This past Christmas, I asked Santa for a Kill-A-Watt. The Kill-A-Watt is a simple device that tells you how much electricty any device in your home is using. Based on that information, you can then figure out how much that item is costing you to use. The Kill-A-Watt is very simple to use and requires only a tiny bit of math to use (which of course is what computers are for).

To use the Kill-A-Watt, you simply plug the device into a wall socket and then plug the device you wish to monitor into the Kill-A-Watt. After that, all you have to do is wait and let the device collect the numbers for you.

The Kill-A-Watt displays a lot of detailed information, like how many watts and amps the plugged-in device is using, but if you are simply interested in figuring out how much the device is costing you in electricity, you need three numbers: the kWh (killowatts per hour usage), how long the device has been running, and how much you pay per killowatt hour. The first two are provided by the Kill-A-Watt; the third is available on your monthly electric bill. According to last month’s OG&E bill, I pay 0.06732 cents per killowatt hour.

The first thing I wanted to test was my furnace blower. Out in the garage, I plugged the Kill-A-Watt into the wall and plugged my HVAC blower into that. I then left it alone for almost 24 hours, to get a good “average” reading. When I came back the following evening, the device had been on for 22:20, and the kWh reading was 18.50. The first thing I had to do was convert the time into minutes.

Time_Minutes = 1340
kWh_Total = 18.50

Then …

kWh_Total / (Time_Minutes / 60) = kWh_per_hour_average.

… or …

18.50 / (1340 / 60) = 0.83 kWh average.

Using my rate of 0.06732 cents per kWh, I did the following calculations:

0.83 * 0.06732 = $0.06. This is the hourly cost.
0.83 * 0.06732 * 24 = $1.34. This is the daily rate.
0.83 * 0.06732 * 24 * 30 = $40.15. This is the monthly (30 day) rate.
0.83 * 0.06732 * 24 * 365 = $488.50. This is the yearly rate.

Of course I didn’t do all of this by hand. I set up a simple Excel spreadsheet to all do the math for me. In fact, all I need to do now is punch in the time and total kWh of each device and Excel does the rest for me. I assigned a cell to hold my own kWh rate, so that it could be easily changed without having to update all of the formulas. I also set aside an area to convert hours and minutes into minutes because yes, I really am that lazy.

Download: Kill-A-Watt.xls (Right-Click/Save As)

The Excel spreadsheet I created is available here. Feel free to modify it as needed. You might want to track the amps or watts each device uses, for example. You might also want to track the dates and times you checked each device. One experiemnt I plan on conducting this weekend is how much electricity my game consoles use when plugged in but turned off. I have been told the numbers may surprise me. We’ll see.

Two downsides to the Kill-A-Watt, or at least the model I own. One is, the device has no internal power of its own, which means the minute you unplug it, the numbers are lost. If (like me) you have a bunch of crap plugged in behind shelves or underneath your computer desk, you’re going to need a flashlight and a piece of paper to write down your findings before unplugging the device. The other downside is that the device only handles normal 120 AC devices, meaning you won’t be able to test things like your washer and dryer.

The Kill-A-Watt is available lots of places. I was told Santa found mine here at ThinkGeek for $25, but feel free to search around.