The definition of Power tells us that it is the amount of Energy supplied during a certain period of time.
In this case 1 kWh is the amount of energy released during a period of 60 minutes to replace a given task.
In practical terms 1kW hour is the equivalent of one thousand Watts, that is
[tex]P = \frac{E}{t}[/tex]
Where
P = Power
E = Energy
t = time
The power is equal to 1000W or 1000J/s through a hour, then
[tex]E = Pt[/tex]
[tex]E = 1000\frac{J}{s}1hour (\frac{3600s}{1hour})[/tex]
[tex]E = 3.6*10^6J[/tex]
Therefore the number of Joules of energy in one Kilowatt hour are [tex]3.6*10^6J[/tex]