watt had
his own reasons for this increase).
So, if an engine can push 33,000 Lb of something one foot in one minute, we
say that is a one-horsepower engine. By the way, I believe that deliverable
power, also known as brake or shaft power, is the one used in automobile
industry in the US, and this indicates the practical ability of the engine,
i.e., engine power minus losses due to friction, compression, heat, etc.
Currently, there are two systems of units used: one is the metric system
used internationally, and the other one is the English systems used mostly
in the US. (That's right; we like the idea of being Royal -and loyal-
subjects!) Horsepower, as a unit of power, belongs to the English system
but I believe it is formally used in the metric system as well, and its
value is 32,549 lb-ft per min. The unit for power in the metric system is
Watt (W). Named after whom?
So, horsepower (abbreviated as hp) is a measure of power, as is Watt (We
come full circle, from Mr. Watt and horses to Horsepower and back to Watt!).
Now, let's for a moment talk about ENERGY. Work and heat are two forms of
energy. If you rub two pieces of metal together and work at it, they heat
up! Or if you take a metal bar and put it end-to end between two heavy
objects and heat up the rod by a torch, the rod will expand and push the
object apart. These two examples show that work and heat are forms of
energy and are convertible to one another. Electricity delivered to our
homes is also a form of energy.
Energy per unit time, as mentioned, is power. Units of power in common use
are horsepower (hp) for work, Btu/hr for heat, and Watt (W) for
electricity. As you expect these units are related. It is due to
historical precedent that terms other than W are used. In the metric
system, we can express all the above in Watts.
In any case, by referring to table of conversions, you will see that one
horsepower is 746 W and 2545 Btu/hr.
Power can be expressed in terms of horsepower, Btu/hr, or Watts. Energy,
which is power multiplied by time, is expressed, correspondingly, in lb-ft,
BTU, and Joules.
In particular, 1 W = 1 Joule per second. I BTU=1055 Joules.
Example: You turn on a 1000 W (=1 kilowatt) electric heater for one hour.
The power of the heater is, of course, 1000 W =1.34 hp = 3412 Btu/hr.
The energy used or the heat produced is commonly referred to as one
kilowatt hour = 1000 W x 3600 seconds = 3600 kilo Joules = 3412 Btu.
I hope the issue is clear now.
Dr. Ali Khounsary
James Watt is said to have developed the definition of a horsepower. The
story goes that in the early days of the industrial revolution in England,
horses were used to help raise coal up from the coal mines. Usually a team
of horses were used. James Watt developed his steam engine and tried to
sell it to the coal companies to replace their horses. So, to compare
apples to apples, he measured how fast it would take a horse to lift a
weight a certain distance, and then he compared it to how fast his steam
engine could lift the same weight the same distance. That way, he could say
that his steam engine has the equivalent power to so many horses, or
Your questions about horsepower and watt and the BTU and the joule have a
common answer. One unit is a measure in the English system of units and the
other is in the metric system (or SI) of units. A horsepower and a watt
both are a measure of power, and the BTU and the joule are a measure of
energy (usually thermal energy). The only difference is the base units.
The English system uses ft, lb, and sec for its base units, whereas the
metric system uses m, kg, and sec for its base. I suggest you look in an
elementary physics book to get a better idea about units in the English and
) James Watt
2) 745.56 Watt = 1 hp (sometimes written as 135.36 Watts = 1 hp, but the
other is more accepted!!)
3) 1 BTU = 1.06 kJ
Watt made the horsepower increase to make his engines look more powerful
than a horse. He made engines to sell, like any business person.
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Update: June 2012