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Updated : 16/10/2016

Energy Measurement Meters


The instantaneous electrical power in an AC circuit can be expressed as:

and is a continuously varying one with time, the average must be obtained by integration. Herein lies the ingenuity of the mechanical electricity meter in that this equation is continuously integrated mechanicaly.

However for practical use the average power is more useful. Averaging the instananeous power equation over one period T of the sinusoidal function will give the average power. The second term in the expression averages to zero since it is an odd function of t and the average of the first term is given by

Since the rms voltage and current are given by and ,the average power can be expressed as

Pavg = VI cosφ

where φ is the phase angle between the current and the voltage and where V and I are understood to be the effective or rms values of the voltage and current. The term cos φ is called the "power factor" for the circuit.

It is important to note that the common use of rms values for AC currents and voltages hides the fact that the maximum values of both are significantly higher. ie for a 240V domestic supply system the peak voltage will be 339V!

Electronic Meters

For a practical measurement, not only voltage and current amplitudes, but also phase angles and harmonic content may change constantly. Thus, simple RMS measurements are inherently inaccurate, and true RMS measurements must be utilized. A modern solid-state electricity Power and Energy Measurement IC such as the 78M6613 functions by emulating the integral operation above, i.e. it processes current and voltage samples through an ADC at a constant frequency. As long as the ADC resolution is high enough and the sample frequency is beyond the harmonic range of interest, the current and voltage samples, multiplied with the time period of sampling will yield an accurate quantity for the momentary energy. Summing up the momentary energy quantities over time will result in accumulated energy.

Power Equation

The above figure shows the shapes of V(t), I(t), the momentary power and the accumulated power, resulting from 50 samples of the voltage and current signals over a period of 20 ms. The application of 240 VAC and 100 A results in an accumulation of 480 Ws (= 0.133 Wh) over the 20 ms period, as indicated by the Accumulated Power curve. The described sampling method works reliably, even in the presence of dynamic phase shift and harmonic distortion.
The largest source of long-term errors in the meter is drift in the preamp, followed by the precision of the voltage reference. Both of these vary with temperature as well, and vary wildly because most meters are outdoors. Characterizing and compensating for these is a major part of meter design.

Efergy Energy monitoring socket 2.0

Efergy ems

By simply plugging the unit into the wall and the appliance into the unit you have installed the energy monitoring socket. The programming requires you to input the unit price you pay for your energy, choose which information you would like to see on the display and the unit will do the rest!

The energy monitoring socket 2.0 allows you to connect your appliances and assess how efficient they are by showing you not only the amount of power they are consuming but also how much they are costing. A large LCD display counts consumption by the Kilowatt-hour.


  • New LCD with more and clearer information
  • Information displayed: in time related, cost related, energy parameter
  • Up to 7 energy parameters displayed (W, kWh, V, A, Hz, PF %, Max)
  • Select up to two tariffs
  • Check your cost by day, by year and so far
  • Up to 5 different currencies available (, , $, Kr, R)
  • If the EMS is on AC power then the screen is always ON
  • If no appliance is connected the EMS will not start counting
  • When battery powered the screen shuts off after 30 seconds of inactivity
  • Programmable to your own energy unit cost.
  • Precision Accuracy to within 2%
  • Memory Battery back up
  • Large display for easy viewing
  • Battery option enables portability around the home
  • Record usage and cost over time
  • Low power consumption


  • The most serious disadvantage of this device is that the cost function does not work correctly! If the device is used to measure the energy used by equipment that is on intermittently (eg, frige, freezer, thermostatically controlled heater) the cost per day, cost per year values are incorrect!

    On trying the device on a domestic fridge I found that most of the results for current, power (kw) and energy (kWh) agree with the fridge manufacturers specifications but there appears to be a problem with the cost measurement function. The cost per year continues to increase the longer the unit is used whereas once the fridge has completed a number of on/off cycles the cost per day or cost per year should stabilise. For example using the unit over a 3 hour period with the Tariff set to R1.00 per kWh to simplify the calculations I obtained the following results from the unit.

    Time Cost/Year Run Time (min) Energy (kWh)
    1h0m R87.6 6 0.012
    2h0m R262.8 20 0.039
    3h0m R438.0 28 0.053

    Given that 0.053 kWh are used in a 3 hour period then @ R1.00/kWh the annual cost should be 0.053/3 * 24 * 365 = R154.76
    It seems that the EMS is rounding the kWh reading to 2 decimal places and multiplying by 24 and 365 and ignoring the time the unit has been running (NOT the run time )to give the following:-

    0.01 *24 *365 = 87.6
    0.03 *24 *365 = 262.8
    0.05 *24 *365 = 438.0

    This is obviously an incorrect way to determine the cost per year.

  • The data within the device cannot be downloaded to a PC for analysis

Mechanical Meters

In the mechanical wattmeter, one field is created by V, the other by I, and the attractive force between them is proportional to V x I (See figure below). Filtering is achieved by the inertia of the meter movement plus mechanical damping; otherwise, the pointer would vibrate rapidly with each cycle.

Energy Meter In the electro-mechanical induction meter two coils, the voltage coil and the current coil interact with a non-magnetic, but electrically conductive, metal disc causing it to rotate at a speed proportional to the power passing through the meter. A worm gear on the spindle of the disc drives a counter mechanism that displays the number of rotations of the disc and hence the amount of power consumed.

The voltage coil consumes a small and relatively constant amount of power, typically around 2 watts, which is not registered on the meter. The current coil similarly consumes a small amount of power in proportion to the square of the current flowing through it, typically up to a couple of watts at full load, which is registered on the meter.

The voltage coil is connected in such a way that it produces a magnetic flux proportional to the supply voltage. This magnetic flux produces eddy currents, within the disc, which interact with the magnetic flux produced by the current coil. Since the field of the voltage coil is delayed by 90 degrees, due to the coil's inductive nature, this interaction results in a rotational force being exerted on the disc which proportional to the product of the instantaneous current, voltage and phase angle (power factor) between them. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc and the equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power or rate of energy usage.

The salient features of the meter are shown in the following photograph and diagramme.

Electricity Meter Mechanism
  1. Voltage coil - many turns of fine wire encased in plastic, connected in parallel with load.
  2. Current coil - three turns of thick wire, connected in series with load.
  3. Stator - concentrates and confines magnetic field.
  4. Aluminum rotor disc.
  5. Rotor brake magnets.
  6. Spindle with worm gear.
  7. Display dials - note that the 1/10, 10 and 1000 dials rotate clockwise while the 1, 100 and 10000 dials rotate counter-clockwise

Energy Meter Mechanism

(A) The Current Stator;
(B) Metal Shunt,
(C) High Inductive Voltage Coil,
(D) Disk


The electromechanical induction meter has been the defacto standard in South Africa for many years. It is the reading produced by such meters which determine how much you will be charged regardless of want you may measure with any other instrument.


The meter dials are difficult to read and the there is no easy way to capture data from the meter for trend or data analysis.


  1. AC Power Measurement
  2. WikiPaedia - Electricity Meter
  3. How The Utility Meter Works
  4. Energy monitoring socket 2.0
  5. HyperPhysics
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