Introduction and Overview
Power is the rate of change of energy within a system per unit time. The formula for power is energy/time. Thus, power can be represented by any energy unit divided by any time unit. Ergs per year may seem a bit ridiculous for most uses, but is practical for measurements of radioactive decay in a sample of uranium. Since energy is actually composed of units of mass, length, and time, an arbitrary unit of power could be formed by an arbitrary collection of the proper units, but it would have little practical application.
Power is typically used in reference to systems that cause motion or do work. Thus, the horsepower was the rate at which a draft horse could do work, such as pulling a cart or driving a grindstone. It came into particular usage when sources such as flowing water started to replace animals as power sources and there was need to represent how useful the alternate source was. In todays world, power usually refers to internal combustion engines or to electric power. But it is quite able to represent the rate of heat transfer from a hot object to a cold object. The rate at which an ice cube melts can be thought to be proportional to the power input to the ice cube. A fire puts out energy at a certain power. A weightlifter who puts 300 lbs. overhead in less than a second is using nearly 4 horsepower or 3 kilowatts. Actually, since the weightlifter does not lift in one continuous motion, his or her peak power may greatly exceed those levels.
Power was not a quantity people dealt with in earlier times, and so the British and metric units are the only ones in common use. This megaConverter converts between the most commonly used power units of today. For explanations of the various units see megaConverters Mass #1, Length #2, Time #4, Energy #7 and Force #8.
For a more comprehensive treatment of measurements, find "NTCs Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library. For a better discussion of power, see any college physics textbook.
There are slight variations in certain units depending on who is using them. For instance,
* Much of our written history still refers to things in common units. The Bible does not refer to meters or kilograms, but to cubits and stadia, or shekels and drachma. Wouldn't it be nice to know what they were talking about way back then? Now you can use megaConverter! For a more complete listing of ancient, foreign, and obsolete measures, download our 'megaSpreadsheet' of conversions in MS Excel format.
Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.