What is 'one unit' or 'one kilowatthour' of electrical energy ?

The unit of electric energy is :

Assertion (A): A 1000 W bulb when used for 1 hour consumes the same number of electrical units as that of 100 W bulb that is used for 10 hours. Reason (R ) : On electrical unit is charged when one kilowatt hour electrical energy is consumed.

Define unit of electric energy?

What is the Si unit of energy?

What is the commercial unit of electrical energy ? Represent it in terms of joules.