Kill A Watt Meter

A great way to save electricity

The Kill A Watt sits between the device you want to measure and plugs into the wall. This is a useful way to hunt down mysterious energy consumers – for instance, I found that my Keurig B70 coffee maker draws 5 watts when it’s turned OFF! Another surprise was the power supply for HexHound’s new Dell Poweredge 1950-III server, each which draw a hefty 15 watts with the server off but plugged in. On the bright side, I found that many of my “wall warts” draw almost nothing when not connected to a device. My cell phone charger registered 0.0 watts without the phone connected and a Dell laptop power supply registered 1.0 watts with the power supply plugged into the wall, but not the laptop. Clearly the Kill A Watt is a great way to trim that electric bill!

Eight different measurements

The Kill A Watt advertises that it “displays eight critical units of measurement.” Below are the measurements available and a brief description of what they are.

• Volts
  • Self-explanatory, volts displays the current line voltage at the outlet the Kill A Watt is plugged into. This is useful in determining line quality and voltage sag if a large energy consumer is plugged in. The Kill A Watt is rated for maximum of 125 volts AC – I have already exceeded that at 126 volts and it was fine.
• Amps
  • Amp is short for ampere or amperage, a measure of current. The wikipedia definition is “a measure of the amount of electric charge passing a point in an electric circuit per unit time.” In real terms, amperage is measure of how heavily a circuit is being loaded. I specifically purchased the Kill A Watt to measure the current draw of the Poweredge server as the colocation center allocates a maximum of 2 amps draw at 110 volts. The Kill A Watt is rated for a maximum of 15 amps.
• Watts
  • Wattage is a measure of power, or work over time. Wattage is a measure of “real” or “active” power as opposed to “apparent” power which we will be getting to next. You are billed on watts – real power – consumed (killowatt-hours) but electrical wiring is spec’d based on apparent power.
• Volt-Amps
  • Volt-Amps, or VA, is a measure of apparent power – the load presented to the electrical system. While this can be the same as the real power (wattage), it is often different, particularly with electronics. When different, VA is always greater than wattage. The Kill A Watt is rated for a maximum of 1875 VA.
• Hertz
  • Hertz, abbreviated Hz, is a measure of how many times the current switches polarity in a second. In the United States, this should measure exactly 60 Hz. This is a measure of line quality and important to certain devices which regulate themselves based on the power frequency (some turntables, electric motors, older TVs, etc).
• Power Factor
  • Power Factor, or PF, is a ratio between real power and apparent power, calculated by dividing the wattage by the quantity of the root-mean-square of the the voltage times the root-mean-square of the amperage (PF = W/(RMS(Volts) * RMS(Amps))). A power factor of 1 means the apparent power and real power are the same. Many new computer power supplies and most servers use a technology called “Active Power Factor Compensation,” or Active PFC, to keep their power factor squarely at 1 and reduce their load on feed circuitry. The Poweredge above shows a PF of .99 to 1 indicating it uses Active PFC. As a side note,  power supplies equipped with Active PFC often don’t play well with older/cheaper uninterruptable power supplies (UPSs) that output “simulated sine wave” power. For this reason, it is important to choose a UPS that outputs “pure sinewave” power if your power supply uses Active PFC.
• Kilowatt-Hours
  • Abbreviated kWh, kilowatt-hours are a measure of power used over time and is what the power companies use to measure and bill electric consumers. The Kill A Watt will track total energy consumption for as long as it is plugged in and powered, up to 9999 kWhs at which time it will reset to 0. This is useful for measuring devices that turn on and off, or otherwise use power in a non-linear fashion. The Keurig is a good example, consuming approximately 7 watts when powered on and idle, and spiking to hundreds of watts when the heating element turns on to keep the water tank hot.
• Hours
  • Finally, the Kill A Watt will display the total time it has been powered on. Again, it will track a maximum of 9999 hours before resetting to 0.