The conception of electrical power plays an important role in Electrical Engineering. Almost all electrical appliances are rated for the voltage of the source on which it can work and also the amount of power required for its operation. For example, an electric lamp is rated as 250 V, 60 W etc.
As has been said earlier, the electric current is similar to the flow of a water through a pipe. The flow of water is measured in liters per second. The power required to keep the water current through the water pipe is equal to the product of water head or pressure and quantity of water flow.
Exactly in a similar way, to keep the electric current flow in the circuit, a power is required which is a product of electric current in amperes (I) and pressure in volts (E) maintaining this current flow. The unit in which the electrical power is measured is called as Watt (W).
Thus the unit of power “Watt” is defined as “that power used when one volt pressure causes one ampere current to flow in the circuit.”
Hence Power (P) = Volts (V) × Current (I) … (1)
or
P = V × I … (2)
Where, V = power in watts
E = pressure in volts
I = current in amperes
Again P = VI
But V = I × R
∴ P = (I×R) × I
∴ P = I2R … (3)
Also I = (V/R)h.
P = V × (V/R)
P = V2R … (4)
So, P = V × I = I2R = V2R … (5)
It is important to note:
There are 3 equations by which the electrical power can be determined. When the voltage and the current in the circuit are known the power is given by equation (2); when the resistance of circuit and the current in it is known then its power can be determined by the use of equation (3); when the pressure in volts and the resistance of the circuit is known the power in the circuit is known by the use of equation (4).
Kilowatt – The unit of power watt is a smaller unit, the bigger unit of power is equal to 1000 watts and is called as kilowatt.
Thus 1 kilowatt (kW) = 1000 W … (6)