How Kill A Watt Works

If you're really looking to monitor your energy use, it helps to think about conservation more than once a month. See more green science pictures.
If you're really looking to monitor your energy use, it helps to think about conservation more than once a month. See more green science pictures.

Each month, a bill arrives to remind us just how far we've come from the days of cooking with fire and cleaning in a stream. Some of us put a couple hundred dollars a month into powering ovens, dishwashers, air conditioning, heat, lighting and all the other wonderful, electron-driven trappings of modern life. And sometimes when we look at that bill, it seems crucial to start line drying instead of using the dryer.

But to actually reduce our electricity costs -- and do some environmental good -- it's necessary to think more than once a month about conserving energy. Energy monitors are one of the easiest, most effective ways to inspire real change in consumption habits. They bring energy costs to your attention on a regular basis, making it a little harder to forget to line dry your laundry.


An energy monitor is simply a device that counts watts. You plug an electrical appliance (or a whole house full of appliances) into an energy monitor, and plug the monitor into a wall outlet. It sits between the appliance plug and the energy supply. As electricity moves through the monitor and into the appliance, the device tracks, displays and records how much electricity the appliance is drawing to operate.

There are a couple of different approaches to monitoring energy in this way. You can monitor your entire home's energy use using a comprehensive system, a single appliance at a time or a particular collection of energy suckers, like the components of a home-theater setup. It all depends on how you want to go about it and how much you want to spend.

In this article, we'll look at several different energy monitors out there and see what they can do. We'll begin by focusing specifically on one of the least expensive devices you can choose: the Kill A Watt.

Kill A Watt is a simple, straightforward way to monitor energy consumption, and it allows for targeted monitoring, which can be great for certain applications.


The Kill A Watt Device

The Kill A Watt EZ will run you about $50.
The Kill A Watt EZ will run you about $50.
Photo courtesy of

In its original form, the Kill A Watt device pretty much does one thing: count watts in kilowatt/hours (kW/h), which is how the electric company does it. It's made up of a three-prong electrical plug-in, a few buttons and a display. You plug the monitor into the wall, and then a 120-volt appliance, like a television, into a monitor to find out exactly how much electricity it uses, including when it's just plugged in but not turned on.

You can make Kill A Watt display the TV's energy use by the day, week, month or year. It's accurate to within 0.2 percent [source: P3]. So you can find out how much energy you'd save by skipping TV for a day. You could also check to see exactly how much energy the TV (or the DVR, stereo or computer) drains when it's plugged in overnight.


This type of "phantom energy" use can add up. Monitoring standby electricity is one of the most common reasons people start monitoring their energy. You might be able to save yourself a few hundred dollars a year just by unplugging the biggest energy suckers when you're not using them.

It has other applications, too, besides giving you the in-your-face information you need to start using less energy. Kill A Watt displays additional measurements like volts and hertz (line frequency), so you can use it to test line quality, determine whether an outlet is working and detect power spikes and disruptions.

That's the original Kill A Watt, which runs about $25. Later versions add some additional functionality. Kill A Watt EZ (about $50) will calculate how much money an appliance is costing you if you just input how much your energy company charges by the kW/h. It will forecast your weekly, monthly or yearly energy costs, too, so you don't have to pull out a calculator at all. And the Kill A Watt PS (about $85), or power strip, has a couple of added bonuses: It protects your components from power surges, and it lets you plug in a whole set of devices so you can see, for instance, how much your entire computer setup is costing you in electricity.

Of course, you could also just plug your current power strip into a Kill A Watt or Kill A Watt EZ to achieve the same feature. Next, we'll check out some other energy monitors and the feature variations they offer.

Other Energy Monitors

Pricier products will help you track your electric usage with constant feedback.
Pricier products will help you track your electric usage with constant feedback.
Photo courtesy of

If you're really just looking to find out how much electricity you're wasting by leaving your computer on all day instead of shutting down when you're not using it, a $25 Kill A Watt is probably all you need. You may be looking for some added frills, though.

Here, a sampling of other energy-monitoring devices and what they have to offer:


  • Watts Up? -- Watts Up ($110) is a single-appliance monitor like Kill A Watt EZ, but it's especially fast. It responds to changes in voltage so quickly that you can see the electrical spike when you first turn an appliance on. It records the highs and lows so you can see power surges and poor line quality later on when you check the device. Watts Up? Pro ($210) will also do some additional calculations for you, including your monthly energy savings and how long it would take you to make up the cost of a more energy-efficient appliance.
  • Doc Wattson -- Wattson ($280) is a whole-home system. There's a monitor that you hook into your home's power supply, and a display that you put wherever you'll notice it most. The monitor wirelessly sends your house's electrical data to the display, where you can see all the ups and downs as you turn appliances on and off. In addition to displaying kW/h and total energy cost, the display glows red when you're using a lot of power and blue when you're using a little, making it even more difficult to ignore your energy consumption.
  • Energy Joule -- This is one of the coolest energy monitors out there, but you can only get it if your power comes from Consumer Powerline energy company. It does what any other energy monitor does but with one very special addition: It knows when the cost of electricity goes up and down, because it gets constant feedback from the power company. So you can see when the best time to do your laundry might be. And you don't even have to guess at it, because Energy Joule counts down to lower-cost power. If you need to do a load, but you check the display and find you've only got nine minutes until reduced kW/h rates, you may decide it's worth it to wait.

Even though it's not widely available, Energy Joule is worth knowing about -- if you and a few thousand of your friends call your own power company and request it (or something like it), maybe you'll be able to start counting down to savings, too.

For more information on energy monitors, Kill A Watt and related topics, look over the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Cut Your Electricity Costs With An Energy Monitor. The Greenest Dollar. March 11, 2009.
  • Energy Monitors. Consumer Reports. March 2009.
  • Kill A Watt. P3 International.
  • Kill A Watt PS. P3 International.
  • Lower Your Monthly Energy Bill and Save Money: Kill A Watt. Smart Home.
  • Monitor and Save Money and Energy: Kill A Watt EZ. SmartHome.