Kill-A-Watt is a simple looking device that plugs into an outlet and offers an outlet for a device you wish to monitor. Physically the Kill-A-Watt is only about 1 1/2 inches thick and the size of a double wall outlet. Sadly the Kill-A-Watt is just large enough that it will block the unused outlet forcing you to use an extension cord if you need two items plugged in.
The Kill-A-Watt is mind-numbingly simple to use but offers invaluable data. After you have your device plugged in simply push the button corresponding to whatever measurement you wish to see; Volts, Amps, Watts, VA, Hertz, PF, KWH or Hour. The Volts and Hertz settings are more of a health check for your house as those should be fairly consistent and steady readings in every outlet, if they vary much you may have some serious wiring issues. The most beneficial readings for the novice user will be the Watt and KWH settings. With the Watt setting you can watch in real time as power usage for a device changes up and down, with KWH you can see total accumulated power used.
I find that the Kill-A-Watt is great for checking vampire loads, or power loads that exist when the device is in its "off" state. What? "Off" doesn't mean "off"? Nope. Many modern electronics are actually in a "ready" state when they are set to "off", this allows a remote control to turn them on or just allows them to start up faster. I know my blu-ray player and DVR take a considerable amount of time to start after a power failure, this means they have vampire loads.
Some power chargers for music players or phones use a small amount of electricity even when not in use. Simply plug the charger into the Kill-A-Watt without the phone or player plugged in and select the Watt setting, if it reads "0.0" then this is a safe device to leave plugged in all the time.
Some devices have varied amounts of electricity throughout the day based on usage, this is where the KWH setting comes in handy. The KWH setting on the Kill-A-Watt will give you an accumulated usage of electricity. Plug in a device and leave it plugged in for a day or two to see how much juice it really uses. Does your blu-ray player suck up electricity like a sponge when you watch Netflix? Find out how much electricity gets used on your kid's X-Box while you are at work. All of these things can add up much higher than you might imagine.
After assessing your devices you can make intelligent decisions on which ones should not be plugged in all the time based on usage and start-up times. For example I have a Wii setup with chargers for the remotes; since I don't have my nieces and nephews over to play the Wii very often, charging the controllers 24/7/365 would be a waste. I simply have a power strip in my entertainment center with all the Wii equipment plugged in that I switch off and it kills all of the vampire load, when they are coming over I switch it on to charge the controllers before they get here. In my back room I have a similar setup with a cable receiver and TV that see little use so a power strip is setup for them too. Most of my chargers have no electrical draw when not in use, I replaced the ones that do since they are so inexpensive.
Most energy providers will give you a cost per KWH, or kilowatt hour, that you can multiply your individual device usage by. Is it actually worth $3.72 in electricity to leave that device I use once a week on all the time? By making yourself an informed electrical consumer you can make informed decisions to reduce your usage and reduce your bills.