The kilowatt-hour is the unit in which usage of electricity is measured in the home.
Now as you know the rating of home appliances is its power and the unit of power is the Watt (W).
Now if a kettle with a rating of 1000 W is used for two hours, then the amount of energy used will be calculated as shown:
Power = Energy used / time taken
Energy used = power * time taken = 1000 * (2 * 3600)
= 7200000 J
= 7.2 MW (Megawatt)
However as you can see such calculation is difficult and beyond someone that is not a Physicist.
Hence in order to make matter more simple we use the Kilowatt-hour.
If we use the example above we would need to use the equation below:
Energy used in kilowatt-hour = power of appliance in Kilowatt x time in hour
Hence the energy used = 2 kW x 2 h
= 4 kilowatt-hour (kWh)
As you can see it is quite simple to determine the the energy used in kilowatt-hour.