__Basic Principle of Energy Measurement:__

Let us take the magnetic flux of the shunt magnet lags by exactly 90° phase difference behind the applied voltage (V) as per the theory of the induction type watt meter. The driving torque T_{d} α VI CosΦ where V = RMS value of voltage and I = RMS value of current of the electrical circuit and Φ = phase angle between voltage and current. Thus the torque is proportional to the power of the electrical circuit. Now, the braking torque T_{B} α N where N = speed of revolution of the disc.

For steady speed, driving torque T_{d} = braking torque T_{B}. Now, N α VI CosΦ or power α Speed revolutions. The number of revolutions which equals N, dt is proportional to the energy supplied.

### Meter Constant of Energy Meter:

The number of revolutions of the disc of the energy meter for the consumption of one kilowatt hour or one unit is called the meter constant of the energy meter. Therefore, the meter constant,

K = number of revolutions of the disc / Kilowatt-hour

If the meter constant of the energy meter is marked 900, it means that for the consumption of 1 kWhr of electrical energy, if the disc of the energy meter will make 900 revolutions. To specify an energy meter, we have to mention the voltage grade, current rating, system and type of supply. The commercial unit of electrical energy is a kilowatt-hour (kWh) or BOT. unit.

If an energy meter isn’t available for the measurement of electrical energy consumption in an electrical circuit, we can measure the energy consumption of the electrical circuit with the help of a wattmeter and a stopwatch. The wattmeter will read the electrical power consumed in watts and the stopwatch will record the time.

Now, we know that

The electrical energy = Electrical Power x time

= Wattmeter reading (in watt) x stopwatch reading in seconds

= watt/1000 kilo-watt x (time in second / 60×60) hour

= Kilowatt hour

= Electrical energy consumed in the electrical circuit