Re: Nice Jag .
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. To summarize: 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