Power & Energy

In this theory, the concept of power, energy and kilo watt hour is given.

Power, Electrical Energy and Kilo Watt Hour

Power 

• It is defined as the rate of doing work.

                                  OR

• It is rate at which electrical energy is consumed or dissipated.
• If W is electric energy consumed in a circuit for a given time, the power

        P = W / t

• Let, the electric energy supplied by a current source for a given time t is

       W = VQ

From equation ( 1 ) and ( 2 )

• P = VQ / t …… ( 3 )
• But I = Q / t …… ( 4 )

From equation ( 3 ) and ( 4 )

• P = VI

According to ohm’s law V = IR

• P = ( IR )R = I²R                           ( As V = IR )

According to ohm’s law I = V / R

• P = VI = V ( V / R ) = V² / R

Equation of Power

VI

I2R

V2 / R

 

The SI unit of power is watt.

Watt = Volt × Ampere

Electrical Energy

• It is defined as the amount of energy consumed in an electrical circuit for a given time. 
• The electrical energy not only depends upon electrical power but also on the time for which electrical power is supplied.
• Electrical energy = Power × Time
• Unit of electrical energy = Watt – Second

Kilo Watt Hour 

• The commercial unit of electrical energy is kilo watt hour ( kWh )/
• One Unit = One kWh

                  = 10³ × Watt × 3600 second

                  = 3.6 × 10⁶ Watt second

                  = 3.6 × 10⁶ Joule

Example of Kilo Watt Hour

How many units are consumed by a lamp of 100 W if it will turn on for twenty hour?

Solution

Rating of lamp = 100 W

Twenty hour

1 kWh = One unit

Number of units consumed by lamp = 100 × 20

                                                                 = 2000 Wh

                                                                = 2 kWh

                                                                = 2 Unit

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