KWH (kilowatt - hour) is defined as the electric energy consumed by an appliance of 1000 watt in one hour.
1 Kilo watt hour = 1 kilowatt x 1 hour
= 1000 watts x 60 x 60 sec
= 36 x ${{10}^{5}}$ W. second
= 36 x ${{10}^{5}}$ joules (watt, second = joule)
= 3.6 x ${{10}^{6}}$ J
.-. KWH = 3.6 x ${{10}^{6}}$ J