Power is the measurement of how much work can be done per unit of time. It can be expressed in either watts or horsepower, depending on whether metric (International System) or English (Imperial) units of measure are used to calculate it. Horsepower is a term first coined by James Watt, who calculated that a single horse can do 33,000 foot-pounds of work in 1 minute.

Step 1

Calculate how much work is done by the device. Work is the measure of force times distance. For instance, raising an object weighing 1 Newton 1 meter results in 1 Newton-meter (or 1 Joule) of work. In English units, raising an object weighing 1 pound 1 foot results in 1 foot-pound of work.

Step 2

Understand the definition of "weight." Weight is a measurement of mass multiplied the acceleration of gravity. In SI units, kilogram is a measurement of mass, although it is commonly used to refer to weight. 1 kilogram of mass multiplied by the acceleration of gravity is 1 Newton of weight. Similarly, in English units, a pound is a measurement of weight, and slugs is a measurement of mass, making a pound equal to 1 slug multiplied by the acceleration of gravity.

Step 3

Learn how to define "work." To do work, you must move an object in the same direction as the force that was applied to it. For instance, if you try to lift a box, but can't, you haven't done any work even though the attempt my tire you. If you lift the box and carry it across the room, only the distance that you lifted the box counts as work.

Step 4

Determine how much time was required to do the work. If it took 1 second to raise the 1-Newton weight 1 meter, the power of the device that raised the weight is 1 Joule per second or 1 watt. In English units, the device that raised a 1-pound weight 1 foot in 1 second is 1 foot-pound per second.

Step 5

Convert foot-pounds per second into horsepower by dividing by 550. You can also convert watts to horsepower if you know that 746 watts equals 1 horsepower.