EGEE 401
Energy in a Changing World

Energy and Power


The terms "energy" and "power" are easily (and often) confused in casual conversation. They are related concepts, but distinctly different. We'll use them carefully throughout this course. We know energy is defined as "the ability to do work." Power is how fast (or slow) the work is done. It may take the same amount of energy to walk or run up a steep hill, but it takes more power to run than to walk.

Power is the time rate at which work is done or energy is transformed. (In calculus terms, power is the derivative of work with respect to time.) From the reading, you learned about some common units of measure for energy: kilowatt-hours, BTUs, Joules (megajoules), and calories.

Power is measured in units of “energy per time.” For example, BTU/min and Joules/sec. A common unit of measure of power is “horsepower.” This measure can be converted to other power units of measure expressed as “energy/time”. For example, 1 horsepower = 2,545 BTU/hour.

A Watt is another measure of power defined as 1 Joule/sec. A Watt-hour or kilowatt-hour is the measure of energy. 1 horsepower = 756 Watts. When we're talking about electricity, we use Watts to describe power and Watt-hrs (or kilowatt hours, kWh) to describe energy.

Maybe this will help...think of a light bulb rated at 100 Watts. This is the power rating for the bulb. It tells us how much electricity the bulb can transform into light and heat over a given amount of time. If left on for one hour, a 25-W bulb will use 25 Watt-hours (Wh) of energy.

one 25 W light bulb and one 100W light bulb as described in text.
Figure 1.1: Power and Energy.
Credit: V. Cole

In the same amount of time, a 100 W bulb will use four times as much energy as a 25 W bulb. If you leave them both on for one hour, the 25-W bulb uses 25 Watt-hours (Wh) of energy and the 100-W uses 100 Watt-hours of energy. After 3 hours, it’s 75 Wh and 300 Wh respectively.

The power company charges you based not on how many light bulbs you have, but for how much energy they use while you have them turned on. The same is true for all electrical loads. You are charged for the energy they use (power x time), usually measured in kilowatt-hours (kWh).


Here's a good site for a wide range of international unit conversions, Unit Converter. Try it, especially for energy and power unit conversions. You may find it helpful for some of the assignments in the course.