Residential Energy Efficiency Monitoring Explained

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In order to monitor energy consumption, two types of devices are required: 1) A power meter and 2) Current Sensors (also called current transformers). The power meter picks up voltage from “hot” wires connected to the meter. However, in order to determine actual consumption, current sensors are needed. These two elements are used to determine kWs consumed. This is determined by the mathematical formula P = I * V, where P = power (measured in watts), I = Current (measured in Amps) and V = Voltage (measured in volts). In a basic residential installation, current sensors measure current in the 2 power lines feeding the house – typically near, or at, the power panel. Residential power panels in the US and many other countries use “split-phased” wiring as shown below. Note that the “A” phase alternates every other circuit breaker row.

Because AC power rises and falls in the form of a sine wave, when A is at its peak, B is at its lowest point (we call this 180 degrees out of phase).

Because A and B are out of phase, and residences are wired in ABAB fashion, devices that require 240 Volts, such as electric kitchen stoves and electric dryers, can receive 240 Volts by combining A and B (the difference between A and B when A peaks is 240 Volts because +120 – -120V = 240 V). If we count the circuit breaker rated amperage for each circuit breaker within A, we can get the maximum amperage, or maximum load, for all of the A wiring. In the example above the maximum amperage for Source A is 450A. The maximum amperage for Source B is 460A. Here’s a real residential power panel.In this example Source A and Source B have a max consumption of 150A and 160A, respectively. In a more detailed installation, users may want to monitor individual circuits within the residence. For example, you may want to know how much power is being consumed by the kitchen, the master bedroom, etc. In order to do this we can place a CT around the power line used by an individual circuit. (show diagram) Finally, the last type of installation users may want is to monitor an individual appliance. This can be done by placing a CT around the source side of the electrical wire.

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