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Power vs. Energy - Understanding the Difference

Some use the terms power and energy interchangeably...

However, these terms represent very different, but related concepts.

Power is the rate at which energy is consumed, expressed in watts or kilowatts.

Energy is the amount of power consumed, expressed in watt-hours or kilowatt-hours (kWh).

To understand energy use, and consequently our utility bills, we must factor in the amount of power devices and appliances use and how long we use them.

Let’s look at the example of a typical light fixture outside your front door with a 60 watt light bulb. Sixty watts is the amount of power the lamp consumes, or the rate at which the lamp uses energy. If you run a 60 watt light bulb from dusk to dawn for 12 hours, you will consume 720 watt-hours of energy (or 0.72 kilowatt-hours).

In Southwest Florida we currently pay around 10 cents per kilowatt-hour, so that light bulb would cost 7.2 cents per night, or $26.28 annually.

If you look at your utility bill, you will see that you are charged for the number of kilowatt-hours (KWH) that you consume.

This is the amount of energy consumed between meter readings. To reduce the energy you use, you must either reduce the amount of power you use, or the amount of time you use that power (or both!)

Energy  =  Power  x Time

Going back to our light bulb example, you could install a light bulb that uses less power, or you could reduce the number of hours it runs. Both courses of action would reduce the energy used and save you money. Here are three different ideas to save energy and money.

1) Replace the bulb with a 13W compact fluorescent with equivalent light output (reduce power).

Energy Saved:                   0.56 kilowatt hours daily, or $20.58 saved annually

2) Put the light on a timer and run for just 6 hours nightly (reduce time).

Energy saved:                  0.36 kilowatt hours daily, or $13.14 saved annuall

3) Do both of the above (reduce power and time).

Energy Saved:                   0.64 kilowatt hours daily, or $23.43 saved annually

While power and energy are intimately connected, they are not the same.

Understanding the difference can help you save money!

In my next article, I will take this concept one step further and explain why you should think twice when trying to be “energy efficient.”