We are confronted every day by the notions of Energy and Power: cars and motors are sold by horsepower, light bulbs by watts, natural gas by therms, electricity by kilowatt-hours, and air conditioners by tons or BTUs per hour. But, do we really know what energy is? Is energy the same as power?

        Of all the scientific terms that I have encountered, the word "energy" would have to be the most abused. This word has a very specific meaning to physicists, but the lay press, and many people who are not familiar with its proper usage, distort its meaning and use it in misleading ways. This is especially true for electric products’ advertisers.

        To answer the question above: No, energy and power aren’t the same concepts but they are two very much related terms. Mixing up the two terms is a very common mistake. For now, let’s think of energy as the capacity to do work or simply a "stuff" we can use to run our appliances. Power is just how fast we are using energy. A 100-watt light bulb uses energy twice as fast as a 50-watt light bulb. This is why it gives more light. So, power always includes time in some way. Thus, the key word to remember is time, because the time interval during which energy is consumed (or produced) is of great importance. Remember that energy is what we pay for. Even though all this might sound a scientific issue, the previous statements also show how the distinction between power and energy has practical significances, if we think in terms of energy economics.

        Electrical power is usually measured in watt (W), kilowatt (kW), megawatt (MW), etc. Electrical energy, however, is not measured in kilowatts, but in kilowatt-hours (kWh). Here lies another problem: we don’t always use the same energy unit. One of the difficulties of reading articles or understanding issues about energy resources is the plethora of units used. In fact, depending on the context, we need to use different units in energy discussions. But, we rarely stop to think about energy equivalency or how the energy numbers we usually hear fit into most common denominators. And I am not talking here about the standard rules of unit conversions.

        For example, for most of us if we hear that 1-calorie (cal) is the energy needed to raise the temperature of one gram of water by one degree Celsius, it probably doesn’t say much. The same is applicable if we hear that the energy content of 1 gallon of gasoline is about 1.2´ 10⁵ BTU (British Thermal Unit). But, what if we say that U.S. annual energy consumption (~ 10²⁰ Joules) is about a million (1,000,000) times more than the energy released in the explosion of the atomic bomb in Hiroshima? Now, the number sounds alarming!

        Something to think about: Every kilowatt-hour (kWh) of electricity we avoid using (by turning off the air conditioner for about 1-1/2 hour) saves more than 1-1/2 pounds of CO₂ from being pumped into the atmosphere.

Source: Rough Values of Power of Various Processes