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Class 9, Science, Chapter-10, Lecture-3, Power (Notes)

Power: 

The rate of consumption of energy with respect to time is termed as power. 

Mathematically: ${\rm{P}} = {{\rm{E}} \over {\rm{t}}}$

where, 
${\rm{E}} = $ Energy consumed or Work done
${\rm{t}} = $ time

SI unit of Power is watt (W)

One Watt: 

The power of an appliance which consumes 1 joule of energy in 1 second is One watt.

Commercial Unit of Energy is kilo watt hour (kWh) 

One kilowatt hour: 

The amount of energy consumed in 1 hour by an appliance having a power of 1 kW is equal to 1 kWh

RELATION BETWEEN COMMERCIAL UNIT AND SI UNIT OF ELECTRICAL ENERGY:

SI unit of energy is joule ( J )
Commercial unit of energy is kilo watt hour ( kWh )
The amount of energy consumed in 1 hour by an appliance having a power of 1 kW is equal to 1 kWh 
Therefore, 
$1{\rm{kwh  =  1kw}} \times {\rm{1h}}$
$ = 1000{\rm{W}} \times 60 \times 60{\rm{s}}$
${\rm{ = 1000 J}}{{\rm{s}}^{ - 1}} \times 3600{\rm{s}}$
$ = 3600000{\rm{ J}}$

$1{\rm{ kwh  =  3}}{\rm{.6}} \times {\rm{1}}{{\rm{0}}^6}{\rm{ J}}$