Energy: The Master Resource by Robert L. Bradley Jr.: What is Energy?

Energy is the capacity to do work. Power is the rate at which work is done, and is calculated by dividing work by the time taken to do the work. The faster the work is done, therefore, the more power is expended.

It is useful to think of energy in terms of work because the whole reason we want to control energy is for the work it can do for us. In fact, the word energy comes from the Greek words en meaning in or at, and ergon meaning work. In the United States, the most common unit of measure is the British Thermal Unit, or BTU. A BTU is defined as the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.

 Another common energy unit is the kilowatt-hour used to measure electricity. One-kilowatt-hour is equivalent to 3,413 BTUs. Note that kilowatts measure capacity or flow, while kilowatt-hours measure quantity; a one kilowatt electrical generator running for one hour produces one kilowatt hour of electricity. It takes about one kilowatt of generating capacity to provide electricity to an average American home.

Energy exists in two basic forms:

1. Potential—Energy at rest, waiting to be used
2. Kinetic—Energy in motion

A boulder sitting on the edge of a cliff is said to have potential energy by virtue of its position in the Earth’s gravitational field. The boulder’s potential energy is converted into kinetic energy when it is pushed over the edge, and gravity causes it to fall.