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
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
1 KWhr = 3.6×106 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