How do you measure power?

One definition of the word power is the "amount of energy required or expended for a given unit of time". As far as the utility company is concerned, they bill you for kilowatts-hours (abbreviated KwH).

What is a kilowatt-hour? 1.0 KwH is using 1000 watts of power for a period of 1 hour.

Because it involves both power (kilowatts) and time (hours) you really have to pay attention to both items. Example: you could use 500 watts (a half-kilowatt) for 2 hours, and the utility will bill you for 1.0 KwH. Likewise, you could use 6000 watts for 10 minutes and you would also be billed for 1.0 KwH.

The utility's power meter keeps a running tally of the total amount of power you consume. The utility charges you for the power you used over the billing period, which is approximately one month. Changes in the climate (how hot/cold, light/dark) and the variability of the billing period (27 to 34 days) can make it frustratingly difficult to compare one month's bill to another.

While the total number is useful, especially in spotting trends, it is too coarse to be of value when trying to do something about the power hogs in your house. To do that you need something called a plug-load power meter. A popular one, just right for the Greener Geek, is the P3 Kill A Watt EZ 4460 meter, which allows you to enter the cost per KwH (18.3 cents here in good-old Rhode Island). A less expensive model (P3 4400) is also available which doesn't tally up the running cost.

The P3 EZ 4460 costs about $38 at Amazon.com, while the less expensive one is about $20 at Amazon.com.

How accurate is the P3 EZ? Well I have some pretty fancy equipment available because of my line of work, traceable to the National Bureau of Standards. I can tell you that the P3 EZ that I own is better than 2% on both current and voltage measurements (mine was worse on current) for loads in the 20 to 400 watt range.


What does the P3 meter do for you? You plug whatever appliance (or Greener Geek accessory) into the front of the meter, and it will measure the power consumed for what ever period of time you have it plugged in (it also will record the total time it has been measuring the power).

I recommend measuring whatever device you have for about a week to get a good representation of cost to operate. The reason for this is that the power consumed may vary wildly by use. For example, a laptop with a fully charged battery might only pull 20 or so watts, but that same laptop with a discharged battery might pull 65 watts while the battery is charging.