Confused about Energy and Power?

When I recently solicited feedback for topics to cover for my upcoming book, several people requested that I discuss the difference between energy and power. Just two weeks ago Congressman Dana Rohrabacher, R-Calif., who is on the House Committee on Science, Space, and Technology, admitted that he’s “not educated enough to know the difference between the terms … energy and power.” You would certainly expect that someone who influences legislation over science and technology would know the difference, but it is true that people commonly get confused between the two.

So this weekend I wrote up a sidebar for Power Plays discussing the differences, which I share below. If you believe that a point could be clearer, or if anything about my explanation is confusing, I would be happy to hear reader feedback.

Energy, Power, and Units of Measurement

There are a number of potentially confusing units of measurement for energy and power. The first thing to understand, however, is the difference between energy and power. Technically speaking, energy refers to the capacity of a system to do work. In this definition, “a system” could be a gallon of gasoline that contains 115,000 British Thermal Units (BTUs) – a unit of energy. In addition to the BTU, some other units of energy are the joule (J), the calorie (cal), and the watt hour (Wh). Multiples of these units have abbreviations like kilo (one thousand) or mega (one million), so one kilowatt hour (kWh) is one thousand watt hours. Each of these units can be converted into the other. One BTU is equal to 1,055 joules, 252 calories, or 0.29 watt hours.

Power is the rate at which energy is consumed or generated. One BTU of gasoline refers to the energy content, but gasoline consumption over time could be measured in BTUs/hour, a unit of power. Other units of power are joules/second, calories/day, or watts.

A 100 megawatt (MW) power plant is capable of generating energy at the rate of 100 megawatt hours (MWh) per hour. People frequently confuse the energy unit of watt hours with the power unit of watts. This is probably because other measures of power are defined per unit of time (e.g., calories/hr), and therefore it would seem logical that a kilowatt hour would be a unit of power. Alas, it is not. However, it should be clear that a joule/second is a unit of power (energy consumed over time), and that’s what a watt actually is: 1 watt = 1 joule/s. Someone thought it would be a good idea to call it a watt instead of a joule/s, and many people have been confused ever since.