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

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The Kill A Watt Device

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.