Energy and Power

As an engineer, I have acquired a pet peeve about people and websites that confuse energy and power. For example, today I read the following on a company website (http://www.microplanet.com/section.asp?catid=142&pageid=33):

What is energy? How does it differ from power and electricity?

The American Heritage Dictionary defines "energy" as (1) usable heat or power, (2) a source of usable power, such as petroleum or coal, (3) the capacity for a physical system to do work. [The website continues] Electricity is a form of energy. "Energy" is often used interchangeably with "power," but they are not the same thing. While energy is the capacity to do work, power is the conversion of this energy into work over time. Electric utilities sell electrical energy, which is converted into power when you use it. Although energy, like power, can be measured in watts, power is measured by the number of watts converted into work over time, or watt/hours. While this may be confusing, the important point is that the rate at which energy is converted into power (and you pay for it) in most devices is dependent on the voltage at which it is delivered.

It is no wonder that most people are confused about energy and power, when not even a company devoted to saving energy can use the terms correctly. Wikipedia gets it right:

On their Electric Energy page: “The term Electric Power is frequently misused as an alternate name for electrical energy. Yet energy and power are two different things.”
On their Electric Power page: “When paired with a unit of time, the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour.

The first definition by American Heritage is incorrect, since “energy” is not “usable heat or power”. Heat is the rate of thermal energy transfer, and power is the rate of mechanical energy transfer. It is true that electric utilities sell electrical energy, but it is converted into electric power at the power plant, not when and where you use it. Energy is not measured in Watts; power is measured in Watts. Energy is measured in Joules, Watt-hours, British thermal units (Btu), ft-lb, hp-hr, erg, etc. Power is expressed as Watts, kW, horsepower (hp), ft-lb/second, etc. Heat is expressed as Watts, kW, Btu per hour, etc. Correctly restating their statement: “power is measured by the number of watt-hours converted into work over time, or watts.”

Okay, I'll step off my podium now. As they say in Congress, I yield back the balance of my time.