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Plug Pass-through Watt-Hour Meters

Some major appliances and heating, ventilation, and air conditioning (HVAC) components are powered on dedicated circuits so that their energy use may be monitored directly at the circuit breaker panel. Often, however, we are interested in monitoring individual appliances and electronics that are plugged into receptacles on shared circuits. One way to make these measurements is to create a stubby extension cord that separates the hot, neutral, and ground wires and has two additional wires to tap the voltage, so that you can easily use a current transformer (CT) to measure the supplied current and a watt transducer to calculate the power in a similar way to how these measurements are made at the circuit breaker panel. This option is suitable when you want to record the data using same data acquisition (DAQ) system you are using for other measurements at the site (for example, using a Campbell Scientific data logger). If you are looking to make a quick measurement or monitor the appliance for a short period of time, and don't require the data to be recorded using the same DAQ as other monitored systems around the house, then a much easier way to measure the electricity use of plug loads is to use portable plug watt-hour meters at the receptacle.

Some common types of meters used to measure plug load power consumption:


Photo of a hand-held Kill-a-Watt meter. Meter has a digital display that reads 10.27.

Kill-A-Watt meter in use
Porkrind, Wikimedia Commons

The Kill-A-Watt is a device that can be used to measure the power consumption of 120V appliances and electronics. Plug the Kill-A-Watt into the wall outlet and plug the appliance into the Kill-A-Watt. The display can show instantaneous power (how many watts your appliance is drawing at any moment) or cumulative energy (how many watt-hours the appliance has consumed since installing the Kill-A-Watt). It also displays voltage, current, and apparent power. These devices are inexpensive (~) and simple to use, but do not have data storage or transfer capabilities, so they are mostly useful for spot checks, rather than long-term data-acquisition. The memory is volatile (i.e. erased each time you unplug it), so if you are interested in the cumulative energy use data, you must be sure the read and record the display before unplugging the device from the outlet.

WattsUp? Pro Power Meter

Photo of a WattsUp? Power meter. Meter has a small digital screen and two small buttons on the front.

Plug the WattsUp? Pro into the wall outlet and plug the appliance into the WattsUp? Pro. The meter is capable of displaying instantaneous power, cumulative energy use, time, root mean square (RMS) voltage and current, power factor, duty cycle, and cost information. It has internal memory so data is stored even when power is lost as well as additional software and Universal Serial Bus (USB) interface to a personal computer so it can be used for real-time data.


Available Products for Measuring Electrical Energy Consumption at the Receptacle

With the advent of Automated Home Energy Management systems, many manufacturers have begun offering products that plug into the receptacle in series with the appliance and provide power consumption and/or cumulative electrical energy use of that appliance. Below are some examples:

  • Upm Energy Meter with LCD Display EM100
  • Belkin Conserve Insight F7C005q Energy-Use Monitor
  • Stanley 77-028 Energy Meter EM100