With energy costs increasing faster than inflation and much faster than income in the United States, many have a desire to conserve. But, without information, our conservation may be more expensive than it is worth and hardly as effective as it could have been. The kill-a-watt electricity usage monitor provides a useful way to be smart about conservation. Increase the efficiency of your home.
I have owned one of these for the past 18 months or so, and have found it to be very enlightening when comparing the power consumption of various devices and appliances. There is no sense in paying for power when we gain nothing from it (standby power), and there is no sense in paying for a power premium on a device that uses much more than other similar systems use. 
The Kill-A-Watt (I have model number P4400) by P3 International is easy to use, although the size and shape of the device leave something to be desired. It is difficult or impossible to utilize the second plug on an outlet when the kill-a-watt is installed. However, what you gain from this monitor far outweighs this inconvenience. And, using it in conjunction with a short extension cord can solve the form factor issue. Be warned that this is not a scienfitic instrument in its accuracy, but it is sufficient for the intended purpose.
So, what have I learned? First, my desktop computer at home typically uses between 200W and 95W depending on what I am doing with it, whether surfing the web (low consumption) or playing a game (high consumption). A new video card added almost another 20W of power consumption. Even when turned off, the computer uses about 12W. My laptop computer, on the other hand draws approximately 22W when operating off of the wall outlet in normal operations, and up to 31W while charging the battery. I imagine I could increase the power consumption by doing some intensive calculations or playing video, but those are not the average situation. Still, that was enough for me to ask why I was leaving them powered on while away from home. [ad name="Adsense Small Horz Banner"] Our very old refrigerator is the largest power consumer outside of the major appliances (air conditioner, electric dryer, dishwasher, etc.) consuming over 1000W while running. The total power consumption of the refrigerator alone is more than 30% of our total electricity usage in Spring and Fall months where the air conditioner or heater is hardly used. For really big numbers watch your toaster or microwave while operating. Of course, the short cycle time of these devices makes their total consumption rather low over the course of a day. The most efficient cooking device in my kitchen is the slow cooker, making a whole meal for the entire family and then some on much less than $1 of energy, while most other methods required significantly more.
The kill-a-watt can provide you with output in watts (useful for instantaneous verification of power consumption), kilowatt-hours (an average over time and the measure by which your electricity bill is calculated), as well as volts and amps (useful if you are troubleshooting an appliance, but otherwise not necessary). If you want to find out how much a certain device is costing you, have the kill-a-watt record the amount of kilowatt-hours over the course of a day or week, then multiply that value times the average cost shown on your energy bill.
The best use of the kill-a-watt however, is finding hidden energy drains. Many items including battery chargers, various transformers for electronic devices, televisions, computer monitors, computers, radios, DVD players, stereos, and other things still consume electricity when turned off. While many of these are token values of 10 Watts or less, some are more, especially on large televisions and other electronics. Adding up all of these values, you may find that you are consuming a few hundred Watts of power whether you are using the devices or not. What a waste!
Make a list and keep track of your own personal energy audit. Empowered with this information, you have make good choices. Adding true electrical switches to outlets will allow you to absolutely cut off power to hungry electronics. Monitoring the energy expended by older appliances will help you decide whether or not to invest in new ones. With follow-through, the cost of this monitor can easily be made up within months.