Th S.I. unit of energy is joul(J).

Let the power (p) of an electrical appliance = P watt

Let it is used for t seconds.

Then Electrical energy consumed (W) = P x t watt-second

i.e. W = Pt Watt-second

or W = Pt Joul (Joul is Mechanical unit equivalent of Watt-second)

In practice bigger unit like Watt-Hour (Wh) or Kilo Watt Hour (KWh) is used for Electrical energy consumption. Watt-Hour : It is the electrical unit consumed by an electrical appliance of P watt that was used for 1 hour. 1 w-hr = 1 watt x 1 hr

= 1 w x (60×60)second

=3600 watt-second

=3600 Joul

1 Whr =3600 J

Kilowatt-Hour: One kilowatt-hour is the electrical energy consumed by an electrical appliance of power rating 1 kw when it used for 1 hour.

1 KWhr = 1 kilowatt x 1 hr

= 1000 w x (60×60)second

=3.6×10^{6} watt-second

=3.6×10^{6} Joul

1 KWhr = 3.6×10^{6} J

The electrical energy consumed in our houses or in industries are measured in KWhr . This is also called ! unit electrical unit consumed. The rate delared by governmet is Rs 8 per unit. It means Rs. 8 per KWhr