Common Units of Power

SI:

Watts (W)

   1000 W = 1 kW

Kilowatt (kW)

   1 kW = 1.341 hp

English

Horsepower (hp)

   1 hp = 0.746 kW

What is Power?

Power is a measure of how quickly work can be done. Using a lever, you may be able to generate 200 ft-lb of torque. But could you spin that lever 3,000 times per minute? That is exactly what your car engine does.

The SI unit for power is the watt. A watt breaks down into other units that we have already talked about. One watt is equal to 1 Newton-meter per second (Nm/s). You can multiply the amount of torque in Newton-meters by the rotational speed in order to find the power in watts. Another way to look at power is as a unit of speed (m/s) combined with a unit of force (N). If you were pushing on something with a force of 1 N, and it moved at a speed of 1 m/s, your power output would be 1 watt.

An interesting way to figure out how much power you can output is to see how quickly you can run up a flight of stairs.

  1. Measure the height of a set of stairs that takes you up about three stories.
  2. Time yourself while you run up the stairs as quickly as possible.
  3. Divide the height of the stairs by the time it took you to ascend them. This will give you your speed.

For instance, if it took you 15 seconds to run up 10 meters, then your speed was 0.66 m/s (only your speed in the vertical direction is important). Now you need to figure out how much force you exerted over those 10 meters, and since the only thing you hauled up the stairs was yourself, this force is equal to your weight. To get the amount of power you output, multiply your weight by your speed.

Power (W) = (height of stairs (m) / Time to climb (s) ) * weight (N)

Power (hp) = [(height of stairs (ft) / Time to climb (s) ) * weight (lb)] / 550